Question

Which of the following gives a positive Libermann-nitroso test?
(A) 2-butanamine
(B) N-ethyl-2-pentanamine
(C) N-methylpiperidine
(D) N,N-dimethylcyclohexylamine

Answer 1

Hint: The Libermann-nitroso test which is used for the test of secondary amines that may be aliphatic or aromatic. In the test, the secondary amines react with nitrous acid and formation of N-nitrosamines takes place which are water soluble yellow oils which convert into violet or greenish blue on further treatment with alkali.

Complete Step by Step Solution:
As we know that only secondary amines will give a positive result to the Libermann-nitroso test. So, among the given options we have to compare the structures of the given amines as follows:
Structure (A): 2-butanamine

Structure (B): N-ethyl-2-pentanamine

Structure (C): N-methylpiperidine

Structure (D): N,N-dimethylcyclohexylamine


Secondary amines are the amines in which the amino group i.e The nitrogen atom is bonded directly to the two carbon atoms of any hybridization but the carbon atoms must not be carbonyl group carbons.

So, among the given options, structure (A) is eliminated because the nitrogen atom of the amino group is bonded to a single carbon atom only. Structures (C) and (D) are eliminated as well because the nitrogen atom of the amino group in both the compounds are bonded to three carbon atoms i.e., the structure (C) and (D) are the tertiary amines.

In structure B, the nitrogen atom is bonded to two carbon atoms and one hydrogen atom and thus, is a secondary amine which shows positive Liberman-nitroso test. Hence, a positive Libermann-nitroso test will be given by N-ethyl-2-pentanamine.
Therefore, option (B) is the correct answer.

Note: It is important to note that the Libermann-nitroso test is also employed for the test of phenol as on reaction, it forms a deep green or blue colour solution which converts to red on dilution with water.