20.5 Reactions of Amines

This section covers the primary chemical reactions of amines, including their interaction with acid halides, alkyl halides, and nitrous acid, leading to the formation of amides, more substituted amines, and diazonium salts, respectively.

1. Reaction of Amine with Acid Halide (Acylation)

When a primary or secondary amine reacts with an acid halide (acyl chloride), an N-substituted amide is formed. This reaction is also known as acylation. The hydrogen atom on the nitrogen is replaced by an acyl group ($R\text{-}C=O$).

The general reaction is:

${\text{R-NH}}_2+\text{R’COCl}\to \text{R-NH-COR’}+\text{HCl}$

(Primary Amine) + (Acid Halide) → (N-substituted Amide) + (Hydrogen Chloride)

Example: Ethylamine reacting with acetyl chloride:

${\text{CH}}_3{\text{CH}}_2{\text{NH}}_2+{\text{CH}}_3\text{COCl}\to {\text{CH}}_3{\text{CO-NH-CH}}_2{\text{CH}}_3+\text{HCl}$

2. Reaction of Amine with Alkyl Halide (Alkylation)

Amines can react with alkyl halides in a process called alkylation. This reaction proceeds in a stepwise manner, where the primary amine is converted into a secondary amine, then a tertiary amine, and finally a quaternary ammonium salt. This sequential substitution makes it difficult to stop the reaction at a specific stage, often resulting in a mixture of products.

Reaction Steps:

1. Primary Amine to Secondary Amine: ${\text{R-NH}}_2+\text{R’-X}\to \text{R-NH-R’}+\text{HX}$

2. Secondary Amine to Tertiary Amine: $\text{R-NH-R’}+\text{R’-X}\to {\text{R-N(R’)}}_2+\text{HX}$

3. Tertiary Amine to Quaternary Ammonium Salt: ${\text{R-N(R’)}}_2+\text{R’-X}\to [{\text{R-N(R’)}}_3{]}^{+}{\text{X}}^{-}$

3. Reaction with Nitrous Acid (Diazotization)

The reaction of primary aromatic amines with nitrous acid ($HN{O}_2$), typically generated in situ from $NaN{O}_2$ and a strong acid like $HCl$, forms a diazonium salt. This reaction is called diazotization and must be carried out at low temperatures (below $10^{\circ }\mathrm{C}$) to prevent the decomposition of the product.

${\text{Ar-NH}}_2+{\text{HNO}}_2+\text{HCl}\stackrel{<10^{\circ }\text{C}}{\to }\underset{\text{(Diazonium Salt)}}{{\text{Ar-N}}_2^{+}{\text{Cl}}^{-}}+2{\text{H}}_2\text{O}$

Stability and Uses of Diazonium Salts

• Alkyl diazonium salts are highly unstable and not practically useful — they decompose immediately even at $0\text{–}{5}^{\circ }\mathrm{C}$, releasing $N_2$ gas.
• Aryl (e.g., benzene) diazonium salts are more stable due to the delocalization of the positive charge over the benzene ring.
  • Benzene diazonium chloride ($C_6{H}_5{N}_2^{+}C{l}^{-}$) is a colorless crystalline solid, stable below $10^{\circ }\mathrm{C}$.

${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NH}}_2+{\mathrm{HNO}}_2\stackrel{\mathrm{HCl}}{\to }\underset{\begin{array}{c}\text{Benzene diazonium}\\ \text{chloride}\end{array}}{{\mathrm{C}}_6{\mathrm{H}}_5\text{-N}\equiv {\text{N}}^{+}{\text{Cl}}^{-}}+2{\mathrm{H}}_2\mathrm{O}$

• Applications: Diazonium salts are important intermediates in the synthesis of azo dyes. For example, when benzene diazonium chloride couples with phenol, an orange-red azo dye is formed. Phenyl azo compounds are used as dyes and pigments in textile, cosmetic, paint, and leather industries. They also exhibit antibacterial and antifungal properties.

4. Identification Tests for Amines

Hinsberg Test

The Hinsberg Test uses benzene sulfonyl chloride ($C_6{H}_{5}S{O}_{2}Cl$) to distinguish between primary, secondary, and tertiary amines:

Carbylamine Reaction

The Carbylamine reaction is a specific test for primary amines only. When a primary amine is heated with chloroform ($CHC{l}_3$) and alcoholic $KOH$, a foul-smelling isocyanide (carbylamine) is produced:

${\text{R-NH}}_2+{\text{CHCl}}_3+3\text{KOH}\stackrel{\Delta }{\to }\text{R-N}\equiv \text{C}+3\text{KCl}+3{\text{H}}_2\text{O}$

Secondary and tertiary amines do not give this reaction.