Name: Isomerism in Amines
Uploaded: 2019-07-19
Duration: 4 min 27 s
Description: Organic chemistry is filled with variations and subtle effects of those variations. Watch this video to learn about the different types of isomers found in a...

Question

Describe the methods of preparation of aromatic monoamines, taking example of aniline. Discuss the relative basic character of aniline and methyl amine. Explain the weak basic character of aniline.

Accepted Answer

Preparation of aromatic amine:

1. Aromatic amines are normally prepared by reduction of the corresponding aromatic nitrocompound. For example, the catalytic hydrogenation of nitrobenzene gives aniline.

$ph - {NO}_{2} + 3 H_{2} \overset{Ni}{\to} ph - {NH}_{2} + 2 H_{2} O$

2. The action of bromine on acid amides in an alkaline solution produces an amine containing one carbon atom less than the number present in the amide. This reaction is called as Hoffmann bromamide reaction. It is used in degradation of carbon chain. For example, action of bromine on benzamide gives aniline.

$ph - {CONH}_{2} \overset{NaOH / {Br}_{2}}{\to} ph - {NH}_{2} + 2 NaBr + {Na}_{2} {CO}_{3} + 2 H_{2} O$

Aniline is less basic than methyl amine. The basicity of the aromatic amine increases when the availability of the lone pair increases. In the case of aniline (aromatic amine), the lone pair undergoes resonance. As a result, lone pair availability decreases as well as the basicity.

On the other hand, methylamine is an aliphatic amine. Basicity of aliphatic amine increases with the increase in inductive effect of the alkyl group. The electron density of the nitrogen increases due to this and as well as the basicity. In case of aniline due to conjugation, the lone pair density is less than that of methylamine. Due to this reason, aniline is less basic than methylamine.

Aniline is a resonance hybrid of the following canonical structures:

Due to the resonance interaction between pi-electron of the benzene ring and the lone pair of electrons on nitrogen atom of the amino group, the lone pair of electrons on the nitrogen atom tends to repel proton. Hence, aniline is a weak base.

Describe the methods of preparation of aromatic monoamines, taking example of aniline. Discuss the relative basic character of aniline and methyl amine. Explain the weak basic character of aniline.

Important Questions on Amines

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