Amines

Notes

Amines

1. Introduction

Amines are organic derivatives of ammonia (NH₃) in which one or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups.

Depending upon the number of hydrogen atoms replaced, amines are classified as primary, secondary and tertiary amines.

2. Classification of Amines

2.1 Primary (1°) Amine

When one hydrogen atom of NH₃ is replaced by an alkyl or aryl group, a primary amine is formed.

General formula: R–NH₂

Examples:

• CH₃NH₂ — Methylamine
• C₂H₅NH₂ — Ethylamine
• C₆H₅NH₂ — Aniline

2.2 Secondary (2°) Amine

When two hydrogen atoms of NH₃ are replaced by alkyl or aryl groups, a secondary amine is formed.

General formula: R₂NH

Example:

(CH₃)₂NH — Dimethylamine

2.3 Tertiary (3°) Amine

When all three hydrogen atoms of NH₃ are replaced by alkyl or aryl groups, a tertiary amine is formed.

General formula: R₃N

Example:

(CH₃)₃N — Trimethylamine

3. Nomenclature of Amines

Common Nomenclature

In the common system, the alkyl or aryl group attached to nitrogen is named followed by the word amine.

Examples:

• CH₃NH₂ → Methylamine
• C₂H₅NH₂ → Ethylamine
• (CH₃)₂NH → Dimethylamine
• (C₂H₅)₂NH → Diethylamine

IUPAC Nomenclature

In IUPAC nomenclature, the suffix –amine is used.

Examples:

• CH₃NH₂ → Methanamine
• CH₃CH₂NH₂ → Ethanamine
• CH₃CH₂CH₂NH₂ → Propan-1-amine

For secondary and tertiary amines, the groups attached directly to nitrogen are indicated using N-.

4. Preparation of Amines

4.1 Reduction of Nitro Compounds

Nitro compounds are reduced to primary amines.

R–NO₂ →[Reduction] R–NH₂

Reducing agents shown include:

• H₂/Ni
• Sn/HCl
• Fe/HCl

For nitrobenzene:

C₆H₅NO₂ →[Sn/HCl] C₆H₅NH₂

Product: Aniline

4.2 Ammonolysis of Alkyl Halides

Alkyl halides react with ammonia to form amines.

RX + NH₃ → RNH₂

The primary amine formed can further react with alkyl halide.

RNH₂ + RX → R₂NH

R₂NH + RX → R₃N

R₃N + RX → R₄N⁺X⁻

Thus, ammonolysis can produce a mixture of:

• Primary amine
• Secondary amine
• Tertiary amine
• Quaternary ammonium salt

An excess of ammonia favours the formation of primary amine.

4.3 Preparation from Alcohols

Alcohols react with ammonia to form amines under suitable conditions.

R–OH + NH₃ → R–NH₂ + H₂O

The reaction is carried out at high temperature in the presence of a suitable catalyst.