AROMATIC

04/21/23 Dr Seemal Jelani Chem-160 1

BENZENBENZENEE

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 known

three disubstituted isomers C6H4Y2 known04/21/23 Dr Seemal Jelani Chem-160 2

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.


•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=CHCCH CH2=C=CHCH2CCH

CH3CH2=C=CHCCCH3 CH2=C=CCCH

CH=CH2

CH2=C=CHCH=C=CH2 CH2=C CCH

CH3CH=C=CHCCH04/21/23 Dr Seemal Jelani Chem-160 5

CH2

H2C CH2

=C=C=CH2

=C=CH2

CH2 CH2

CH2

=CH2HC2=


=CH2

Which of these structures is benzene?


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=CHCCH CH2=C=CHCH2CCH five + four + CH3CH2=C=CHCCCH3 CH2=C=CCCH three + three + CH=CH2

CH2=C=CHCH=C=CH2 CH2=C four + CCH four +

CH3CH=C=CHCCH four +04/21/23 Dr Seemal Jelani Chem-160 9

CH2

H2C CH2

=C=C=CH2

=C=CH2

CH2 CH2

CH2

=CH2HC2=

one possible

two

three +

two +

two


=CH2

three one

two


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: 1,3,5-cyclohexatriene

Br

BrBr

Br

Br

Br

Br

Br

Br

Br

Br

Br


Interesting features of benzeneInteresting features of benzene


•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.


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. Sulfonation

C6H6 + H2SO4, SO3 C6H5SO3H + H2O

3. Halogenation

C6H6 + X2, Fe C6H5X + HX

4. Freidel-Crafts alkylation

C6H6 + RX, AlCl3 C6H5R + HX

substitutions04/21/23 Dr Seemal Jelani Chem-160 18

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!



C—C single bonds 1.50 Å

C = C double bonds 1.34 Å

The bonds in benzene are all equal and 1.39 Å

but 1,3,5-cyclohexatriene has three double bonds and three single bonds!


Facts about benzene:

a)Formula = C6H6

b)Isomer number:

one monosubstituted isomer C6H5Y known

three 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.



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).


Aliphatic

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.


Aromatic

Nomenclature for benzene:Monosubstituted benzenes: Special names:

CH3 NH2 OH

CO2H SO3H

toluene aniline phenol

benzoic acid benzenesulfonic acid


Br NO2 Cl

others as substituted benzenes

bromobenzene nitrobenzene chlorobenzene

Mercedes Benzene


Disubsituted benzenes:

ortho- meta- para-

1,2- 1,3- 1,4-


Br

Br

NO2

Cl

CH3

Br

o-dibromobenzene m-chloronitrobenzene p-bromotoluene

1,2-dibromobenzene 3-chloro-1-nitrobenzene 4-bromotoluene

Br

Br

If more than two groups on the ring, use numbers!

Br NH2

Br

Br

Br

1,2,4-tribromobenzene 2,4,6-tribromoaniline04/21/23 Dr Seemal Jelani Chem-160 29

napthalene anthracene phenanthrene

Some other aromatic hydrocarbons:


NOTE:

H2C

H2CCH2

CH2

CH2

H2C

C6H12

cyclohexane

HC

HCCH

CH

CH

HC

C6H6

benzene04/21/23 Dr Seemal Jelani Chem-160 31

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