Acetaldehyde reacts with hydrogen cyanide by nucleophilic addition to give acetaldehyde cyanohydrin, which is then hydrolysed by heating with dilute hydrochloric acid. What is the final product of the reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Lactic acid
The reaction runs in two steps: nucleophilic addition of HCN to the carbonyl group, then acid hydrolysis of the –CN group to –COOH.
Addition of HCN: the cyanide ion adds to the carbonyl carbon of acetaldehyde, giving acetaldehyde cyanohydrin (2-hydroxypropanenitrile).
\( \displaystyle \ce{CH3CHO + HCN} \xrightarrow{\ce{OH^-}} \ce{CH3CH(OH)CN} \)
Hydrolysis: heating with dilute HCl hydrolyses the nitrile group to a carboxyl group; the nitrogen leaves as ammonium chloride.
\( \displaystyle \ce{CH3CH(OH)CN + 2H2O + HCl} \xrightarrow{\Delta} \ce{CH3CH(OH)COOH + NH4Cl} \)
The product, \( \ce{CH3CH(OH)COOH} \) (2-hydroxypropanoic acid), is lactic acid. The cyanide carbon becomes the carboxyl carbon, so the two-carbon aldehyde gives a three-carbon α-hydroxy acid.
Why the other options are incorrect
Option (1) Mandelic acid — Incorrect. Mandelic acid, \( \ce{C6H5CH(OH)COOH} \), carries a phenyl group; it comes from benzaldehyde cyanohydrin, not from acetaldehyde.
Option (3) Tartaric acid — Incorrect. Tartaric acid, \( \ce{HOOC-CH(OH)-CH(OH)-COOH} \), has four carbons and two –COOH groups; one HCN addition to acetaldehyde gives only three carbons and one –COOH group.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “(a) Addition of hydrogen cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN−) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.”
8.7 Methods of Preparation of Carboxylic Acids – “3. From nitriles and amides
Nitriles are hydrolysed to amides and then to acids in the presence of H+ or OH− as catalyst.”