Question

Give one chemical test to distinguish between methylamine and dimethylamine.

Answer 1

Hint:Methylamine, and dimethylamine are the members of amines family having functional group $ - N{H_2}$. Methylamine is a primary amine with formula $C{H_3} - N{H_2}$ and dimethylamine is a secondary amine with formula $C{H_3} - NH - C{H_3}$.

Complete step by step answer:
Step (1):
Carbylamine test - This test is given by $1^\circ - $ amines or primary amines. It is also called an isocyanide test because it gives isocyanide as the end product. Primary amine is treated with chloroform in a basic medium. This is heated to form isocyanide, Potassium chloride and water.
The reaction will be:
Alkyl amine + Chloroform + Alcoholic potassium hydroxide + Heat $ \to $ Isocyanide + Potassium Chloride + Water
$R - N{H_2} + CHC{l_3} + {\text{3KOH}}\xrightarrow{\Delta }{R^ \oplus } \equiv N{C^\Theta } + {\text{3KCl}} + 3{H_2}O$

Step (2):
Since methylamine, $C{H_3} - N{H_2}$, is a primary amine, it will give a carbylamine reaction.
Mechanism will be: The hydrogen of the chloroform will be taken by the base ${\text{KOH}}$ to form an intermediate dichlorocarbene, which is an electron deficient molecule called electrophile. Methylamine donates its electron to this dichlorocarbene. Then base acts on the product to form isocyanide. Its formation can be observed by a foul smell. This foul smell is due to the isocyanide formed. Dimethylamine does not give this test as it is a secondary amine or $2^\circ - $ amine. So, there will be no foul smell observed after doing a carbylamine test. Thus, methylamine gives carbylamine test whereas dimethylamine does not.

Hence, methylamine, and dimethyl amine can be distinguished using carbylamine test.

Note:Dimethylamine or any secondary amine has only one active hydrogen attached to nitrogen, $R - NH - R$ , where ${\text{R}}$ is any alkyl or aryl group, which will give a little yield of unstable inner salt during carbylamine test as it cannot give further elimination due to presence of only one active hydrogen.