ctronic Structure d Covalent Bonding

ctronic Structured Covalent Bonding

Ethane Ethene

Ethyne

y alive, early humans must have been able to tell the difference betweenkinds of materials in their world. "You can live on roots and berries," they

~{ have said, "but you can't live on dirt. You can stay warm by burning tree{you can't burn rocks."

early eighteenth century, scientists thought they had grasped the nature ofence. Compounds derived from living sources were believed to contain an

_;c: , zale vital force-the essence of life. Because they came from organisms,~ called "organic" compounds. Compounds derived from minerals-those

vital force-were "inorganic."hemists could not create life in the laboratory, they assumed they could

compounds that had a vital force. Since this was their mind-set, you can. surprised chemists were in 1828 when Friedrich Wohler producedpound known to be excreted by mammals-by heating ammoniumorganic mineral.

NH40CN ~ammonium cyanate

oII

/C"H2N NH2

urea

rime, an "organic" compound had been obtained from something other= organism and certainly without the aid of any kind of vital force. Clearly,eeded a new definition for "organic compounds." Organic compounds are

compounds that contain carbon._ entire branch of chemistry devoted to the study of carbon-containing

~-' We study organic chemistry because just about all of the molecules that

CHAPTER 1

BIOGI(AI'II"

German chemist FriedrichWohler (1800-1882) began hisprofessional life as C! physicianand later became a-professor ofchemistry at the UniversityofGottingen. WoMer codiscov-ered the fact that two differentchemicals could have the samemolecular formula. He also de-veloped methods of purifyingaluminum-at the time, themost expensive metal onEarth-and beryllium.

r-

1

2 CHAP T ER 1 Electronic Structure and Covalent Bonding

make life possible-proteins, enzymes, vitamins, lipids, carbohydrates, and nucleiacids-contain carbon; thus, the chemical reactions that take place in living systemsincluding our own bodies, are reactions of organic compounds. Most of the com-pounds found in nature-those we rely on for food, medicine, clothing (cotton, wool.silk), and energy (natural gas, petroleum)-are organic compounds as well.

Organic compounds are not, however, limited to those found in nature. Chemistshave learned to synthesize millions of organic compounds never found in nature, in-cluding synthetic fabrics, plastics, synthetic rubber, medicines, and even things likephotographic film and Super Glue. Many of these synthetic compounds prevent short-ages of naturally occurring products. For example, it has been estimated that if syn-thetic materials were not available for clothing, all of the arable land in the UnitedStates would have to be used for the production of cotton and wool just to provideenough material to clothe us. Currently, there are about 16 million known organiccompounds, and many more are possible.

What makes carbon so special? Why are there so many carbon-containing com-pounds? The answer lies in carbon's position in the periodic table. Carbon is in thecenter of the second row of elements. The atoms to the left of carbon have a tendencyto give up electrons, whereas the atoms to the right have a tendency to accept electrons(Section 1.3).

Li FBe B C

the second row of the periodic table

Because carbon is in the middle, it neither readily gives up nor readily accepts elec-trons. Instead, it shares electrons. Carbon can share electrons with several differentkinds of atoms, and it can also share electrons with other carbon atoms. Consequently,carbon is able to form millions of stable compounds with a wide range of chemicalproperties simply by sharing electrons.

When we study organic chemistry, we study how organic compounds react. Whenan organic compound reacts, some existing bonds break and some new bonds form.Bonds form when two atoms share electrons, and bonds break when two atoms nolonger share electrons. How readily a bond forms and how easily it breaks depend onthe particular electrons that are shared, which, in turn, depend on the atoms to whichthe electrons belong. So if we are going to start our study of organic chemistry at thebeginning, we must start with an understanding of the structure of an atom-whatelectrons an atom has and where they are located.

NATURAL VERSUS SYNTHETICIt is a popular belief that natural substances-those made in nature-are superior to syntheticones-those made in the laboratory. Yet when a

chemist synthesizes a compound, such as penicillin, it is ex-actly the same in all respects as the compound synthesizedin nature. Sometimes chemists can improve on nature. Forexample, chemists have synthesized analogs of morphine-compounds with structures similar to but not identical to thatof morphine-that have painkilling effects like morphinebut, unlike morphine, are not habit forming. Chemists havesynthesized analogs of penicillin that do not produce the al-lergic responses that a significant fraction of the populationexperiences from naturally produced penicillin, or that donot have the bacterial resistance of the naturally producedantibiotic.

A field of poppies growing in Afghanistan. Commercialmorphine is obtained from opium, the juice obtained fromthis speciesof poppy.

Section 1.2 How the Electrons in an Atom Are Distributed 3

THE STRUCTURE OF AN ATOMconsists of a tiny dense nucleus surrounded by electrons that are spread

~~ut a relatively large volume of space around the nucleus. The nucleus con-sitively charged protons and neutral neutrons, so it is positively charged. The

~-5 are negatively charged. Because the amount of positive charge on a proton·•..e amount of negative charge on an electron, a neutral atom has an equal num-;:mtons and electrons. Atoms can gain electrons and thereby become negatively

_=-'C. or they can lose electrons and become positively charged. However, the nurn-tons in an atom does not change.and neutrons have approximately the same mass and are about 1800 times

ssive than an electron. This means that most of the mass of an atom is in itsHowever, most of the volume of an atom is occupied by its electrons, and that

_ our focus will be because it is the electrons that form chemical bonds.nomic number of an atom equals the number of protons in its nucleus. Thezamber is also the number of electrons that surround the nucleus of a neutral

~ example, the atomic number of carbon is 6, which means that a neutral car-has six protons and six electrons.

number of an atom is the sum of its protons and neutrons. All carbone the same atomic number because they all have the same number of pro-.- do not all have the same mass number because they do not all have the

-ber of neutrons. For example, 98.89% of naturally occurring carbon atomseutrons-giving them a mass number of 12-and 1.11% have sevengivinf them a mass number of 13. These two different kinds of carbon

-=c~and 3C) are called isotopes. Isotopes have the same atomic number (thate number of protons), but different mass numbers because they have differ-

-- - - -- of neutrons ..- occurring carbon also contains a trace amount of 14C, which has six pro-ghr neutrons. This isotope of carbon is radioactive, decaying with a half-life

-Z"S. (The half- life is the time it takes for one-half of the nuclei to decay.) Ast or an animal is alive, it takes in as much 14C as it excretes or exhales .. it no longer takes in 14C, so the 14C in the organism slowly decreases.age of an organic substance can be determined by its 14C content.

lc weight (or atomic mass) of a naturally occurring element is theof its atoms. For example, carbon has an atomic weight of 12.011 atom-

s. The molecular weight of a compound is the sum of the atomic weightsin the molecule.

~1,.·"

- three isotopes with mass numbers of 16,17, and 18. The atomic number ofHowmany protonsand neutronsdoes each of the isotopes have?

THE ELECTRONS IN AN ATOMDISTRIBUTEDan atom can be thought of as occupying a set of shells that surround

+- >"F::e way in which the electrons are distributed in these shells is based on-~~ .• ~ by Einstein. The first shell is the smallest and the one closest to the

md shell is larger and extends farther from the nucleus; and the thirdred shells extend even farther out. Each shell consists of subshells

-0: H=r=ic orbitals. The first shell has only an s atomic orbital; the second shellp atomic orbitals; and the third shell consists of s, p, and d atomic

21,38

S,p, d1,3,518


I

Table 1.1 Distribution of Electrons in the First Three Shells That Surround the NucleusI

First shell Second shell Third shell

Atomic orbitalsNumber of atomic orbitalsMaximum number of electrons

S s.p

Table 1.2 The Electronic Configurations of the Smallest Atoms

Name of AtomicAtom element number Is 2s 2px 2py 2pz 3s

H Hydrogen 1 1He Helium 2 uLi Lithium 3 u 1Be Beryllium 4 iJ, uB Boron 5 u u 1C Carbon 6 u u 1 1N Nitrogen 7 iJ, iJ, 1 1 10 Oxygen 8 iJ, iJ, 11 1 1F Fluorine 9 iJ, iJ, u u iNe Neon 10 iJ, n iJ, iJ, iJ,Na Sodium 11 iJ, iJ, iJ, u iJ, i

Each shell contains one s orbital. The second and higher shells-in addition to theirs orbital-each contain three p orbitals. The three p orbitals have the same energy. Thethird and higher shells-in addition to their sand p orbitals-also contain five d or-bitals. Because an orbital can contain no more than two electrons (see below), the firstshell, with only one atomic orbital, can contain no more than two electrons. The sec-ond shell, with four atomic orbitals=-one s and three p-can have a total of eight elec-trons. Eighteen electrons can occupy the nine atomic orbitals-one s, three p, and fived-of the third shell.

An important point to remember is that the closer the atomic orbital is to the nucle-us, the lower is its energy. Because the s orbital in the first shell (called a Is orbital) iscloser to the nucleus than is the s orbital in the second shell (called a 2s orbital), the Is

The closer the orbital is to the nucleus, the orbital is lower in energy. Comparing orbitals in the same shell, we see that an s orbitallower is its energy. is lower in energy than a p orbital, and a p orbital is lower in energy than a d orbital.

