Aromaticity

hydrocarbons

aliphatic aromatic

alkanes alkenes alkynes

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 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.

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=CHCCH CH2=C=CHCH2CCH

CH3CH2=C=CHCCCH3 CH2=C=CCCH

CH=CH2

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

CH3CH=C=CHCCH

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 +

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:

Br

BrBr

Br

Br

Br

Br

Br

Br

Br

Br

Br

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. Sulfonation

C6H6 + H2SO4, SO3 C6H5SO3H + H2O

3. Halogenation

C6H6 + X2, Fe C6H5X + HX

4. Freidel-Crafts alkylation

C6H6 + RX, AlCl3 C6H5R + HX

substitutions

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 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!

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 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.

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.

and

2) 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 atomic orbitals

.

0 pi-electrons 1 pi-electron 2 pi-electrons

2 pi-electrons

annulenes: monocyclic compounds with the formula:

-[-CH=CH-]n-

HC

HC CH

CH

4 pi electrons 6 pi electrons 8 pi electrons

10 pi electrons 12 pi electrons

aromatic

aromatic

aromatic

aromatic

•

•::

4 e-

4 e- 5 e-

does not have p-orbitals on all atoms

8 e- 7 e-

6 e-

6 e-

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-tribromoaniline

MD

MD

MD

MD

MD

MD

DDS

DDS

DDS

DDS

orthodoxparadox metaphysicians

orthodontists periodontists

napthalene anthracene phenanthrene

Some other aromatic hydrocarbons:

NOTE:

H2C

H2CCH2

CH2

CH2

H2C

C6H12

cyclohexane

HC

HCCH

CH

CH

HC

C6H6

benzene