ALKANES (PARAFFINS)

Organic Chemistry (1) Dr./Mohamed Abdellatif Zein .

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Hydrocarbons

Organic compounds contain only two elements hydrogen and

oxygen are known as hydrocarbons, which divided into: Hydrocarbons

AliphaticAromatic

Alkanes Alkenes Alkynes Cyclic aliphatic

ALKANES

(PARAFFINS)

Alkanes are saturated hydrocarbons and they have

assign (RH) and a general molecular formula CnH2n+2. All alkanes

are ended with the suffix “ane “. The simplest member of the

alkane family is methane CH4. All carbon – carbon bonds are

single bonds and their bonding orbitals are SP3 (hybrid

orbitals).

Industrial source of alkanes:

The primary source of alkanes is petroleum.

REFINING

1. The first step in refining petroleum is distillation.

2. More than 500 different compounds are contained in the

petroleum distillates boiling below 200°C, and many have almost

the same boiling points.

3. Mixtures of alkanes are suitable for uses as fuels, solvents,


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and lubricants.

4. Petroleum also contains small amounts of oxygen, nitrogen,

and sulfur-containing compounds.

Table. Typical Fractions Obtained by distillation of

Petroleum

Boiling Range of

Fraction (°C)

Number of

Carbon Atoms

per Molecules

Use

Below 20 C1–C4 Natural gas, bottled gas,

petrochemicals

20–60 C5–C6 Petroleum ether, solvents

60–100 C6–C7 Ligroin, solvents

40–200 C5–C10 Gasoline (straight-run

gasoline) 175–325 C12–C18 Kerosene and jet fuel

250–400 C12 and higher Gas oil, fuel oil, and diesel

oil Nonvolatile

liquids C20 and higher

Refined mineral oil,

lubricating oil, grease

Nonvolatile

solids

C20 and higher Paraffin wax, asphalt, and

tar

CRACKING

1. Catalytic cracking: When a mixture of alkanes from the gas

oil fraction (C12 and higher) is heated at very high

temperature (500 °C) in the presence of a variety of

catalysts, the molecules break apart and rearrange to

smaller, more highly branched alkanes containing 5-10 carbon

atoms.

2. Thermal cracking: tend to produce unbranched chains


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which have very low “octane number”.

3. Octane Number: Ratio of isooctane a mixture of isooctane

and heptane.

1) Isooctane: 2,2,4-trimethylpentane burns very smoothly in

internal combustion engines and has an octane number of 100.

C CH3C C CH3

CH3

CH3

CH3

H

H

H Isooctane (100 octane number)

2) Heptane: CH3CH2CH2 CH2CH2 CH2CH3(Zero octane number)

PHYSICAL PROPERTIES OF ALKANES

A series of compounds, where each member differs from

the next member by a constant unit, is called a homologous

series. Members of a homologous series are called homologs.

At room temperature (25°C) and 1 atm pressure, the

C1-C4 unbranched alkanes are gases; the C5-C17

unbranched alkanes are liquids; the unbranched alkanes with

18 or more carbon atoms are solids.

BOILING POINTS

1. The boiling points of the unbranched alkanes show a

regular increase with increasing molecular weight.


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1. Branching of the alkane chain lowers the boiling point.

Boiling points of C6H14: hexane (68.7°C);

2-methylpentane(60.3°C);3-methylpentane(63.3°C);

2,3-dimethylbutane(58°C);2,2-dimethylbutane (49.7 °C).

2. As the molecular weight of unbranched alkanes

increases, so too does the molecular size, and even more

importantly molecular surface areas. Increasing surface

area increasing the van der Waals forces between

molecules more energy (a higher temperature) is

required to separate molecules from one another and

produce boiling.

3. Chain branching makes a molecule more compact,

reducing the surface area and with it the strength of the

van der Waals forces operating between it and adjacent

molecules lowering the boiling.


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MELTING POINTS

1. There is an alteration as one progress from an unbranched

alkane with an even number of carbon atoms to the next one

with an odd number of carbon atoms.

Melting points: ethane (–183°C); propane (–188°C); butane

(–138°C); pentane (–130 °C).

2. X-ray diffraction studies have revealed that alkane chains

with an even number of carbon atoms pack more closely in

the crystalline state Attractive forces between individual

chains are greater and melting points are higher.

3. Branching produces highly symmetric structures results in abnormally

high melting points.

2, 2, 3, 3,-Tetramethylbutane (mp 100.7 °C); (bp 106.3 °C).

C C

CH3

CH3

CH3

H3C

CH3

CH3


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DENSITY

The alkanes and cycloalkanes are the least dense of

all groups of organic compounds.

SOLUBILITY

Alkanes and cycloalkanes are almost totally insoluble in

water because of their very low polarity and their inability to

form hydrogen bonds.

Liquid alkanes and cycloalkanes are soluble in one another,

and they generally dissolve in solvents of low polarity.

Free rotation about the carbon – carbon single bond

(Conformation).

