Amines are reactive in nature, mainly due to difference in electronegativity between Nitrogen and Hydrogen and, due to lone pair of electrons on Nitrogen. Amines behaves like nucleophiles and as good Lewis bases and there are various reactions to support this. Different degree of amines differ in their reactivity due to steric hinderance and electronic effects. Aromatic amines are less basic due to +R effect, hence have higher \(pK_b\) value than the aliphatic amines. Basic strength of amines also depend on stability of corresponding conjugate acid in aqueous medium.

'A' compound is treated with Hinsberg's reagent, subsequent addition of alkali to it gives a clear solution. Compound 'A' is:

Primary amine

The correct answer is option 1. Primary amine.

Let us dive into the details of how the Hinsberg test works and why the correct answer is a primary amine.

The Hinsberg test is a classical method used to distinguish between primary, secondary, and tertiary amines based on their different chemical behaviors with Hinsberg's reagent (benzenesulfonyl chloride, \( C_6H_5SO_2Cl \)).

Reactions with Hinsberg's Reagent:

1. Primary Amine:

A primary amine \((R-NH_2)\) reacts with Hinsberg's reagent to form a sulphonamide derivative:

The sulphonamide formed still contains a hydrogen atom attached to the nitrogen. This hydrogen is slightly acidic. When treated with an alkali like NaOH, this sulphonamide dissolves because the acidic hydrogen reacts with the base, forming a soluble salt:

The solution becomes clear due to the solubility of the resulting salt in the alkali.

2. Secondary Amine:

A secondary amine \((R_2NH)\) reacts with Hinsberg's reagent to form a sulphonamide as well:

However, this sulphonamide lacks the acidic hydrogen that could make it soluble in alkali. When treated with NaOH, this sulphonamide does not dissolve because it cannot form a soluble salt. The solution remains cloudy or forms a precipitate.

3. Tertiary Amine:

A tertiary amine \((R_3N)\) does not react with Hinsberg's reagent at all, because it lacks any hydrogen attached to the nitrogen:

There is no product to dissolve or react with NaOH.  No change; no clear solution forms.

4. Aldehyde:

Aldehydes are carbonyl compounds and do not contain the amine functional group, so they do not react with Hinsberg's reagent. No reaction; no clear solution forms.

Conclusion:

The key observation here is that compound 'A' forms a clear solution upon treatment with alkali after reacting with Hinsberg's reagent. This outcome is specific to primary amines, as they form a sulfonamide that dissolves in alkali due to the formation of a soluble salt.

Therefore, compound 'A' is a primary amine.