Read the passage carefully and answer the Questions.

Amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecules. Aliphatic amines are a stronger bases due to +1 effect of an alkyl groups. Intermolecular hydrogen bonding in primary and secondary amines affects the boiling points of amines. The intermolecular association is more in primary amines than in secondary amines as there are more number of hydrogen atoms available for hydrogen bond formation. Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in an acidic medium. Gabriel synthesis is used for the preparation of primary amines. Primary amines react with nitrous acid to give unstable diazonium-salts which can be used to synthesize a variety of compounds.

Identify the amine possessing least basic strength

p-Nitroaniline

The correct answer is Option (2) → p-Nitroaniline

Basicity of amines depends on the availability of the lone pair on nitrogen for protonation. Electron-donating groups (–CH₃, –OCH₃) increase basicity, while electron-withdrawing groups (–NO₂) decrease it.

Analysis:

1. Aniline (C₆H₅NH₂) → NH₂ attached to benzene → lone pair delocalized into the ring → moderately basic.
2. p-Nitroaniline (p-NO₂–C₆H₄–NH₂) → NO₂ is strongly electron-withdrawing → lone pair less available → least basic.
3. p-Toluidine (p-CH₃–C₆H₄–NH₂) → CH₃ is electron-donating → increases basicity.
4. p-Methoxyaniline (p-OCH₃–C₆H₄–NH₂) → OCH₃ is electron-donating → increases basicity further.