Relative energies of atomic orbitals: Is < 2s < 2p < 3s < 3p < 3d

The electronic configuration of an atom describes what orbitals the electron •.occupy. The following three rules are used to determine an atom's electronicconfiguration:

1. An electron always goes into the available orbital with the lowest energy.2. No more than two electrons can occupy each orbital, and the two electrons must

be of opposite spin. (Notice in Table 1.2 that spin in one direction is designatedby i, and spin in the opposite direction by J.) .

From these first two rules, we can assign electrons to atomic orbitals for atoms thatcontain one, two, three, four, or five electrons. The single electron of a hydrogen atomoccupies a Is orbital, the second electron of a helium atom fills the Is orbital, the thirdelectron of a lithium atom occupies a 2s orbital, the fourth electron of a berylliumatom fills the 2s orbital, and the fifth electron of a boron atom occupies one of the 2porbitals. (The subscripts x, y, and z distinguish the three 2p orbitals.) Because the three

Section 1.2 How the Electrons in an Atom Are Distributed 5

ALBERT EINSTEINAlbert Einstein (1879-1955) was born in Germany. When he was in high school,his father's business failed and his family moved to Milan, Italy. Although Ein-stein wanted to join his family in Italy, he had to stay behind because German

law required compulsory military service after high school. To help him, his high schoolmathematics teacher wrote a letter saying that Einstein could have a nervous breakdownwithout his family and also that there was nothing left to teach him. Eventually, Einstein wasasked to leave the school because of his disruptive behavior. Popular folklore says he left be-cause of poor grades in Latin and Greek, but his grades in those subjects were fine.

Einstein was visiting the United States when Hitler came to power, so he accepted a po-sition at the Institute for Advanced Study in Princeton, becoming a U.S. citizen in 1940. Al-though a lifelong pacifist, he wrote a letter to President Roosevelt warning of ominousadvances in German nuclear research. This led to the creation of the Manhattan Project,which developed the atomic bomb and tested it in New Mexico in 1945.

td

sp orbitals have the same energy, the electron can be put into anyone of them. Beforewe can continue to atoms containing six or more electrons, we need the third rule:

3. When there are two or more orbitals with the same energy, an electron willoccupy an empty orbital before it will pair up with another electron.

The sixth electron of a carbon atom, therefore, goes into an empty 2p orbital, ratherthan pairing up with the electron already occupying a 2p orbital (Table 1.2). There isone more empty 2p orbital, so that is where the seventh electron of a nitrogen atomgoes. The eighth electron of an oxygen atom pairs up with an electron occupying a 2porbital rather than going into a higher energy 3s orbital.

Electrons in inner shells (those below the outermost shell) are called coreelectrons. Electrons in the outermost shell are called valence electrons. Carbon, forexample, has two core electrons and four valence electrons (Table 1.2).

Lithium and sodium each have one valence electron. Elements in the same column,f the periodic table have the same number of valence electrons. Because the number

of valence electrons is the major factor determining an element's chemical properties,elements in the same column of the periodic table have similar chemical properties.You can find a periodic table inside the back cover of this book.) Thus, the chemicalbehavior of an element depends on its electronic configuration.

Itndnpe

How many valence electrons do the following atoms have?a. carbon b. nitrogen c. oxygen d. fluorine

Table 1.2 shows that lithium and sodium each have one valence electron. Find potassium(K) in the periodic table and predict how many valence electrons it has.

PROBLEM-SOLVING STRATEGYWrite the ground-state electronic configuration for chlorine.The periodic table in the back of the book shows that chlorine has 17 electrons. Now weneed to assign the electrons to orbitals using the rules that determine an atom's electronicconfiguration. Two electrons are in the Is orbital, two are in the 2s orbital, six are in the 2porbitals, and two are in the 3s orbital. This accounts for 12 of the 17 electrons. The remain-ing five electrons are in the 3p orbitals. Therefore, the electronic configuration is writtenas: Is2, 2s2, 2p6, 3s2, 3p5_.ow continue on to Problem 4.

Tutorial:,k Electrons in orbitals1fJ~··CV-:..

Shown is a bronze sculpture ofEinstein on the grounds of theNational Academy of Sciences inWashington, D.C. It measures 21 feetfrom the top of the head to the tip ofthe feet and weighs 7000 pounds.In his left hand, Einstein holds themathematical equations that repre-sent his three most important contri-butions to science: the photoelectriceffect, the equivalency of energy andmatter, and the theory of relativity.At his feet is a map of the sky.

"


1I10GI{APIIY

American chemist GilbertNewton Lewis (1875-1946).was born in Weymo"'iith,Massa-chusetts, and received a Ph.D.from Harvard in 1899. He wasthe first person to prepare"heavy water," which has'

-, deuterium atoms in place ofthe usual hydrogen atoms (D20versus H20). Because heavywater can be used as a moder-ator of neutrons, it became im-portant in the development ofthe atomic 'bomb, Lewis startedhis career as a professor atthe Massachusetts Institute ofTechnology and joined thefaculty at the University-of .California, Berkeley, in 1912,

How many valence electrons do chlorine, bromine, and iodine have?

a. Compare the ground-state electronic configurations for nitrogen and phosphorus andlook at their relative positions in the periodic table, How many valence electrons doeseach have? How many core electrons does each have?

b. Compare the ground-state electronic configurations for oxygen and sulfur and look attheir relative positions in the periodic table, How many valence electrons does eachhave? How many core electrons does each have?

1.3 IONIC AND COVALENT BONDSIn trying to explain why atoms form bonds, O. N. Lewis proposed that an atom is moststable if its outer shell is either filled or contains eight electrons and it has no electronsof higher energy. According to Lewis's theory, an atom will give up, accept, or shareelectrons in order to achieve a filled outer shell or an outer shell that contains eightelectrons. This theory has come to be called the octet rule (even though the filledouter shell of hydrogen has only two electrons).

Lithium (Li) has a single electron in its 2s orbital. If it loses this electron, the lithi-um atom ends up with a filled outer shell-a stable configuration. Lithium, therefore,loses an electron relatively easily. Sodium (Na) has a single electron in its 3s orbital,so it too loses an electron easily.

When we draw the electrons around an atom, as in the following equations, coreelectrons are not shown; only valence electrons are shown because only valence elec-trons are used in bonding. Each valence electron is shown as a dot. Notice that whenthe single valence electron of lithium or sodium is removed, the resulting atom-nowcalled an ion--carries a positive charge.

lithium has lostan electron

u- Li+ + e-a lithium iona lithium atom

Na' Na+ + e-a sodium iona sodium atom

Fluorine and chlorine each have seven valence electrons (Table 1.2 and Problem 5). Con-sequently, they readily acquire an electron in order to have an outer shell of eight electrons.

fluorine has gainedan electron

:f.' + ea fluorine atom a fluoride ion

:~l' + e-a chlorine atom

~ :~l:-a chloride ion

Ionic Bonds Are Formed by the Attraction Between Ionsof Opposite ChargeWe have just seen that sodium gives up an electron easily and chlorine readily acquiresan electron. Therefore, when sodium metal and chlorine gas are mixed, each sodiumatom transfers an electron to a chlorine atom, and crystalline sodium chloride (table

alt) is formed as a result. The positively charged sodium ions and negatively chargedhloride ions are independent species held together by the attraction of oppositeharges (Figure 1.1). A bond is an attractive force between two ions or between twoatoms. A bond that results from the attraction between ions of opposite charge iscalled an ionic bond.

an ionic bond is theattraction between ionsof opposite charges

:~r:Na+f:~l:Na+:~l: Na+

:~l: Na+:~l:

sodium chloride

»stInsire~tted

a. b.

hi-re.:aI.

)reec-tenow valent Bonds Are Formed by Sharing Electrons

stead of giving up or acquiring electrons, an atom can achieve a filled outer shell (orter shell of eight electrons) by sharing electrons. For example, two fluorine atomseach attain a filled second shell by sharing their unpaired valence electrons. Aformed as a result of sharing electrons is called a covalent bond.

a covalent bond is formedby sharing electrons.... ~:R' + 'R: ~ :R:R:

on-ns. ydrogen atoms can form a covalent bond by sharing electrons. As a result of co-

bonding, each hydrogen acquires a stable, filled first shell.

H' + 'H ~ H:H

srly, hydrogen and chlorine can form a covalent bond by sharing electrons. In~ 50. hydrogen fills its only shell and chlorine achieves an outer shell of eight

H' + '~l: ~ H:~l:

ireiumtble

gen atom can achieve a completely empty shell by losing an electron. Loss ofelectron results in a positively charged hydrogen ion. A positively charged hy-ion is called a proton because when a hydrogen atom loses its valence

~_.~ only the hydrogen nucleus-which consists of a single proton-remains.

Section 1.3 Ionic and Covalent Bonds 7

<411Figure 1.1(a)Crystalline sodium chloride.(b)The electron-rich chloride ions arered,and the electron-poor sodium ionsare blue. Eachchloride ion is surroundedby six sodium ions,and each sodium ionis surrounded by six chloride ions. Ignorethe sticks holding the balls together;they are there only to keep the modelfrom falling apart.

3-D Molecule:

~ Sodium chloride lattice

8 C HA PT ER 1 Electronic Structure and Covalent Bonding

A hydrogen atom can achieve a filled outer shell by gaining an electron, thereby form-ing a negatively charged hydrogen ion, called a hydride ion.