1. Conformations: the temporary molecular shapes that

result from rotations of groups about single bonds.

2. Conformational analysis: the analysis of the energy changes

that a molecule undergoes as groups rotate about single bonds.

a) The staggered conformation of ethane. b) The Newman projection formula

for the staggered conformation.

3. Staggered conformation: allows the maximum separation of

the electron pairs of the six C—H bonds has the lowest

energy most stable conformation.

4. Newman projection formula:


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5. Sawhorse formula:

Newman projection formula Sawhorse formula

The hydrogen atoms have been omitted for clarity

4. Eclipsed conformation: maximum repulsive interaction between the

electron pairs of the six C—H bonds has the highest

energy least stable conformation.

a) The eclipsed conformation of ethane.

b)The Newman projection formula for the eclipsed conformation.

7. Torsional barrier: the energy barrier to rotation of a single

bond.

1) In ethane the difference in energy between the staggered

and eclipsed Conformations are 12 kJ mol−1 (2.87 kcal mol−1).


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Potential energy changes that accompany rotation of groups

about the carbon-carbon bond of ethane.

2) Unless the temperature is extremely low (−250 °C), many

ethane molecules (at any given moment) will have enough energy

to surmount this barrier.

3) An ethane molecule will spend most of its time in the lowest

energy, staggered conformation, or in a conformation very

close to being staggered. Many times every second, it will

acquire enough energy through collisions with other

molecules to surmount the Torsional barrier and will rotate

through an eclipsed conformation.

4) In terms of a large number of ethane molecules, most of

the molecules (at any given moment) will be in staggered or

nearly staggered conformations.

8. Substituted ethanes, GCH2CH2G (G is a group or atom other

than hydrogen):


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The barriers to rotation are far too small to allow

isolation of the different staggered conformations or

conformers, even at temperatures considerably below room

temperature.

G

HH

HH

G

G

HH

GH

H These conformers cannot be isolated except at extremely low

temperatures.

CONFORMATIONAL ANALYSIS OF BUTANE

1. Ethane has a slight barrier to free rotation about the C—C

single bond.

This barrier (Torsional strain) causes the potential

energy of the ethane molecule to rise to a maximum when

rotation brings the hydrogen atoms into an eclipsed

conformation.

2. Important conformations of butane I – VI:


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CH3

HH

HH3C

H

CH3

HH

HH

CH3

CH3

HH

CH3H

H

H3C

HH

CH3

HH

H

HH3C

CH3

HH

H

CH3H

CH3

HH

I III

VIV

II

VI

An anticonformation

A gaucheconformation

An eclipsedconformation


A gaucheconformation


1) The anti-conformation (I): does not have Torsional strain

most stable.

2) The gauche conformations (III and V): the two

methyl groups are close enough to each other the van

der Waals forces between them are repulsive the

Torsional strain is 3.8 kJ mol−1 .

3) The eclipsed conformation (II, IV, and VI): energy

maxima II, and IV have Torsional strain and van der

Waals repulsions arising from the eclipsed methyl group and

hydrogen atoms; VI has the greatest energy due to the large


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van der Waals repulsion force arising from the eclipsed

methyl groups.

4) The energy barriers are still too small to permit isolation

of the gauche and anti-conformations at normal temperatures.

Energy changes that arise from rotation about the

C2–C3 bond of butane.

Van der Waals forces can be attractive or repulsive:

1) Attraction or repulsion depends of the distance that

separates the two groups.

2) Momentarily unsymmetrical distribution of electrons in one

group induces an opposite polarity in the other when


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the opposite charges are in closet proximimity lead to

attraction between them.

3) The attraction increases to a maximum as the internuclear

distance of the two groups decreases The internuclear

distance is equal to the sum of van der Waals radii of the

two groups.

4) The van der Waals radius is a measure of its size.

5) If the groups are brought still closer — closer than the sum

of van der Waals radii --- the interaction between them

becomes repulsive their electron clouds begin to penetrate

each other, and strong electron-electron interactions begin to

occur.

NOMENCLATURE OF ALKANES

A- Common name:

The names methane, ethane, propane, butane are used for

alkanes which contain 1, 2, 3, and 4 carbon atoms respectively

but alkanes which contain more than 4 carbon atoms have a

name derived from the Latin prefix for the number of carbons

in the alkane.


