Distinguish between primary, secondary and tertiary amines.

Explanation

Reaction of an amine with nitrous acid (HO - N =O) is the distinguishing test between primary, secondary and tertiary amines as they react differently with nitrous acid.
Nitrous acid is unstable, hence prepared instantly by the action of cold, dilute, hydrochloric acid on sodium nitrite.
`underset("Sodium nitrite")(NaNO_(2)) + HCI overset("cold")underset("dil.")to NaCI + underset(" nitrous acid")(HNO_(2))`
(1) Action of nitrous acid on primary amine : All primary amines, except methly amine react with nitrous acid in cold to give alcohol and liberate nitrogen gas.
For example, ethyl amine on reaction with nitrous acid in cold, forms ethyl alcohol and nitrogent gas.
`underset("ethylamine")(C_2H_5 -NH_2)+underset("nitrous acid")(HO-N=O) overset(NaNO_(2)+ " dill. HCI")underset("cold")to underset("ethyl alcohol")(C_(2)H_(5)-OH)H_(2)O+underset("nitrogen gas")(N_(2))`
(2) Action of nitrous acid on secondary amines : Tertiary amine on reaction with nitrous acid forms N-nitrosoamine which is a pale yellow oil.
For example, diethyl amine on reaction with nitrous acid in cold froms pale yellow oily N-nitrosodiethylamine.
`underset("diethylamine")((C_(2)H_(5))_(2)NH)+H-O-H=O overset(NaNO_(2)+ " dil.HCI") underset("cold")to underset("N-nitrosodiethylamine")((C_(2)H_(5))_(2)N-N=O + H_(2))O`
(3) Action of nitrous acid on tertiary amines , Tertiary amine on reaction with nitrous acid forms water - soluble nitrite salt. As no visible change , is observed , it is said that there is no reaction.
For example, triethylamine reacts with nitrous acid in cold and forms water soluble triethyl ammonium nitrite salt.
`underset("triethyamine nitrous acid")((C_(2)H_(5))_(3)N+HO-N=O) overset(NaNO_(2)+ " dil. HCI")underset("cold")to underset("triethyl ammonium nitrite")([(c_(2)H_(5))_(3)NH^(+)]NO_(2)^(-))`