H'a hydrogen atom

W + ea proton

Because oxygen has six valence electrons, it needs to form two covalent bonds toachieve a filled outer shell. Nitrogen, with five valence electrons, must form three co-valent bonds, and carbon, with four valence electrons, must form four covalent bondsto achieve a filled outer shell. Notice that all the atoms in water, ammonia, andmethane have filled outer shells.

a hydrogen atom a hydride ion

Nonpolar Covalent Bonds and Polar Covalent BondsThe atoms that share the bonding electrons in the F- For the H - H covalent bondare identical. Therefore, they share the electrons equally; that is, each electron spendsas much time in the vicinity of one atom as in the other. Such a bond is called anonpolar covalent bond.

In contrast, the bonding electrons in hydrogen chloride, water, and ammonia aremore attracted to one atom than to another because the atoms that share the electronsin these molecules are different and have different electronegativities. Electro-negativity is a measure of the ability of an atom to pull the bonding electrons towarditself. The electronegativities of some of the elements are shown in Table 1.3. The

oxygen has formed

2H' 'Q: H:O: 2 covalent bonds+ ----;.

itwater nitrogen has formed

3H' .~. H:N:H 3 covalent bonds+ ----;.

itammonia

H carbon has formed

4H' -c- H:C:H 4 covalent bonds+ ----;.

itmethane

Table 1.3 The Electronegativities of Selected Elementsa

IAI

IIAI IB I

liB I iliA IIVA I VA I VIA I VilA

H2.1-- f----

Li Be B C N 0 F1.0 1.5 2.0 2.5 3.0 3.5 4.0

-- -- -- -- -- -- --Na Mg Al Si P S CI0.9 1.2 1.5 1.8 2.1 2.5 3.0-- --K Ca Br0.8 1.0 2.8

--I

2.5

aElectronegativity values are relative. not absolute. As a result, there are several scales of electronegativities.The electronegativities listed here are from the scale devised by Linus Pauling.

II

oI-

.sd

Iddsa

rens0-

rdbe

bonding electrons in hydrogen chloride, water, and ammonia are more attracted to theatom with the greater electronegativity. A polar covalent bond is a covalent bond be-tween atoms of different electronegativities. Notice that electronegativity increasesfrom left to right across a row of the periodic table or going up any of the columns.

A polar covalent bond has a slight positive charge on one end and a slight negativecharge on the other. Polarity in a covalent bond is indicated by the symbols 0+ and5-, which denote partial positive and partial negative charges, respectively. The neg-ative end of the bond is the end that has the more electronegative atom. The greaterthe difference in electronegativity between the bonded atoms, the more polar the

nd will be. (Notice that a pair of shared electrons can be shown as a line betweentwo atoms.)

6+ 0-H-G!:0+ Ii-;H-O:

IHc5+

ii+ 1-; 0+H-N-H

IH6+

You can think of ionic bonds and nonpolar covalent bonds as being at the opposites of a continuum of bond types. At one end is an ionic bond, a bond in which thereno sharing of electrons. At the other end is a nonpolar covalent bond, a bond inich the electrons are shared equally. Polar covalent bonds fall somewhere in be-een, and the greater the difference in electronegativity between the atoms formingbond, the closer the bond is to the ionic end of the continuum. C - H bonds aretively nonpolar, because carbon and hydrogen have similar electronegativitiestronegativity difference = 0.4; see Table 1.3). N - H bonds are relatively polartronegativity difference = 0.9), but not as polar as 0- H bonds (electronegativi-ifference = 1.4). Even closer to the ionic end of the continuum is the bond betweenium and chloride ions (electronegativity difference = 2.1), but sodium chloride isas ionic as potassium fluoride (electronegativity difference = 3.2).

continuum of bond types

tonicbondK+P- Na+Cl-

polarcovalent bondO-H N-H

nonpolarcovalent bondC-H C-C

Section 1.3 Ionic and Covalent Bonds 9

Tutorial:~ Electronegativity differences andt!J'f..~ bond types

fIQ~!J~.§)~-:

L---- -----,

. h bond is more polar?

H-CH3 or CI-CH3

H-OH or H-H

c. H-CI or H-P

d. CI-Cl or CI-CH3

1l£O~~~i7,J1

h of the following hasmost polar bond? b. the least polar bond?

_ .:11 LiBr Cl2 KCI

I•..qnw»~ymbols 8+ and 8- to show the direction of polarity of the indicated bond in each

0+ 0-

following compounds (for example, H3C-OH)

b. H3C-NH2 c. HO-Br d. I-CI

/

/


Electrostatic potential maps (often simply called potential maps) are models thatshow how charge is distributed in the molecule under the map. The potential maps forLiH, H2, and HF are shown below.

3-D Molecules:

~ LiH;H2;HF

.»:

LiH HF

The colors indicate the distribution of charge in the molecule: red signifies electron-rich areas (negative charge); blue signifies electron-deficient areas (positive charge);and green signifies no charge. For example, the potential map for LiH indicates thatthe hydrogen atom is more negatively charged than the lithium atom. By comparingthe three maps, we can tell that the hydrogen in LiH is more negatively charged thana hydrogen in H2, and the hydrogen in HF is more positively charged than a hydrogenin H2.

attracts positivecharge

attracts negativechargered • orange • yellow • green • blue

most negativeelectrostatic potential

most positiveelectrostatic potential

PROBLEM 9.

After examining the potential maps for LiH, HF, and H2, answer the following questions:a. Which compounds are polar?h. Which compound has the most positively charged hydrogen?

1.4 HOW THE STRUCTURE OF A COMPOUNDIS REPRESENTED

First we will see how compounds are drawn using Lewis structures. Then we will look atthe representations of structures that are used more commonly for organic compounds.

Lewis StructuresThe chemical symbols we have been using, in which the valence electrons are repre-sented as dots, are called Lewis structures (or electron dot structures) after G. N.Lewis (Section 1.3). Lewis structures are useful because they show us which atoms arebonded together and tell us whether any atoms possess lone-pair electrons or have aformal charge, two concepts we describe below. The Lewis structures for H20, H30+,HO-, and H202 are

H)a formal charge !. )a formal charg~.I.•

H:Q:H H:Q:- H:Q:Q:Hwater hydronium ion hydrogen peroxidehydroxide ion

1

tgnn

~

]

: atds.

re-N.aree a)+,

Section 1.4 How the Structure of a Compound Is Represented 11

When you draw a Lewis structure, make sure that hydrogen atoms are surroundedby no more than two electrons and that C, 0, N, and halogen (P, CI, Br, I) atoms areurrounded by no more than eight electrons, in accordance with the octet rule. Valenceelectrons not used in bonding are called nonbonding electrons, lone-pair electrons,or simply lone pairs.

Once you have all the atoms and the electrons in place, you must examine eachatom to see whether a formal charge should be assigned to it. A formal charge is thedifference between the number of valence electrons an atom has when it is not bonded(0 any other atoms and the number of electrons it "owns" when it is bonded. An atom"owns" all of its lone-pair electrons and half of its bonding (shared) electrons.

formal charge = number of valence electrons - (number oflone-pair electrons + 1/2 number of bonding electrons)

For example, an oxygen atom has six valence electrons (Table 1.2). In water (H20),oxygen "owns" six electrons (four lone-pair electrons and half of the four bondingelectrons). Because the number of electrons it "owns" is equal to the number of its va-nce electrons (6 - 6 = 0), the oxygen atom in water does not have a formal charge.

The oxygen atom in the hydronium ion (H30+) "owns" five electrons: two lone-pairelectrons plus three (half of six) bonding electrons. Because the number of electrons it"owns" is one less than the number of its valence electrons, its formal charge is-1 (6 - 5 = 1). The oxygen atom in hydroxide ion HO- "owns" seven electrons:. lone-pair electrons plus one (half of two) bonding electron. Because it "owns" oneore electron than the number of its valence electrons, its formal charge is

-1 (6 - 7 = -1).

H30+ H20 HO-

PROBLEM10.The formal charge does not necessarily indicate that the atom has greater or less electrondensity than other atoms in the molecule without formal charges. You can see this by ex-amining the potential maps for H20, H30+, and HO-.a. Which atom bears the formal negative charge in the hydroxide ion?b. Which atom has the greater electron density in the hydroxide ion?c. Which atom bears the formal positive charge in the hydronium ion?d. Which atom has the least electron density in the hydronium ion?

Nitrogen has five valence electrons (Table 1.2). Prove to yourself that the appropri-~ formal charges have been assigned to the nitrogen atoms in the following Lewis

tures:

H:N:Hii

ammonia

H+H:N:Hii

ammonium ion

H:N:N:Hiiii

hydrazine

H:N:-ii

amide anion

Carbon has four valence electrons. Take a moment to understand why the carbonin the following Lewis structures have the indicated formal charges:

H H H H HHH:C:H H:C+ H:C:- H:C' H:C:C:Hii ii ii ii iiii

methane methyl cation methyl anion methyl radical ethanea carbocation a carbanion

tI!f'.VI

Movie:Formal charge

A species containing a positively charged carbon atom is called a carbocation, and aspecies containing a negatively charged carbon atom is called a carbanion. (Recallthat a cation is a positively charged ion and an anion is a negatively charged ion.) Aspecies containing an atom with a single unpaired electron is called a radical (oftencalled a free radical).

