Give one chemical test to distinguish between the following pairs of compounds .
i. Methylamine and dimethylamine
ii. Secondary and tertiary amines
iii. Ethylamine and aniline
iv. Aniline and benzylamine
v. Aniline and N-methylaniline

Explanation

(i) These can be distinguished by carbylamine test. When heated with alcoholic solution of KOH and chloroform, methyl- amine gives foul smell of methyl isocyanide.
`underset("Methylamine")(CH_(3)NH_(2)) + CHCl_(3) + underset(alc.)(3KOH) to underset("foul smell")underset("Methyl isocyanide")(CH_(3)NC) + 3KCl + 3H_(2)O`
Dimethylamine does not give this test.
`underset("Dimethlamine")((CH_(3))_(2)NH) underset(Delta)overset(CHCl_(3),KOH)to` No reaction
(ii) Secondary amines give Libermann nitrosoamine test while `3^(@)` amines do not. `2^(@)` amine on treatment with `HNO_(2)` (prepared in situ by the action of HCl on `NaNO_(2)`) gives yellow coloured oily N-nitrosoamine.
`underset("Diethylamine")((CH_(3)CH_(2))_(2)NH) + HO-N=O to underset("(Yellow colour)")underset("N-Nitrosodiethylamine")(CH_(3)CH_(2))_(2) N-N=O + H_(2)O`
N-Nitrosodiethylamine on warming with a crystal of phenol and conc. `H_(2)SO_(4)` gives a green solution which when made alkaline with aqueous NaOH turns deep blue and then red on dilution. Tertiary amines donot give this test.
(iii) These can be distinguished by azo dye test.
Dissolve the compound in conc. HCl and add ice-cold solution of `HNO_(2) (NaNO_(2) + "dil" HCI)` and then treat it with an alkaline solution of 2-naphthol. Appearance of brilliant orange or red dye indicates aniline.

Ethylamine does not form dye. It will give brisk effervescence due to the evolution of `N_(2)` but solution remains clear.
(iv) These can be distinguished by azo dye test.
Aniline reacts with `HNO_(2)` ( `NaNO_(2)` + dil. HCI) at 273-278K to form stable benzene diazonium chloride which on treat ment with an alkaline solution of 2-naphthol gives an orange dye (as given in reaction (iii)).
Benzylamine does not give azo dye test.
(v) These can be distinguished by carbylamine test. Aniline being primary amines gives carbylamine test i.e., when heated with an alcoholic solution of KOH and `CHCl_(3)` ,it gives foul smell of phenyl isocyanide.
`underset("Aniline")(C_(6)H_(5)NH_(2) + CHCl_(3) + underset("(alc.)")(3KOH) overset(Delta)(to) underset("Phenylisocyanide")(C_(6)H_(5)NC) + 3KCl + 3h_(2)O`
`underset("N-Methylaniline")(C_(6)H_(5)NH–CH_(3)) underset(Delta)overset(CHCl_(3), KOH)(to)` No reaction