Read the Passage carefully and answer the questions.
The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.
The source of nucleophile in the base catalyzed cyanohydrin formation is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → HCN
Aldehydes and ketones add hydrogen cyanide across the C=O bond to give cyanohydrins. The base takes a proton from HCN and generates the cyanide ion, the real nucleophile:
\( \displaystyle \ce{HCN + OH^- <=> CN^- + H2O} \)
The cyanide ion adds to the carbonyl carbon, and the alkoxide ion formed takes a proton from another HCN molecule. This gives the cyanohydrin and gives back the cyanide ion, so only a catalytic amount of base is needed:
\( \displaystyle \ce{R2C=O + CN^- -> R2C(CN)-O^-} \)
\( \displaystyle \ce{R2C(CN)-O^- + HCN -> R2C(CN)-OH + CN^-} \)
So in the base catalysed reaction the nucleophile \( \ce{CN^-} \) comes from HCN; the base only acts as the catalyst that releases it.
Why the other options are incorrect
Option (2) KCN — Incorrect. KCN is a salt that already contains \( \ce{CN^-} \); it does not need a base to release the nucleophile, so it is not the reagent of the base catalysed addition of HCN described here.
Option (3) NaCN — Incorrect. Like KCN, it is an ionic cyanide that supplies \( \ce{CN^-} \) directly; the reaction described is the addition of HCN in the presence of a base.
Option (4) \(CH_3CN\) — Incorrect. In ethanenitrile the CN group is bonded to carbon by a strong covalent bond, so it does not give cyanide ions.
NCERT (2026-27 edition):
Addition of Hydrogen Cyanide (HCN)
