(S)-Alanine, CH3-CH(NH2)-COOH, is treated with NaNO2 in dilute sulfuric acid to give the diazonium ion, CH3-CH(N2+)-COOH, which then reacts with water. The predominant product, lactic acid, is

(S)-lactic acid, formed with retention through carboxyl participation

(S)-lactic acid, formed by front-side attack of water on the diazonium ion

racemic lactic acid, formed through a free secondary cation

(R)-lactic acid, formed with inversion by direct SN2 attack of water

MolSynapse / Question bank / Organic Reaction Mechanisms III: Nucleophilic Substitution / Neighbouring group participation

JEE Advanced levelSingle correct optionDifficulty 3 of 3o3158

(S)-Alanine, $CH X_{3} - CH (NH X_{2}) - COOH$ , is treated with $NaNO X_{2}$ in dilute sulfuric acid to give the diazonium ion, $CH X_{3} - CH (N X_{2} X^{+}) - COOH$ , which then reacts with water. The predominant product, lactic acid, is

A(S)-lactic acid, formed with retention through carboxyl participation
B(S)-lactic acid, formed by front-side attack of water on the diazonium ion
Cracemic lactic acid, formed through a free secondary cation
D(R)-lactic acid, formed with inversion by direct SN2 attack of water

Answer: A

The diazonium group is a superb leaving group but the neighbouring carboxyl oxygen displaces

N X_{2}

from the back, giving an alpha-lactone (first inversion). Water opens it at the same carbon from the back (second inversion), so the net result is retention. Priorities are

N / O > COOH > CH X_{3} > H

for the amine, diazonium and alcohol, so the product is (S)-lactic acid.

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on Neighbouring group participation

Previous questionNext question

(S)-Alanine, CHX3​−CH(NHX2​)−COOH, is treated with NaNOX2​ in dilute sulfuric acid to give the diazonium ion, CHX3​−CH(NX2​X+)−COOH, which then reacts with water. The predominant product, lactic acid, is

Answer: A

More questions on Neighbouring group participation

(S)-Alanine, $CH X_{3} - CH (NH X_{2}) - COOH$ , is treated with $NaNO X_{2}$ in dilute sulfuric acid to give the diazonium ion, $CH X_{3} - CH (N X_{2} X^{+}) - COOH$ , which then reacts with water. The predominant product, lactic acid, is