Question
Describe a method for the identification of primary, secondary and tertiary amines. Also write chemical equations of the reaction involved.
✓
Answer
Identification of primary, secondary and tertiary amines are performed by these methods.
(i) Carbyl amine reaction : Aliphatic and aromatic primary amine are heated with chloroform and ethanolic potassium hydroxide then pungent smelling substance isocyanide or carbyl amine is formed. This reaction is not shown by secondary and tertiary amines.
(ii) Reaction with Hinsberg reagent : In the Hinsberg method mixture of $1^{\circ}, 2^{\circ}$ and $3^{\circ}$ amines is treated with Hinsberg reagent $\left( C _6 H _5 SO _4\right)$ then product obtained from $1^{\circ}$ amine is soluble in alkali and the product obtained from $2^{\circ}$ amine is insoluble in alkali $( NaOH )$ while $3^{\circ}$ amine does not react with Hinsberg reagent.
When steam distillation of reaction mixture is done then $3^{\circ}$ amine is distilled and remaining part is filtered so $C _6 H _5 SO _2 NR _2$ insoluble in $NaOH$ is separated. So $\left( C _6 H _5 SO _2 \stackrel{-}{ NR }\right) \stackrel{+}{ Na }$ is left. Both of these are treated with $25 \% HCl$ followed by hydrolysis then free $1^{\circ}$ and $2^{\circ}$ amines are obtained.
(iii) Hoffman method (Reaction with oxalic ester) : In Hoffman method mixture of amines is treated with oxalic esters $3^{\circ}$-amine does not react, liquid product from $2^{\circ}$-amine and solid product is formed from $1^{\circ}$-amine.
When reaction mixture is distilled then unreacted $3^{\circ}-$ amine is distilled. Remaining residue is filtered and solid and liquid products are separated. Then these are distilled with KOH or NaOH separately so $1^{\circ}$ and $2^{\circ}$ amine become free.
(i) Carbyl amine reaction : Aliphatic and aromatic primary amine are heated with chloroform and ethanolic potassium hydroxide then pungent smelling substance isocyanide or carbyl amine is formed. This reaction is not shown by secondary and tertiary amines.
(ii) Reaction with Hinsberg reagent : In the Hinsberg method mixture of $1^{\circ}, 2^{\circ}$ and $3^{\circ}$ amines is treated with Hinsberg reagent $\left( C _6 H _5 SO _4\right)$ then product obtained from $1^{\circ}$ amine is soluble in alkali and the product obtained from $2^{\circ}$ amine is insoluble in alkali $( NaOH )$ while $3^{\circ}$ amine does not react with Hinsberg reagent.
When steam distillation of reaction mixture is done then $3^{\circ}$ amine is distilled and remaining part is filtered so $C _6 H _5 SO _2 NR _2$ insoluble in $NaOH$ is separated. So $\left( C _6 H _5 SO _2 \stackrel{-}{ NR }\right) \stackrel{+}{ Na }$ is left. Both of these are treated with $25 \% HCl$ followed by hydrolysis then free $1^{\circ}$ and $2^{\circ}$ amines are obtained.
(iii) Hoffman method (Reaction with oxalic ester) : In Hoffman method mixture of amines is treated with oxalic esters $3^{\circ}$-amine does not react, liquid product from $2^{\circ}$-amine and solid product is formed from $1^{\circ}$-amine.
When reaction mixture is distilled then unreacted $3^{\circ}-$ amine is distilled. Remaining residue is filtered and solid and liquid products are separated. Then these are distilled with KOH or NaOH separately so $1^{\circ}$ and $2^{\circ}$ amine become free.
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