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The unbranched Alkanes

The prefix (n- ) has been used for any alkane with a continuous

chain of no branching e.g.:

CH3CH2CH2CH3 CH3CH2CH2CH2CH3 CH3CH2CH2CH2CH2CH3

n-butane n- pentane n- hexane


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The prefix ( iso- ) is use for alkanes in which all C atoms are

continues except the end is (CH3)2CH- e.g.:

CH

CH3

H3C CH3 CH

CH3

H3C CH3

H2C

Iso- butane Iso- pentane

The prefix ( neo- ) is use for alkanes in which all C atoms are

continues except the end is (CH3)3C- e.g.:

C

CH3

H3C CH3 C

CH3

H3C CH3

H2C

neo- pentane neo- hexane

CH3CH3

NOMENCLATURE OF ALKYKL GROUPS

1. Alkyl groups: -ane -yl (alkane alkyl) R

CH3- Methyl

C2H5- CH3CH2- Ethyl

C3H7- CH3CH2CH2- n- propyl

CH

CH3

H3C

iso-propyl


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C4H9- CH3CH2CH2CH2- n- butyl

C CH2H3C CH3

H

sec- butyl

CH CH2

CH3

H3C

isobutyl

CH3C

CH3

CH3

ter-butyl

C5H11- CH3CH2CH2CH2CH2- n- pentyl

C CH2H3C CH2

H

CH3

sec- pentyl

CH CH2

CH3

H3C CH2

iso- pentyl

CH3C CH2

CH3

CH3

ter- pentyl

C CH2H3C

CH3

CH3

neopentyl

B- IUPAC (International Union of Pure and Applied

Chemistry) names of Alkanes

1. Locate the longest continuous chain of carbon atoms; this

chain determines the parent name for the alkane. CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3


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2. Number the longest chain beginning with the end of

the chain nearer the substituent.

CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3

123456 7

1

2

3456

Substituent

Substituent

3. Use the numbers obtained by application of rule 2 to

designate the location of the substituent group. The parent

name is placed last; the substituent group, preceded by

the number indicating its location on the chain, is placed first.

CH3CH2CH2CH2CHCH3

CH3

CH3CH2CH2CH2CHCH3

CH2

CH3

123456 7

1

2

3456

Substituent

Substituent

2-Methylhexane 3-Methylheptane

4. When two or more substituents are present, give each

substituent a number corresponding to its location on the

longest chain.

CH3CHCH2CHCH2CH3

1 2 3 4 5 6

CH3 CH2

CH3 4-Ethyl-2-methylhexane

a) The substituent groups are listed alphabetically.

b) In deciding on alphabetically order disregard multiplying

prefixes such as “di” and “tri”.

5. When two substituents are present on the same carbon,


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use the number twice.

CH3CH2CCH2CH2CH3

1 2 3 4 5 6

CH2

CH3

CH3 3-Ethyl-3-methylhexane

6. When two or more substituents are identical, indicate

this by the use of the prefixes di-, tri-, tetra-, and so on.

CH3CH CH

CH3CH3

CH3CH3CH CH

CH3CH3

CH3CH

CH3

CH3C

CH3

CH3

CH2 C

CH3

CH3

CH3

2,3-Dimethylbutane 2,3,4-Trimethylpentane 2,2,4,4-

Tetramethylpentane

7. When two chains of equal length compete for selection as the

parent chain, choose the chain with the greater number of

substituents.

12345

CH3CH2 C C

CH3

H

CH2

H

C C

CH3

H

CH3

H

CH3

CH2

CH3

67

2,3,5-Trimethyl-4-propylheptane

8. When branching first occurs at an equal distance from

either end of the longest chain, choose the name that

gives the lower number at the first point of

difference.


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12345

C CH

CH3

H H

C C

CH3

H

CH3

H

CH3

6H3C

2,3,5-Trimethylhexane (not 2,4,5-trimethylhexane)

Numerical Prefixes Commonly Used in Forming Chemical Names

Numeral Prefix Numeral Prefix Numeral Prefix

1/2 hemi-

1 mono-

3/2 sesqui-

2 di-, bi-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-, ennea-

10 deca-

11 undeca-, hendeca-

12 dodeca-

13 trideca-

14 tetradeca-

15 pentadeca-

16 hexadeca-

17 heptadeca-

18 octadeca-

19 nonadeca-

20 eicosa-

21 heneicosa-

22 docosa-

23 tricosa-

24 tetracosa-

25 pentacosa-

26 hexacosa-

27 heptacosa-

28 octacosa-

29 nonacosa-

30 triaconta-

40 tetraconta-

50 pentaconta-

60 hexaconta-

70 heptaconta-

80 octaconta-

90 nonaconta-

100 hecta-

101 henhecta-

102 dohecta-

110 decahecta-

120 eicosahecta-

200 dicta-


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NOMENCLATURE OF ALKYL HALIDES (Haloalkanes).

CH3CH2Cl CH3CH2CH2F CH3CHBrCH3

Chloroethane 1-Fluoropropane 2-Bromopropane

Ethyl chloride n-Propyl fluoride Isopropyl bromide

1) When the parent chain has both a halo and an alkyl

substituent attached to it, number the chain from the end

nearer the first substituent.

2) Common names for simple haloalkanes are accepted by the

IUPAC alkyl halides (radicofunctional nomenclature).

(CH3)3CBr CH3CH(CH3)CH2Cl (CH3)3CCH2Br

2-Bromo-2-

methylpropane

1-Chloro-2-

methylpropane

1-Bromo-2,2-

dimethylpropane

tert-Butyl bromide Isobutyl chloride Neopentyl bromide

ALKANES (PARAFFINS)

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