Post on 12-Mar-2023
Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.
Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.
Benzene. This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946.
Facts about benzene:a) Formula = C6H6
b) Isomer number:one monosubstituted isomer C6H5Y knownthree disubstituted isomers C6H4Y2 known
c) Benzene resists addition reaction, undergoes substitution reactions.
d) Heats of hydrogenation and combustion are far lower than they should be.
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
a) Formula = C6H6
Max. number of H’s for 6-carbons = 14. Benzene only has 6 hydrogens. Given one degree of unsaturation (double bond or ring) for every two missing hydrogens less than the maximum, benzene has 4 degrees of unsaturation; that is four combinations of pi-bonds and rings.
CH3CC-CCCH3 HCC-CC-CH2CH3
HCCCH2CCCH3 HCCCH2CH2CCH
CH3
HCCCHCCH CH2=CHCCCH=CH2
CH2=CHCH=CHCCHCH2=C=CHCH2CCH
CH3CH2=C=CHCCCH3 CH2=C=CCCH
CH=CH2CH2=C=CHCH=C=CH2 CH2=C CCH
CH3CH=C=CHCCH
b) Isomer number. There is only one monosubstituted benzene of any type: only one bromobenzene C6H5Br, only one nitrobenzene C6H5NO2, etc.
CH3CC-CCCH3 HCC-CC-CH2CH3
one possible three possibles
HCCCH2CCCH3 HCCCH2CH2CCH three possible two
possible
CH3
HCCCHCCH CH2=CHCCCH=CH2 three two +
CH2=CHCH=CHCCHCH2=C=CHCH2CCH five + four + CH3CH2=C=CHCCCH3 CH2=C=CCCH three + three + CH=CH2CH2=C=CHCH=C=CH2 CH2=C four + CCH four +
CH3CH=C=CHCCH four +
CH3CC-CCCH3
There are three disubstituted benzenes of any type: three dibromobenzenes C6H4Br2, etc.CH2
CH2 CH2two possible
four
four
No classical valence bond structure for C6H6 correctly explains the existence of only one monosubstituted benzene and three disubstituted benzenes. Kekulé (1890) proposed that the following were in rapid equilibrium:
BrBr
Br
Br
Br
Br
BrBr
BrBr
BrBr
If benzene is 1,3,5-cyclohexatriene as Kekulé proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles.But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons!
KMnO4 oxidation
no reaction
Br2/CCl4 addition no reaction
HI addition no reaction
H2/Ni reduction
no reaction
Reagent Cyclohexene Benzene
Benzene + 3 H2, Ni, room temp. NR
Benzene + 3 H2, Ni, 200oC, 1500 psi cyclohexane
Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.
Reactions of benzene:1. Nitration
C6H6 + HNO3, H2SO4 C6H5NO2 + H2O
2. SulfonationC6H6 + H2SO4, SO3 C6H5SO3H + H2O
3. HalogenationC6H6 + X2, Fe C6H5X + HX
4. Freidel-Crafts alkylation C6H6 + RX, AlCl3 C6H5R
+ HXsubstitutions
d) Heats of hydrogenation and combustion are far lower than they should be.
cyclohexene + H2, Ni cyclohexane + 28.6 Kcal/mole
1,3-cyclohexadiene + 2 H2, Ni cyclohexane + 55.4 Kcal/mole
(predicted value = 2 X 28.6 = 57.2 Kcal/mole)
benzene + 3 H2, Ni, heat, pressure cyclohexane + 49.8 Kcal/mole
(predicted value = 3 X 28.6 = 85.8 Kcal/mole)
Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
C—C single bonds 1.50 ÅC = C double bonds 1.34 ÅThe bonds in benzene are all equal and 1.39 Å
but 1,3,5-cyclohexatriene has three double bonds and three single bonds!
Resonance!We can draw more than one classic structure that differ only in where the electrons are. The two structures are of the same energy, so resonance is important. The molecule cannot be adequately represented by classic structures but must be thought of as a hybrid of the contributing structures. Additionally, the hybrid is more stable than any of the contributing structures (resonance stabilization energy).
Facts about benzene:a) Formula = C6H6
b) Isomer number:one monosubstituted isomer C6H5Y knownthree disubstituted isomers C6H4Y2 known
c) Benzene resists addition reaction, undergoes substitution reactions.
d) Heats of hydrogenation and combustion are far lower than they should be.
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
Aliphatic hydrocarbons are open-chain and ring compounds that react like open chain compounds:saturated: alkanes and cycloalkanes (typical reaction = substitution)unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical reaction = addition).Aromatic hydrocarbons are unsaturated ring compounds that resist the typical addition reactions of aliphatic unsaturated compounds, instead undergoing substitution reactions. They are also much more stable than they should be.
Can we predict which compounds will be aliphatic and which ones will be aromatic like benzene? Yes.
In order to be aromatic, the compound must be:
1) cyclic with p-atomic orbitals on all members of the ring.
and2) have 4i + 2 electrons in the p-
orbitals of the ring (where i = 0, 1, 2, 3, …).
[ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]
pi-electrons sp2 hybridized carbonsp atom ic orbitals
.
0 pi-electrons 1 pi-electron 2 pi-electrons
2 pi-electrons
annulenes: monocyclic compounds with the formula:
-[-CH=CH-]n-HCHC CH
CH
4 pi electrons 6 pi electrons 8 pi electrons
10 pi electrons 12 pi electrons
arom atic
arom atic
Nomenclature for benzene:monosubstituted benzenes:Special names:
CH3 NH2 OH
CO 2H SO 3H
toluene aniline phenol
benzoic acid benzenesulfonic acid
Br
Br
NO 2
Cl
CH3
Br
o-dibrom obenzene m-chloronitrobenzene p-brom otoluene1,2-dibrom obenzene 3-chloro-1-nitrobenzene 4-brom otoluene
Br
Br
If m ore than two groups on the ring, use num bers!
Br NH2Br
Br
Br
1,2,4-tribrom obenzene 2,4,6-tribrom oaniline