Hydrogen has one valence electron, and each halogen (F, Cl, Br, I) has seven va-lence electrons, so the following species have the indicated formal charges:

'2 C H APT ER 1 Electronic Structure and Covalent Bonding

b.H-C-HIH

H HI Id.H-N-B-HI IH H

H+ H:- H· :~r:- :Br- :Br:Br: :~l:~l:hydrogen hydride hydrogen bromide bromine bromine chlorine

ion ion atom ion atoma radical a radical

PROBLEM11.Give each atom the appropriate formal charge:

In studying the molecules in this section, notice that when the atoms do not bear aformal charge or an unpaired electron, hydrogen and the halogens always have one co-valent bond, oxygen always has two covalent bonds, nitrogen always has three cova-lent bonds, and carbon hasfour covalent bonds. Atoms that have more bonds or fewerbonds than the number required for a neutral atom will have either a formal charge oran unpaired electron. These numbers are very important to remember when you arefirst drawing structures of organic compounds because they provide a quick way torecognize when you have made a mistake.

HI .•H-C-Br:I "H

HI .•H-C-O-HI ..H

H HI .. IH-C-O-C-HI •• IH H

HI ••H-C-N-HI IH H

H HI .. IH-C-N-C-HI I IH H H

:f.- :~J- :0- -N- IH- -C-:x- :~r- I I Ione bond one bond two bonds three bonds four bonds

In the following Lewis structures, notice that each atom has a filled outer shell.Also notice that since none of the molecules has a formal charge or an unpaired elec-tron, C forms four bonds, N forms three bonds, 0 forms two bonds, and Hand Br eachform one bond.

HH:C:Br:i-i ..

HH:C:O:H

i-i"H H

H:C:O:C:Hi-i"i-i

HH:C:N:H

i-ii-iH H

H:C:N:C:Hi-ii-ii-i

Because a pair of shared electrons can be shown as a line between two atoms, comparethe preceding structures with the following ones:

PROBLEM12.

Draw the Lewis structure for each of the following:+a.CH3NH3 b.T2HS c.NaOH d.NH4CI

Section 1.4 How the Structure of a Compound Is Represented 13

a1\o

Kekule StructuresIn Kekule structures, the bonding electrons are drawn as lines and the lone-pair elec-trons are usually left out entirely, unless they are needed to draw attention to somechemical property of the molecule. (Although lone-pair electrons are not shown, youshould remember that neutral nitrogen, oxygen, and halogen atoms always have them:one pair in the case of nitrogen, two pairs in the case of oxygen, and three pairs in thecase of a halogen.)

l-

HI

H-C-N-HI IH H

H HI I

H-C-N-C-HI I IH H H

HI

H-C-BrIH

HI

H-C-O-HIH

H HI I

H-C-O-C-HI IH H

Ie

Condensed StructuresFrequently, structures are simplified by omitting some (or aU) of the covalent bondsand listing atoms bonded to a particular carbon (or nitrogen or oxygen) next to it withubscripts as necessary. These structures are called condensed structures. Comparethe following examples with the Kekule structures shown above:

CH30H CH30CH3CH3Br CH3NH2 CH3NHCH3

'a0-

You can find more examples of condensed structures and the conventions common-used to create them in Table 1.4.

'a-'erorrreto

, ~!t.Q_BL~_M'13~Draw the lone-pair electrons that are not shown in the following structures:

a. CH3CH2NH2 c. CH3CH20H e. CH3CH2CI

b. CH3NHCH3 d. CH30CH3 f. HONH2

·R;ij4u.~'im)~U.ec-ich

Draw condensed structures for the compounds represented by the following models(black = C, white = H, red = 0, blue = N, green = Cl):

a. c.

are

d.b.

-H

] PROBL.~'~M_~~Which of the atoms in the molecular models in Problem 14 havea. three lone pairs? b. two lone pairs? c. one lone pair? d. no lone pairs?

v/

v

/

Table 1.4 Kekule and Condensed Structures

Condensed structures

14 CHA PT ER 1 Electronic Structure and Covalent Bonding

Kekule structure

Atoms bonded to a carbon are shown to the right of the carbon. Atoms other than H can be shown hanging from the carbon.

Repeating CHzgroups can be shown in parentheses.

Groups bonded to a carbon can be shown (in parentheses) to the right of the carbon, or hanging from the carbon.

Groups bonded to the far-right carbon are not put in parentheses.

Two or more identical groups considered bonded to the "first" atom on the left can be shown (in parentheses) to the left of that atom,or hanging from the atom.

H H H H H HI I I I I I

H-C-C-C-C-C-C-HI I I I I IH Br H H CI H

H H H H H HI I I I I I

H-C-C-C-C-C-C-HI I I I I IH H H H H H

HHH HHHI I I I I I

H-C-C-C-C-C-C-HI I I I I IH H CH3H OHH

H H CH3H HI I I I I

H-C-C-C-C-C-HI I I I IH H CH3H OH

H H H HI I I I

H-C--N-C-C-C-HI I I I IH H-C-H H H H

IH

H H H H HI I I I I

H-C--C-C-C-C-HI I I I IH H-C-H H H H

IH

CH3I

CH3CHzCCH?CHzOHI -CH3

(CH3hNCHzCH2CH3

PROBLEM 16

Which of the following molecular formulas are not possible for an organic compound?

"

1,

Section 1.5 Atomic Orbitals 15

-:a. Draw two Lewis structures for C2H60.b. Draw three Lewis structures for C3HsO.

(Hint: The two Lewis structures in part a are constitutional isomers; they have thesame atoms, but differ in the way the atoms are connected; see page 143. The threeLewis structures in part b are also constitutional isomers.)

PROBLEM 18 '

Expand the following condensed structures to show the covalent bonds and lone pairs:a. CH3NH(CH2hCH3 c. (CH3hCOHb. (CH3hCHCl d. (CH3hCCCH2hCH(CH3h

1.5 ATOMIC ORBITALSWe have seen that electrons are distributed into different atomic orbitals (Table 1.2)._-\11 orbital is a three-dimensional region around the nucleus where an electron is most.. ely to be found. But what does an orbital look like? The s orbital is a sphere with theucleus at its center. Thus, when we say that an electron occupies a Is orbital, we

mean that there is a greater than 90% probability that the electron is in the space de-fined by the sphere.

An orbital tells us the volume of spacearound the nucleus where an electron ismost likely to be found.

yy lyZI z

~x t x

/(, ;

\

/11s orbital 2s orbital

Because the second shell lies farther from the nucleus than the first shell (Section.Z), the average distance from the nucleus is greater for an electron in a 2s orbital thanan electron in a Is orbital. A 2s orbital, therefore, is represented by a larger sphere.ause of the greater size of the 2s orbital, its average electron density is less than the

z=erage electron density of a Is orbital.Unlike s orbitals, which resemble spheres, a p orbital has two lobes. Generally, thes are depicted as teardrop-shaped, but computer-generated representations reveal

t they are shaped more like doorknobs. In Section 1.2, we saw that the second andgher numbered shells each contain three p orbitals, and the three p orbitals have thee energy. The Px orbital is symmetrical about the x-axis, the Ps orbital is symmetricalut the y-axis, and the Pz orbital is symmetrical about the z-axis. This means that each

orbital is perpendicular to the other two p orbitals. The energy of a 2p orbital is greaterthat of a 2s orbital because the average location of an electron in a 2p orbital iser away from the nucleus.

y y y

z . I., z z, ~

rX ./ x

I \0. ,_. jP"l .••••••..0'

J /'\

] i 2px orbital 2pyorbital 2pz orbital computer-generated2p orbital


Movie:H2 bond formation

Maximum stability corresponds tominimum energy.

~ Figure 1.2The change in energy that occurs astwo1s atomic orbitals approach each other.The internuclear distance at minimumenergy is the length of the H- Hcovalent bond.

1.6 HOW ATOMS FORM COVALENT BONDSHow do atoms form covalent bonds in order to form molecules? Let's look first at thebonding in a hydrogen molecule (H2)' The covalent bond is formed when the Is orbitalof one hydrogen atom overlaps the Is orbital of a second hydrogen atom. The covalentbond that is formed when the two orbitals overlap is called a sigma (0') bond.

·H H:H H:H

1s atomicorbital

15 atomicorbital

Why do atoms form covalent bonds? As the two orbitals start to overlap to form thecovalent bond, energy is released (and stability increases) because the electron in eachatom is attracted both to its own nucleus and to the positively charged nucleus of theother atom (Figure 1.2). Thus atoms form covalent bonds because the covalentlybonded atoms are more stable than the individual atoms. The attraction of the nega-tively charged electrons for the positively charged nuclei is what holds the atomstogether. The more the orbitals overlap, the more the energy decreases until the atomsare so close together that their positively charged nuclei start to repel each other. Thisrepulsion causes a large increase in energy. Maximum stability (that is, minimum en-ergy) is achieved when the nuclei are a certain distance apart. This distance is thebond length of the new covalent bond. The length of the H - H bond is 0.74 A.

As Figure 1.2 shows, energy is released when a covalent bond forms. Whenthe H - H bond forms, 105 kcal/mo1 or 439 kJ/mol of energy is released(1 kcal = 4.184 kJ). * Breaking the bond requires precisely the same amount of ener-gy. Thus, the bond strength-also called the bond dissociation energy-is the ener-gy required to break the bond, or the energy released when the bond is formed. Everycovalent bond has a characteristic bond length and bond strength.

