Question

Draw the products formed when A and B are treated with each of the following reagents:

(a)${\text{Br}}_{\text{2}}$,${\text{FeBr}}_{\text{3}}$; (b)${\text{HNO}}_{\text{3}}{\text{,H}}_{\text{2}}{\text{SO}}_{\text{4}}$l3; (c) ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{COCl, AlCl}}_{\text{3}}$

Short answer

The groups that activate the benzene ring will direct the incoming electrophile to ortho and para positions. They are called ortho-para directors.

e.g., ${\text{- NH}}_{\text{2}}\text{,} {\text{- OCH}}_{\text{3}}\text{,} \text{- OH,} {\text{- CH}}_{\text{3}}$

The group which deactivates the benzene ring will direct the incoming electrophile to meta position. They are called meta directors.

e.g.,${\text{- NO}}_{\text{2}}\text{,} \text{- COOH,} \text{- CHO}$

Step-by-step solution

Activating and deactivating groups

The groups that activate the benzene ring will direct the incoming electrophile to ortho and para positions. They are called ortho-para directors.

e.g., ${\text{- NH}}_{\text{2}}\text{,} {\text{- OCH}}_{\text{3}}\text{,} \text{- OH,} {\text{- CH}}_{\text{3}}$

The group which deactivates the benzene ring will direct the incoming electrophile to meta position. They are called meta directors.

e.g.,${\text{- NO}}_{\text{2}}\text{,} \text{- COOH,} \text{- CHO}$

Effect of substituents on the benzene ring

• The structure of the given three-dimensional models can be represented as

• Both the given compounds are disubstituted benzene derivatives.

• The electrophilic substitution reactions on these compounds depend on the directing effects of the two substituents on the benzene ring.

Compound A

• The substituents are an o-isopropyl group which is an ortho-para director, and a CHO group, which is a meta director.

• If the directing effect of one substituent opposes the directing effect of another substituent, then the position of the incoming electrophile is determined by the dominant activator group. Therefore, the isopropyl group determines the position of the incoming electrophile.

• Also, no substitution can take place between two meta substituents due to crowding.

Compound B

The substituents are an ${\text{NHCOCH}}_{\text{3}}$ group, which is an ortho-para director, and a Br group, a weak ortho-para director.

As the ${\text{NHCOCH}}_{\text{3}}$group is a strong activator, it determines the position of the incoming electrophiles, which is ortho to the ${\text{NHCOCH}}_{\text{3}}$group.

Reaction with the reagents

a. The given reagent is ${\text{Br}}_{\text{2}}$,${\text{FeBr}}_{\text{3}}$. This reagent is used in the bromination of aromatic compounds. The product formed on bromination of compounds A and B are:

b. The given reagents are ${\text{HNO}}_{\text{3}}{\text{,H}}_{\text{2}}{\text{SO}}_{\text{4}}$. This reagent is used in the nitration of aromatic compounds. The product formed on nitration of compounds A and B are:

c. The given reagents are ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{COCl, AlCl}}_{\text{3}}$. This reagent is used in Friedel-Craft’s acylation of aromatic compounds. The product formed on Friedel-Craft’s acylation of compounds A and B are: