Read the following passage given below and answer the question.
Aldehydes and ketones undergo nucleophilic addition reactions. A nucleophile attacks the electrophilic carbon atom of polar carbonyl group from perpendicular to plane of \(sp^2\) hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from \(sp^2\) to \(sp^3\) and a tetrahedral alkoxide intermediate is formed. This captures \(H^+\) to give neutral product. In carboxylic acids, carbonyl carbon is less electrophilic than carbonyl carbon of aldehydes and ketones.
How will you convert acetaldehyde to lactic acid?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → treating with HCN followed by hydrolysis
Lactic acid is 2-hydroxypropanoic acid, \( \ce{CH3CH(OH)COOH} \). It has one carbon more than acetaldehyde and an –OH group on the carbon next to –COOH. Both features come from a cyanohydrin. First, HCN adds to the C=O group (nucleophilic addition, catalysed by a base):
\( \displaystyle \ce{CH3CHO + HCN -> CH3CH(OH)CN} \)
Then acid hydrolysis turns the –CN group into –COOH, giving lactic acid:
\( \displaystyle \ce{CH3CH(OH)CN + 2H2O + H+ -> CH3CH(OH)COOH + NH4+} \)
Option (2) treating with CH3OH molecules twice — Incorrect. Two molecules of methanol (with dry HCl) give an acetal; no new carbon or –COOH group is formed.
\( \displaystyle \ce{CH3CHO + 2CH3OH} \xrightarrow{\text{dry }\ce{HCl}} \ce{CH3CH(OCH3)2 + H2O} \)
Option (3) treating with sodium bisulphite and then hydrolyzing — Incorrect. The bisulphite addition product gives back acetaldehyde on hydrolysis with dilute acid or alkali. This is used to purify aldehydes, not to make acids.
\( \displaystyle \ce{CH3CHO + NaHSO3 <=> CH3CH(OH)SO3Na} \)
Option (4) treating with CH3MgBr followed by hydrolysis — Incorrect. The Grignard reagent adds a methyl group and gives a secondary alcohol, propan-2-ol, not an acid.
\( \displaystyle \ce{CH3CHO} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CH3MgBr}} \ce{CH3CH(OH)CH3} \)
NCERT (2026-27 edition):
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. Cyanohydrins are useful synthetic intermediates.”