+

hydrogenatoms are farapart

hydrogenatoms are closetogether

,------'

>-01•..Q)cQ)

m'';::;cQ)

bc..

II I I

o ·······:~bOOd:leogth'}~i#,;;:::'a.74ft..

Internuclear distance

*Joules are the Systeme International (SI) units for energy, although many chemists use calories. We willuse both in this book.

Section 1.7 How Single Bonds Are Formed in Organic Compounds 17

1.7 HOW SINGLE BONDS ARE FORMEDIN ORGANIC COMPOUNDS

We will begin the discussion of bonding in organic compounds by looking at thebonding in methane, a compound with only one carbon atom. Then we will examinethe bonding in ethane, a compound with two carbons attached by a carbon---carbonsingle bond.

The Bonds in MethaneMethane (CH4) has four covalent C - H bonds. Because all four bonds have the samength and all the bond angles are the same (l09.5°), we can conclude that the four

C - H bonds in methane are identical. Four different ways to represent a methaneolecule are shown here.

r

H

IC···"ltH

H---- \ )109.50

HssS

I-

e

rspectlve formulaof methane

ball-and-stick modelof methane

space-filling modelof methane

electrostatic potentialmap for methane

In a perspective formula, bonds in the plane of the paper are drawn as solid lines,nds protruding out of the plane of the paper toward the viewer are drawn as solidedges, and those protruding back from the plane of the paper away from the viewerdrawn as hatched wedges.The potential map of methane shows that neither carbon nor hydrogen carries much

- a charge: there are neither red areas, representing partially negatively chargedtoms, nor blue areas, representing partially positively charged atoms. (Compare this

with the potential map for water on page 11). The absence of partially chargednns is due to the similar electronegativities of carbon and hydrogen, which causeill to share their bonding electrons relatively equally. Methane is therefore ampolar molecule.You may be surprised to learn that carbon forms four covalent bonds since youw that carbon has only two unpaired electrons in its electronic configuration

Table 1.2). But if carbon formed only two covalent bonds, it would not complete itst. We therefore need to come up with an explanation that accounts for carbon's

erming four covalent bonds.If one of the electrons in carbon's 2s orbital were promoted into the empty 2p, 'tal, the new electronic configuration would have four unpaired electrons; thus,

covalent bonds could be formed.

n:dr-r-~

-- ,01"- •.' ......,,

i i -, '.u p p p

s

ip

ip

ippromotion i

sbefore promotion after promotion

will

- carbon used an s orbital and three p orbitals to form these four bonds, the bonded with the s orbital would be different from the three bonds formed with p

itals. What could account for the fact that the four C- H bonds in methane aretical if they are made using one s and three p orbitals? The answer is that carbon

5 hybrid orbitals.Hybrid orbitals are mixed orbitals that result from combining atomic orbitals. Theept of combining orbitals was first proposed by Linus Pauling in 1931. If the one

3-D Molecule:fJff' Methane..,.!~

I

IIJOGRAPIIY

.»:Linus Carl Pauling

. (1901-19.94) was born in Port-land, OregonrAfriend's homechemistry laboratory sparkedPauling's early interest in sci-ence. He received a Ph.D. from -the California Institute of Tech-nology and remained. there formost of his academi~ career:He received the Nobel Prize inchemistry in 1954 for his workon molecular structure. LikeEinstein, Pauling was a pad-

, fist, winning the 1962'NobelPeace Prize for his work onbehalf of nuclear disarmament.

is

ip

18 C HAP T E R 1 Electronic Structure and Covalent Bonding

~ Figure 1.3An 5 orbital and three p orbitalshybridize to form four Sp3 orbitals.An Sp3 orbital is more stable than a porbital, but not as stable as an 5 orbital.

~ Figure 1.4(a) The four Sp3 orbitals are directedtoward the corners of a tetrahedron,causing each bond angle to be l09S.(b) An orbital picture of methane,showing the overlap of each Sp3 orbitalof carbon with the 5 orbital of ahydrogen. (For clarity, the smaller lobesof the Sp3 orbitals are not shown.)

Electron pairs stay as far from each otheras possible.

s and three p orbitals of the second shell are all combined and then apportioned intofour equal orbitals, each of the four resulting orbitals will be one part s and three partsp. Therefore, each orbital has 25% s character and 75% p character. This type of mixedorbital is called an sp3 (stated "s-p-three" not "s-p-cubed") orbital. (The superscript 3means that three p orbitals were mixed with one s orbital to form the hybrid orbitals.)Each of the four sp3 orbitals has the same energy.

ip

ip hybrid ization

Like a p orbital, an sp3 orbital has two lobes. Unlike the lobes of a p orbital, the twolobes of an sp3 orbital are not the same size (Figure 1.3). The larger lobe of the sp3 or-bital is used in covalent bond formation.

hybridization

p p p

s

The four sp3 orbitals adopt a spatial arrangement that keeps them as far away fromeach other as possible (Figure 1.4a). They do this because electrons repel each other, andmoving the orbitals as far from each other as possible minimizes the repulsion. When fourorbitals move as far from each other as possible, they point toward the comers of a regu-lar tetrahedron (a pyramid with four faces, each an equilateral triangle). Each of the fourC - H bonds in methane is formed from overlap of an sp3 orbital of carbon with the s or-bital of a hydrogen (Figure lAb). This explains why the four C- H bonds are identical.

b.H

c HH

The angle between any two bonds that point from the center to the comers of atetrahedron are 109.5°. The bond angles in methane therefore are 109.5°. This is calleda tetrahedral bond angle. A carbon, such as the one in methane, that forms covalentbonds using four equivalent sp3 orbitals is called a tetrahedral carbon.

Hybrid orbitals may appear to have been contrived just to make things fit-and thatis exactly the case. Nevertheless, they give us a very good picture of the bonding inorganic compounds.

Note to the studentIt is important to understand what molecules look like in three dimensions. There-fore be sure to visit the textbook's website (http://www.chemplace.com) and look atthe three-dimensional representations of molecules that can be found in the mole-cule gallery prepared for each chapter.

i3)

mlduru-urir-al.

faledwt

hatin

I -=--Ie-ate-

~

Section 1.8 Howa Double Bond Is Formed:The Bonds in Ethene 19

The Bonds in EthaneThe two carbon atoms in ethane (CH3CH3) are tetrahedral. Each carbon uses four sp3orbitals to form four covalent bonds:

..., 3-D Molecule:

,J Ethane;:~,..,.,.... .•••JH H

I IH-C-C-H

I IH Hethane

One sp3 orbital of one carbon overlaps an sp3 orbital of the other carbon to form theC-C bond. Each of the remaining three sp3 orbitals of each carbon overlaps the sorbital of a hydrogen to form a C - H bond. Thus, the C - C bond is formed bysp3-sp3 overlap, and each C- H bond is formed by sp3-s overlap (Figure 1.5).

H' H~

H\. .•.....

H 1

'"0

~,H'

I '"

·1 " j

r.,

H" (~

C -H bond formed

----7

r~ r ..._

I

by sp3-s overlap.. --.. -- / I-...HHI

Figure 1.5.•••, orbital picture of ethane. The (-( bond is formed by Sp3_Sp3 overlap, and each (- H bond is=ormed by Sp3_S overlap. (The smaller lobes of the Sp3 orbitals are not shown.)

Each ofthe bond angles in ethane is nearly the tetrahedral bond angle of 109.5°, andzae length of the C - C bond is 1.54 A. Ethane, like methane, is a nonpolar molecule.

H 109.60 IIUc~H~l""'l f\ :~H~H

perspective formulaof ethane

'"'ball-and-stick model

of ethanespace- filling model

of ethaneelectrostatic potential

map for ethane

All the bonds in methane and ethane are sigma (0') bonds. We will see that all. gle bonds found in organic compounds are sigma bonds.

PROBLEM19.What orbitals are used to form the 10 covalent bonds in propane (CH3CH2CH3)?

.8 HOW A DOUBLE BOND IS FORMED:THE BONDS IN ETHENE

h of the carbon atoms in ethene (also called ethylene) forms four bonds, but each isded to only three atoms:

H H\ /C=C/ \H Hetheneethylene

20 C H A PT E R 1 Electronic Structure and Covalent Bonding

3-D Molecule:Ethene

To bond to three atoms, each carbon hybridizes three atomic orbitals: an s orbitaland two of the p orbitals). Because three orbitals are hybridized, three hybrid orbitalsare formed. These are called sp2 orbitals. After hybridization, each carbon atom hasthree identical sp2 orbitals and one p orbital:

ii Ps

ip

ip i

sp2isp2

iphybridization i

sp2

The axes of the three sp2 orbitals lie in a plane (Figure 1.6a). To minimize electronrepulsion, the three orbitals need to get as far from each other as possible. Thereforethe bond angles are all close to 120°. The unhybridized p orbital is perpendicular to theplane defined by the axes of the sp2 orbitals (Figure 1.6b).

~ Figure 1.6(a) The three Sp2 orbitals lie in a plane.(b) The unhybridized p orbital isperpendicular to the plane. (The smallerlobes of the Sp2 orbitals are not shown.)

h.a.

top view side view

Side-to-side overlap of two p atomicorbitals forms a 'TT bond.

