Q26 / 150 · MHT CET 2026 (18 April Shift 1) · Chemistry
Which from following statements is NOT correct regarding Hofmann's exhaustive alkylation?
- AIn this quaternary ammmonium salts are obtained.
- Correct answerBTwo moles of alkylhalide when heated with one mole primary amine forms tetraalkyl ammonium halide.
- CDepending on the quantity of alkyl halides treated with primary amine, the mixture of secondary and tertiary amines obtained.
- DThe reaction is carried out in presence of $\text{NaHCO}_3$
Solution
Hofmann's exhaustive alkylation is the process of converting an amine into a quaternary ammonium salt by treating it with an excess of alkyl halide.
When a primary amine ($RNH_2$) is heated with an alkyl halide ($RX$), it undergoes successive nucleophilic substitution reactions:
$RNH_2 + RX \rightarrow R_2NH + HX$ (Secondary amine)
$R_2NH + RX \rightarrow R_3N + HX$ (Tertiary amine)
$R_3N + RX \rightarrow R_4N^+X^-$ (Quaternary ammonium salt)
To convert one mole of a primary amine into a tetraalkyl ammonium halide, three moles of alkyl halide are required, not two. Therefore, the statement that two moles of alkyl halide form a tetraalkyl ammonium halide is incorrect.
The reaction is typically carried out in the presence of a mild base like $\text{NaHCO}_3$ to neutralize the acid ($HX$) formed, which prevents the protonation of the amine and maintains its nucleophilicity. Depending on the stoichiometric ratio of the reactants, a mixture of secondary and tertiary amines can also be obtained.
Answer: Two moles of alkylhalide when heated with one mole primary amine forms tetraalkyl ammonium halide.
When a primary amine ($RNH_2$) is heated with an alkyl halide ($RX$), it undergoes successive nucleophilic substitution reactions:
$RNH_2 + RX \rightarrow R_2NH + HX$ (Secondary amine)
$R_2NH + RX \rightarrow R_3N + HX$ (Tertiary amine)
$R_3N + RX \rightarrow R_4N^+X^-$ (Quaternary ammonium salt)
To convert one mole of a primary amine into a tetraalkyl ammonium halide, three moles of alkyl halide are required, not two. Therefore, the statement that two moles of alkyl halide form a tetraalkyl ammonium halide is incorrect.
The reaction is typically carried out in the presence of a mild base like $\text{NaHCO}_3$ to neutralize the acid ($HX$) formed, which prevents the protonation of the amine and maintains its nucleophilicity. Depending on the stoichiometric ratio of the reactants, a mixture of secondary and tertiary amines can also be obtained.
Answer: Two moles of alkylhalide when heated with one mole primary amine forms tetraalkyl ammonium halide.
Hence, the correct answer is option: 'B'.