Question

Acetanilide when treated with bromine in acetic acid mainly gives:
A. o-Bromoacetanilida
B. N-Bromoacetanilide
C. p-Bromoacetanilide
D. m-Bromoacetanilide

Answer 1

Hint: We know that the aromatic compounds consist of conjugated pi bonds due to which an electron cloud is formed within the compound. Because of this electron cloud, the compound gains a nucleophilic character and tends to undergo electrophilic attack. Hence, when the compound reacts with an electrophile, the substitution reaction is observed which is known as electrophilic aromatic substitution reaction.

Complete answer:
Aromatic compounds may be brominated when reacted with bromine in the presence of Lewis acid catalysts like ferric chloride. But in case of very electron rich aromatic rings like aryl amines or aryl amide, the reaction may proceed in the absence of any acid catalyst. Due to the presence of nitrogen atoms, the amide groups are ortho para directing. The acetamido group i.e., $ - NHCOC{H_3}$ present in acetanilide is a large group which effectively blocks the ortho position of the aromatic ring and thus only formation of para products is observed. The mechanism for the reaction is as follows:
Step-1: Attack of bromine as an electrophile at para position of the ring:
The double bond present in the ring consists of an electron cloud which behaves as nucleophile and attacks bromine molecules and removal of bromide ion takes place. The loss in the aromaticity of the molecule is observed in this step. The reaction takes place as follows:

Step-2: Removal of hydrogen ion:
The molecule always tries to achieve its aromatic character due to greater stability. Thus, the hydrogen ion from the para position will leave the molecule and formation of p-Bromoacetanilide takes place along with the removal of hydrogen bromide. The reaction proceeds as follows:

Hence, when acetanilide is treated with bromine in acetic acid, it produces p-Bromoacetanilide.
So, option (C) is the correct answer.

Note:
It is important to note that the acetamide group is less powerful ortho-para directing than amine group due to the presence of electron-withdrawing carbonyl group which renders the less nucleophilic character of the ring. In case of aromatic amines, the monosubstituted products are not formed due to high nucleophilic character. Hence, to obtain desired products, the amine group is converted to acetamide group in some organic reactions.