The carbons in ethene are held together by two bonds. Two bonds connecting twoatoms is called a double bond. The two carbon-carbon bonds in the double bond arenot identical. One of them results from the overlap of an sp2 orbital of one carbon withan sp2 orbital of the other carbon; this is a sigma (0') bond. Each carbon uses its othertwo sp2 orbitals to overlap the s orbital of a hydrogen to form the C - H bonds (Figure1.7a). The second carbon-carbon bond results from side-to-side overlap of the two un-hybridized p orbitals (Figure 1.7b). Side-to-side overlap of p orbitals forms a pi (1T)bond. Thus, one of the bonds in a double bond is a 0' bond, and the other is a tr bond.All the C - H bonds are 0' bonds.

The two p orbitals that overlap to form the -tt bond must be parallel to each other formaximum overlap to occur. This forces the triangle formed by one carbon and two hy-drogens to lie in the same plane as the triangle formed by the other carbon and two

a. a bond formedby sp2-s overlap

H

H

A Figure 1.7(a) One (-( bond in ethene is a a bond formed by Sp2_Sp2 overlap, and the C - H bonds areformed by Sp2_S overlap.(b) The second (-( bond is a 'TT bond formed by side-to-side overlap of a p orbital of one carbonwith a p orbital of the other carbon.(c) There is an accumulation of electron density above and below the plane containing the twocarbons and four hydrogens.

I

J

e11~et-

.)i

Jf

Section 1.9 How aTriple Bond Is Formed:The Bonds In Ethyne 21

hydrogens. As a result, all six atoms of ethene lie in the same plane, and the electronsin the p orbitals occupy a volume of space above and below the plane (Figure 1.7c).The potential map for ethene shows that it is a nonpolar molecule with a slight accu-mulation of negative charge (the pale orange area) above the two carbons. (If youcould turn the potential map over to show the hidden side, a similar accumulation ofgative charge would be found there.)

H 121.70 H

"<;t C/)116.60

I;JL;\H~H

a double bond consists ofo: bond and one tt bond

ball-and-stick modelof ethene

••space-filling model electrostatic potential map

of ethene for ethene

Four electrons hold the carbons together in a carbon-carbon double bond; only twotrons hold the carbons together in a carbon-carbon single bond. This means that an-carbon double bond is stronger and shorter than a carbon-carbon single bond.

DIAMOND AND GRAPHITE: SUBSTANCES THAT CONTAIN ONLY CARBON ATOMSDiamond is the hardest of all substances.Graphite, in contrast, is a slippery, soft solid mostfamiliar to us as the "lead" in pencils. Both mate-

rials, in spite of their very different physical properties, con-zain only carbon atoms. The two substances differ solely ine nature of the bonds holding the carbon atoms together.

Diamond consists of a rigid three-dimensional network of

atoms, with each carbon bonded to four other carbons via sp3orbitals. The carbon atoms in graphite, on the other hand, aresp2 hybridized, so each bonds to only three other carbons.This planar arrangement causes the atoms in graphite to lie inflat, layered sheets that can shear off of neighboring sheets.When you write with a pencil, sheets of carbon atoms shearoff to leave a thin trail of graphite.

9 HOW A TRIPLE BOND IS FORMED:THE BONDS IN ETHYNEarbon atoms in ethyne (also called acetylene) are each bonded to only two

=s-a hydrogen and another carbon:H-C=C-H

ethyneacetylene

~ each carbon forms covalent bonds with two atoms, only two orbitals (an sandhybridized. Two identical sp orbitals result. Each carbon atom in ethyne, there-two hybridized sp orbitals and two unhybridized p orbitals (Figure 1.8).

"ip hybridization

sp sp~

ip

ip

ip

ipi i

s~______ ~A~ -.

orbitals are hybridized Iaimize electron repulsion, the two sp orbitals point in opposite directions_ 1.8).arbons in ethyne are held together by three bonds. Three bonds connectingis called a triple bond. One of the sp orbitals of one carbon in ethyne over-

sp orbital of the other carbon to form a carbon-carbon (J" bond. The other sp or-each carbon overlaps the s orbital of a hydrogen to form a C -- H (J" bond

~\

~"\ ~@j l.J ~.•. Figure 1.8The two sp orbitals are oriented 1800

away from each other, perpendicularto the two unhybridized p orbitals.(The smaller lobes of the sp orbitalsare not shown.)

22 C H A PT E R 1 Electronic Structure and Covalent Bonding

(Figure 1.9a). Because the two sp orbitals point in opposite directions, the bond anglesare 1800• The two unhybridized p orbitals are perpendicular to each other, and both areperpendicular to the sp orbitals. Each of the unhybridized p orbitals engages in side-to-side overlap with a parallel p orbital on the other carbon, with the result that two 'TT

bonds are formed (Figure 1.9b) .

..•.Figure 1.9(a) The (-( (T bond in ethyne is formed by sp-sp overlap, and the (- H bonds are formed bysp-s overlap.The carbon atoms and the atoms bonded to them are in a straight line.(b) The two 1T bonds are formed by side-to-side overlap of the p orbitals of one carbon with thep orbitals of the other carbon.(c) The triple bond has an electron-dense region above and below and in front of and in back ofthe internuclear axis ofthe molecule.

A triple bond therefore consists of one (J' bond and two 'TT bonds. Because the twounhybridized p orbitals on each carbon are perpendicular to each other, there is a re-gion of high electron density above and below, and in front of and in back of, the inter-nuclear axis of the molecule (Figure 1.9c). The potential map for ethyne shows thatnegative charge accumulates in a cylinder that wraps around the egg-shaped molecule.

1800

H QC-H

~a triple bond consists of one

(T bond and two 1T bonds

•ball-and-stick model

of ethynespace-filling model

of ethyneelectrostatic potential map

for ethyne

Because the two carbon atoms in a triple bond are held together by six electrons, atriple bond is stronger and shorter than a double bond.

3-D Molecule:Ethyne

PROBLEM 20

For each of the following species:a. Draw its Lewis structure.b. Describe the orbitals used by each carbon atom in bonding and indicate the approxi-

mate bond angles.1. HCOH 2. CCl4 3. HCN

SOLVED

Solution to 20a Because HCOH is neutral, we know that each H forms one bond, theoxygen forms two bonds, and the carbon forms four bonds. Our first attempt at a Lewisstructure (drawing the atoms in the order given by the Kekule structure) shows that carbonis the only atom that does not form the needed number of bonds.

H-C-O-H

If we place a double bond between the carbon and the oxygen and move an H, all theatoms end up with the correct number of bonds. All the C and H atoms have filled outershells but lone-pair electrons need to be added to give the oxygen atom a filled outer shell.When we check to see if any atom needs to be assigned a formal charge, we find that noneof them does.

:0:II

H-C-H

Section 1.10 Bonding in the Methyl Cation, The Methyl Radical, and the Methyl Anion 23

~s:e}-

Solution to 20b Because the carbon atom forms a double bond, we know that carbonuses sp2 orbitals (as it does in ethene) to bond to the two hydrogens and the oxygen. It usesits "leftover" p orbital to form the second bond to oxygen. Because carbon is sp2 hy-bridized, the bond angles are approximately 120°.

:0'·120rll~120°

yC--J'H~H

1200

tt

.10 BONDING IN THE METHYL CATION,THE METHYL RADICAL, ANDTHE METHYL ANION

_-ot all carbon atoms form four bonds. A carbon with a positive charge, a negativege, or an unpaired electron forms only three bonds. Now we will see what orbitals

::ubon uses when it forms three bonds.

\'0"C-

.r-tatIe.

e Methyl Cation (+CH3)~ e positively charged carbon in the methyl cation is bonded to three atoms, so it hy-

dizes three orbitals-an s orbital and two p orbitals. Therefore, it forms its three co-nt bonds using sp2 orbitals. Its unhybridized p orbital remains empty. Theitively charged carbon and the three atoms bonded to it lie in a plane. The p orbitalds perpendicular to the plane.

,H".' ,\

lap

:. a+CH3

methyl cation ball-and-stick models of the methyl cation

angled side view top view

le Methyl Radical (·CH3)

arbon atom in the methyl radical is also Sp2 hybridized. The methyl radical dif-J oy one unpaired electron from the methyl cation. That electron is in the p orbital.ice the similarity in the ball-and-stick models of the methyl cation and the methylcal. The potential maps, however, are quite different because of the additional

n in the methyl radical.e

np orbital contains theunpaired electron

e:rle

angled side view top view

ball-and-stick models of the methyl radical

.CH3methyl radical

electrostatic potential mapfor the methyl cation

electrostatic potential mapfor the methyl radical

bond formed bysp3-s overlap


The Methyl Anion (:CH3)

The negatively charged carbon in the methyl anion has three pairs of bonding electronsand one lone pair. The four pairs of electrons are farthest apart when the four orbitalscontaining the bonding and lone-pair electrons point toward the corners of a tetra-hedron. In other words, a negatively charged carbon is sp3 hybridized. In the methylanion, three of carbon's sp3 orbitals each overlap the s orbital of a hydrogen, and thefourth sp3 orbital holds the lone pair.

lone-pair electronsare in an sp3 orbital

H

:CH3methyl anion ball-and-stick model of the methyl anion electrostatic potential map

for the methyl anion

Take a moment to compare the potential maps for the methyl cation, the methyl rad-ical, and the methyl anion.

