Identify the compound that will react with Hinsberg’s reagent to give a solid which dissolves in alkali.

The correct option is Option 2, which corresponds to ethylamine (${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{NH}}_2$).

Hinsberg's Test Explanation:
Hinsberg's reagent is benzenesulfonyl chloride ($C_6{H}_5{\mathrm{SO}}_2\mathrm{Cl}$). It is used to distinguish between primary, secondary, and tertiary amines based on their reactivity and the solubility of the resulting product in an alkali solution.

1. Reaction with Primary Amines (1° Amines):
A primary amine, such as ethylamine (${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{NH}}_2$), reacts with benzenesulfonyl chloride to form N-ethylbenzenesulfonamide (a solid sulfonamide precipitate).
$C_6{H}_5{\mathrm{SO}}_2\mathrm{Cl}+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{NH}}_2\to C_6{H}_5{\mathrm{SO}}_2\mathrm{NH}-{\mathrm{CH}}_2{\mathrm{CH}}_3+\mathrm{HCl}$
The hydrogen atom attached to the nitrogen in this sulfonamide is strongly acidic because it is attached to a highly electronegative nitrogen atom which is adjacent to the strongly electron-withdrawing sulfonyl group ($-{\mathrm{SO}}_2-$). Due to this acidity, the sulfonamide reacts with aqueous alkali (such as NaOH or KOH) to form a soluble sodium salt, which dissolves completely to form a clear solution:
$C_6{H}_5{\mathrm{SO}}_2\mathrm{NH}-{\mathrm{CH}}_2{\mathrm{CH}}_3+\mathrm{NaOH}\to {\mathrm{Na}}^{+}[C_6{H}_5{\mathrm{SO}}_{2}N-{\mathrm{CH}}_2{\mathrm{CH}}_3{]}^{-}+H_{2}O$

2. Comparison with other classes of compounds:
• Secondary Amines (e.g., $({\mathrm{CH}}_3{\mathrm{CH}}_2{)}_2\mathrm{NH}$) react to form an N,N-dialkylbenzenesulfonamide. Since this compound lacks any acidic hydrogen atom attached to nitrogen, it is completely insoluble in alkali.
• Tertiary Amines (e.g., $({\mathrm{CH}}_3{\mathrm{CH}}_2{)}_{3}N$) do not react with Hinsberg's reagent because they do not have a replaceable hydrogen atom on the nitrogen.
• Nitroethane (${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{NO}}_2$) does not react with Hinsberg's reagent under these conditions.