Reactions of Aromatic Compounds As seen aromatic compounds are

62 Pages·2014·2.83 MB·English

by  Michael Biewer

Reactions of Aromatic Compounds

Aromatic compounds are stabilized by this “aromatic stabilization” energy

Due to this stabilization, normal S 2 reactions observed with alkanes

N
do not occur with aromatic compounds

(S 2 reactions never occur on sp2 hybridized carbons!)

N
In addition, the double bonds of the aromatic group do not behave similar to alkene reactions
Aromatic Substitution

While aromatic compounds do not react through addition reactions seen earlier

Br
Br
Br Br
2 2
FeBr
3 Br
With an appropriate catalyst, benzene will react with bromine

The product is a substitution, not an addition

(the bromine has substituted for a hydrogen)

The product is still aromatic
Electrophilic Aromatic Substitution

Aromatic compounds react through a unique substitution type reaction

Initially an electrophile reacts with the aromatic compound to generate an arenium ion

(also called sigma complex)

The arenium ion has lost aromatic stabilization

(one of the carbons of the ring no longer has a conjugated p orbital)
Electrophilic Aromatic Substitution

In a second step, the arenium ion loses a proton to regenerate the aromatic stabilization

The product is thus a substitution

(the electrophile has substituted for a hydrogen)

and is called an Electrophilic Aromatic Substitution
Energy Profile

Transition
states

Transition
states

Intermediate

Potential
E
energy

H
Starting
material

E
Products

Reaction Coordinate

The rate-limiting step is therefore the formation of the arenium ion
The properties of this arenium ion therefore control electrophilic aromatic substitutions

(just like any reaction consider the stability of the intermediate

formed in the rate limiting step)

1)

The rate will be faster for anything that stabilizes the arenium ion

2)

The regiochemistry will be controlled by the stability of the arenium ion

The properties of the arenium ion will predict the outcome of

electrophilic aromatic substitution chemistry
Bromination

To brominate an aromatic ring need to generate an electrophilic source of bromine

In practice typically add a Lewis acid (e.g. FeBr ) to bromine

3
In presence of aromatic ring this electrophilic bromine source will react

Br
2
Br
FeBr Br
3
H
With unsubstituted benzene the position of reaction is arbitrary

With a monosubstituted aromatic ring, however, can obtain three possible products

How to predict which is favored?

Controlled by stability of arenium ion intermediate
The stability is different depending upon placement of carbocation
Alkyl groups are electron donating

Therefore toluene will favor electrophilic substitution at ortho/para positions

(only ortho/para substitution places carbocation adjacent to alkyl group on ring)

Often obtain more para than ortho due to sterics