1.11 THE BONDS IN WATER:O-HIHwater

The oxygen atom in water (H20) forms two covalent bonds and has two lone pairs.Because the electronic configuration of oxygen shows that it has two unpaired elec-

trons (Table 1.2), oxygen does not need to promote an electron to form the number(two) of covalent bonds required to complete its octet. If we assume that oxygen usesp orbitals to form the two 0 - H bonds, as predicted by oxygen's electronic configu-ration, we would expect a bond angle of about 90° because the two p orbitals are atright angles to each other. However, the experimentally observed bond angle is 104.5°.In addition, we would expect the lone pairs to be chemically different because one pairwould be in an s orbital and the other would be in a p orbital. The lone pairs, however,are known to be identical.

To explain the observed bond angle and the fact that the lone pairs are identical,oxygen must use hybrid orbitals to form covalent bonds-just as carbon does. The sorbital and the three p orbitals must hybridize to produce four identical sp3 orbitals.

3-D Molecule:

~ Water

The bond angles in a molecule indicatewhich orbitals are used in bond formation.

second-shell electronsof oxygen

u l' l' it 1'1 l' i1'1 p p p hybridization sp3 sp3 sp3 sp3S

Each of the two 0- H bonds is formed by the overlap of an sp3 orbital of oxygenwith the s orbital of a hydrogen. A lone pair occupies each of the two remaining sp3orbitals.

The bond angle in water (104.5°) is a little smaller than the bond angle in methane(109.5°) presumably because each of the lone pairs is held by only one nucleus, whichmakes a lone pair more diffuse than a bonding pair that is held by two nuclei and istherefore relatively confined between them. Consequently, there is more electronrepulsion between lone pairs, causing the 0 - H bonds to squeeze closer together,thereby decreasing the bond angle.

illS

alsra-nylthe

mapn

rad-

s..lec-nberusesfigu-re at4.5°.;pairever,

tical,rhesdtals.

cygen19 sp3

.thanewhichand isectron~ether,

Section 1.12 The Bonds in Ammonia and in the Ammonium Ion 25

bond formed by the overlapof an sp3 orbital of oxygen withthe 5 orbital of hydrogenlone-pair electrons

are in sp3 orbitals

H20

water ball-and-stick model of water electrostatic potential mapfor water

Compare the potential map for water with that for methane. Water is a polar mole-ule; methane is nonpolar.

PRJ~_~~[~~The bond angles in H30+are greater than and less than _

WATER-A UNIQUE COMPOUNDWater is the most abundant compound found in liv-ing organisms. Its unique properties have allowedlife to originate and evolve. Its high heat of fusion

(the heat required to convert a solid to a liquid) protects organ-isms from freezing at low temperatures because a lot of heatmust be removed from water to freeze it. Its high heat capacity(the heat required to raise the temperature of a substance a

given amount) minimizes temperature changes in organisms,and its high heat of vaporization (the heat required to convert aliquid to a gas) allows animals to cool themselves with a mini-mal loss of body fluid. Because liquid water is denser than ice,ice formed on the surface of water floats and insulates thewater below. That is why oceans and lakes don't freeze fromthe bottom up. It is also why plants and aquatic animals cansurvive when the ocean or lake they live in freezes.

.12 THE BONDS IN AMMONIAAND IN THE AMMONIUM ION

e nitrogen atom in ammonia (NH3) forms three covalent bonds and has onee pair.Because nitrogen has three unpaired electrons in its electronic configuration

Table 1.2), it can form three covalent bonds without having to promote an electron.experimentally observed bond angles in NH3 are 107.3°, indicating that nitrogen

-- uses hybrid orbitals when it forms covalent bonds. Like carbon and oxygen, thes and three p orbitals of the second shell of nitrogen hybridize to form four identi-sp3 orbitals:

H-N-HIH

ammonia

second-shell electronsof nitrogen

i i i H i i i~1 p p p hybridization sp3 sp3 sp3 sp3S

Each of the N - H bonds in NH3 is formed by the overlap of an sp3 orbital of nitro-with the s orbital of a hydrogen. The single lone pair occupies an sp3 orbital. Thedangle (107.3°) is smaller than the tetrahedral bond angle (109.5°) because of theively diffuse lone pair. Notice that the bond angles in NH3 (107.3°) are larger than


bond formed by the overlapof an sp3 orbital of nitrogen withthe 5 orbital of hydrogen

the bond angles in H20 (104.5°) because nitrogen has only one lone pair, whereasoxygen has two lone pairs.

,...-----N···I//H

H \H)107.30

NH3ammonia ball-and-stick model of ammonia electrostatic potential map

for ammonia

3-D Molecule:Ammonia

Because the ammonium ion (+NH4) has four identical N - H bonds and no lone pairs.all the bond angles are 109.5°, just like the bond angles in methane.

H

1+,...-----N···""H

H \H)109S

+NH4ammonium ion ball-and-stick model of the ammonium ion electrostatic potential map

for the ammonium ion

PROBLEM 23+

PROBLEM 22.

electrostatic potentialmap for methane

According to the potential map for the ammonium ion, which atom(s) has (have) the leastelectron density?

Compare the potential maps for methane, ammonia, and water. Which is the most polarmolecule? Which is the least polar?

electrostatic potential mapfor ammonia

electrostatic potential mapfor water

Predict the approximate bond angles in the methyl carbanion.

1.13 THE BOND IN A HYDROGEN HALIDEFluorine, chlorine, bromine, and iodine are known as the halogens; HF, Hel, HBr, andHI are called hydrogen halides. Bond angles will not help us determine the orbitalsthat form the hydrogen halide bond, as they did with other molecules, because a

•

rnd:alse a

Section 1.13 The Bond in a Hydrogen Halide 27

hydrogen halide has only one bond and therefore no bond angle. We do know, howev-er, that a halogen's three lone pairs are identical and that lone pairs positionthemselves to minimize electron repulsion (Section 1.7). Both of these facts suggestmat a halogen's three lone pairs are in sp3 orbitals. Therefore, we will assume that ahydrogen-halogen bond is formed by the overlap of an sp3 orbital of the halogen withme s orbital of hydrogen.

hydrogen fluoride

H-f.:hydrogen fluoride

hydrogen chloride

ball-and-stickmodel of hydrogen fluoride

electrostatic potential mapfor hydrogen fluoride

In the case of fluorine, the sp3 orbital used in bond formation belongs to the see-nd shell of electrons. In chlorine, the sp3 orbital belongs to the third shell of elec-ns. Because the average distance from the nucleus is greater for an electron ine third shell than for an electron in the second shell, the average electron density- less in a 3sp3 orbital than in a 2sp3 orbital. This means that the electron densitythe region where the s orbital of hydrogen overlaps the sp3 orbital of the halogenreases as the size of the halogen increases (Figure 1.10). Therefore, the

_drogen-halogen bond becomes longer and weaker as the size (atomic weight) ofhalogen increases (Table 1.5).

hydrogen bromide

hydrogen iodide

.•••Figure 1.10There is greater electron density in theregion of overlap of an 5 orbital with a 2Sp3orbital than in the region of overlap of an sorbital with a 3Sp3 orbital.

The greater the electron density in theregion of orbital overlap, the stronger isthe bond.

The shorter the bond, the stronger it is.

Table 1.5 Hydrogen-Halogen Bond lengths and Bond Strengths

Bond!ength Bond strengthHydrogen halide (A) kcal/mol kJ/mol

~H-F J:I~ 0.917 l36 571j\""""F"

H-Cl J:! ~ 1.275 103 432""CIH"'\

H-Br\! 'L' 1.415 87 366H.... Br

H-I \¢. ., 1.609 71 298"-' I

PROBLEM25+Predict the relative lengths and strengths of the bonds in Cl2 and Br2'Predict the relative lengths and strengths of the bonds in HF, HCI, and HBr.

NOB,L!.~tJ§IlWhich bond is longer? b. Which bond is stronger?

1. C-Cl or C-Br 2. C-C or C-H 3. H-Cl or H-H


1.14 SUMMARY: HYBRIDIZATION, BOND LENGTHS,BOND STRENGTHS, AND BOND ANGLES

All single bonds found in organiccompounds are sigma bonds.

The hybridization of a C,O, or Nis Sp(3 minus the number of 7T bonds)

All single bonds are (T bonds. All double bonds are composed of one (T bond and one 7T

bond. All triple bonds are composed of one (T bond and two 7T bonds. The easiest wayto determine the hybridization of a carbon, oxygen, or nitrogen atom is to look at thenumber of 7T bonds it forms: if it forms no 7T bonds, it is sp3 hybridized; if it forms one7T bond, it is sp2 hybridized; if it forms two 7T bonds, it is sp hybridized. The exceptionsare carbocations and carbon radicals, which are Sp2 hybridized-not because they forma 7T bond, but because they have an empty or a half-filled p orbital (Section 1.10).

'(J'~Sp2CH3

IIC

CH3-NH2\ ....

CH3-QH./ <,

:p=C=Q:l=N-NH2 CH3-C=N: CH3 CH3CH3

i i i i i i i i i i i i i i i i isp3 sp3 sp3 sp2 sp2 sp3 sp3 sp sp sp3 sp3 sp3 sp2 sp3 sp2 sp sp2

A '1T bond is weaker than a a bond.

In comparing the lengths and strengths of carbon-carbon single, double, and triplebonds, we see that the more bonds holding two carbon atoms together, the shorter andstronger is the carbon-carbon bond (Table 1.6): Triple bonds are shorter and strongerthan double bonds, which are shorter and stronger than single bonds.

A double bond (a (T bond plus a 7T bond) is stronger than a single bond (a (T bond),but it is not twice as strong. We can conclude, therefore, that a 7T bond is weaker thana (T bond.

You may wonder how an electron "knows" what orbital it should go into. In fact,electrons know nothing about orbitals. They simply arrange themselves around atomsin the most stable manner possible. It is chemists who use the concept of orbitals to ex-plain this arrangement.

PROBLEM27.

Which of the bonds of a carbon-carbon double bond has more effective orbital-orbitaloverlap, the a bond or the 7T bond?

Table 1.6 Comparison of the Bond Angles and the Lengths and Strengths of the Carbon-Carbon Bonds in Ethane,Ethene, and Ethyne

Hybridization Length of <;- C Strength of C - C bondMolecule of carbon Bond angles bond (A) (kcal/mol) (kJ/mol)

H HI I

sp3H-C-C-H 109.5° 1.54 90 377I IH Hethane

H H\ /

sp2C=C 120° 1.33 174 728/ \

H Hethene

H-C=C-H sp 180° 1.20 231 967ethyne

,1

ed:r

),lD.

:t,IS

)t-

]

PROBLEM 28+a. What is the hybridization of each of the carbon atoms in the following compound?

CH3CHCH =CHCH2C=CCH3ICH3

b. What is the hybridization of each of the carbon, oxygen, and nitrogen atoms in the fol-lowing compounds?

CHlOHI

HO-CH10

He' 'C90\ IC=C/ \

HO OHvitamin C

CH.p- tH 0

H9 II IIHC C C

~CH/ "C/ ""OCHlCH3HlC/ 'CHl

I IHlC, /CHl

NICH3

demerolan analgesic

PROBLEM-SOLVING STRATEGYPredict the approximate bond angle of the C - N - H bond in (CH3hNH.

First we need to determine the hybridization of the central atom. Because the nitrogenatom forms only single bonds, we know it is sp3 hybridized. Next we look to see if thereare lone pairs that will affect the bond angle. A neutral nitrogen has one lone pair. Based onthese observations, we can predict that the C - N - H bond angle will be about 107.30

,

the same as the H - N - H bond angle in NH3, another compound with a neutral sp3hybridized nitrogen.

Now continue on to Problem 29.

PROBLEM 29+Predict the approximate bond angles:

+a. the C- N -C bond angle in (CH3hNHlb. the C- N -C bond angle in CH3CHzNHzc. the H - C - N bond angle in (CH3hNHd. the C-O-C bond angle in CH30CH3

PROBLEM'30~:_~~:

Describe the orbitals used in bonding and the approximate bond angles in the followingompounds. (Hint: See Table 1.6.)

a. CH30H b. HONH2 d. N2c. HCOOH

:SUMMARYanic compounds are compounds that contain carbon.

_~e atomic number of an atom is the number of protons innucleus. The mass number of an atom is the sum of itstons and neutrons. Isotopes have the same atomic num-but different mass numbers.

Summary 29

An atomic orbital indicates where there is a high proba-bility of finding an electron. The closer the atomic orbital isto the nucleus, the lower is its energy. Electrons are assignedto atomic orbitals according to three rules: an electron goesinto the available orbital with the lowest energy; no more


than two electrons can be in an orbital; and an electron willoccupy an empty orbital before pairing up with an electronin an orbital with the same energy.

The octet rule states that an atom will give up, accept,or share electrons in order to fill its outer shell or attainan outer shell with eight electrons. The electronic con-figuration of an atom describes the orbitals occupied bythe atom's electrons. Electrons in inner shells are calledcore electrons; electrons in the outermost shell arecalled valence electrons. Lone-pair electrons are va-lence electrons that are not used in bonding. A bondformed as a result of the attraction of opposite charges iscalled an ionic bond; a bond formed as a result of shar-ing electrons is called a covalent bond. A polar cova-lent bond is a covalent bond between atoms withdifferent electronegativities.

Lewis structures indicate which atoms are bonded to-gether and show lone-pair electrons and formal charges.A carbocation has a positively charged carbon, a

PROBLEMS

carbanion has a negatively charged carbon, and a radicalhas an unpaired electron.

Bond strength is measured by the bond dissociation en-ergy. A a bond is stronger than a 1T bond. All single bondsin organic compounds are sigma (o-) bonds. A double bondconsists of one a bond and one 1T bond, and a triple bondconsists of one a bond and two 1T bonds. Carbon-carbontriple bonds are shorter and stronger than carbon-carbondouble bonds, which are shorter and stronger thancarbon-carbon single bonds. To form four bonds, carbopromotes an electron from a 2s to a 2p orbital. C, N, 0, anthe halogens form bonds using hybrid orbitals. Thehybridization of C, N, or ° depends on the number of 1i

bonds the atom forms: no 1T bonds means that the atom is sp=hybridized, one 1T bond indicates that it is sp2 hybridized.and two 1T bonds signifies that it is sp hybridized. Excep-tions are carbocations and carbon radicals, which are sp:hybridized. Bonding and lone-pair electrons around anatom are positioned as far apart as possible.

......•.31.Draw a Lewis structure for each of the following species:a. H2C03 h. co,': c. H2CO d. CH3NH2

32.

/33.

How many valence electrons do the following atoms have?a. carbon and silicon h. nitrogen and phosphorus c. neon and argon d. magnesium and calcium

For each of the following species, give the hybridization of the central atom and indicate the bond angles:a. NH3 h. +NH4 c. -CH3 d. CCCH3)4 e.. CH3 f. +CH3 g. HCN h. H30+

/

34. Use the symbols 8+ and 8- to show the direction of polarity of the indicated bond in each of the following compounds:a. F- Br h. H3C-Cl c. H3C- MgBr d. H2N -OH

35. Draw the condensed structure of a compound that contains only carbon and hydrogen atoms and that hasa. three sp3 carbons.b. one sp3 carbon and two sp2 carbons.c. two sp3 carbons and two sp carbons.

36. Predict the approximate bond angles:a. the H -C-O bond angle in CH30Hc. the H-C-H bond angle in H2C=O

37. Give each atom the appropriate formal charge:a. H:R: b. H:R'

b. the C - 0 - H bond angle in CH30Hd. the C - C - N bond angle in CH3C=N

c. H-N-H d. H-C-H

38. Write the electronic configuration for the following species (carbon's electronic configuration is written as ls2 2s2 2p2):a. Ca b. Ca2+ c. Ar d. Mg2+

"'-39. Only one of the following formulas describes a compound that exists. Fix the other formulas so they also describe compounds thatexist.

b. CHs

./' 40. List the bonds in order of decreasing polarity (that is, list the most polar bond first).a. C-O, C-F, C-N b. C-Cl, C-I, C-Br c. H-O, H-N, H-C d. C-H, C-C, C-N

•Problems 31

__ .n. Write the Kekule structure for each of the following compounds:a. CH3CHO c. CH3COOHb. CH30CH3 d. (CH3)3COH

~2. Assign the missing formal charges.

e. CH3CH(OH)CH2CNf. (CH3hCHCH(CH3)CH2C(CH3)3

H HI I

a. H-C-C:I IH H

H HI I

b. H-C-CI IH H

H HI I

c. H-C-C'I IH H

H :0: HI I I

d. H-C-C-C-HI I IH H H

1,...........43. Draw the missing lone pairs and assign the missing formal charges.

H H HI I I

a. H-C-O-H b. H-C-O-H c. H-C-OI I I IH H H H

HI

d. H-C-N-HI IH H

3~r3

I,44. Account for the difference in the shape and color of the potential maps for ammonia and the ammonium ion in Section 1.12 .

-. What is the hybridization of the indicated atom in each of the following compounds?

....,2

n

1a. CH3CH=CH2

o v--II

b. CH3CCH3

1c. CH3CH20H

1e. CH3CH=NCH3

1d. CH3C=N

1f. CH30CH2CH3

. a. Which of the indicated bonds in each compound is shorter?b. Indicate the hybridization of the C, 0, and N atoms in each of the compounds.

o1. CH3CH

1CHC

1CH 2. CH;~CH):-OH 3. cH3NHlcH2cH2N

1CHCH3

. Which of the following have a tetrahedral geometry?

H20 H30+ +CH3 NH3 +N14 TH3

" Do the two sp2 hybridized carbons and the two indicated atoms lie in the same plane?

\.CHo\.CH H3\ /

C=C/ \

H CH3~

\.CH H3\ /

C=C/ \

H CH2CH3~

{

0CH3

~~

""'. For each of the following compounds, indicate the hybridization of each carbon atom and give the approximate values of all thebond angles:a. CH3C-CH b. CH3CH=CH2 c. CH3CH2CH3 d. CH2=CH-CH=CH2

__. Sodium methoxide (CH30Na) has both ionic and covalent bonds. Which bond is ionic? How many covalent bonds does it have?EL a. Why is a H - H bond (0.74 A) shorter than a C-C bond (1.54 A)?

b. Predict the length of a C - H bond.

~. Explain why the following compound is not stable::hat

H HH \/\ CH-C""'- 'cI III

H-C, .....-C/ CH /\H H

ctronic Structure d Covalent Bonding

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