Nucleophiles such as ammonia and its derivatives, \(H_2N-Z\), add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours product formation due to rapid dehydration of the intermediate to form \(-C=N-Z\), where Z = alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
Match List-I with List-II.
| List-I (Reagent used with an aldehyde or a ketone) | List-II (Result) |
|---|---|
| (A) HCN, in the presence of a base | (I) The carbonyl group is reduced to a CH2 group |
| (B) Sodium hydrogensulphite | (II) A yellow, orange or red solid used to characterise the carbonyl compound |
| (C) Hydrazine, followed by heating with KOH in ethylene glycol | (III) A water-soluble addition compound, useful for the separation and purification of aldehydes |
| (D) 2,4-Dinitrophenylhydrazine | (IV) A product with –OH and –CN groups on the same carbon, useful as a synthetic intermediate |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A-IV, B-III, C-I, D-II
Each reagent is a nucleophile that adds to the carbonyl carbon. What happens next decides the use of the reaction.
(A) HCN, in the presence of a base — This matches (IV) a product with –OH and –CN on the same carbon. The base gives the cyanide ion, a stronger nucleophile than HCN, and the product is a cyanohydrin:
\( \displaystyle \ce{R2C=O + HCN} \xrightarrow{\ce{OH-}} \ce{R2C(OH)CN} \)
(B) Sodium hydrogensulphite — This matches (III) a water-soluble addition compound. The bisulphite addition compound is crystalline and dissolves in water; dilute mineral acid or alkali gives back the carbonyl compound, so aldehydes can be separated and purified this way:
\( \displaystyle \ce{R2C=O + NaHSO3 <=> R2C(OH)OSO2Na} \)
(C) Hydrazine, followed by heating with KOH in ethylene glycol — This matches (I) the carbonyl group is reduced to a CH2 group. Hydrazine first forms the >C=N–NH2 derivative; heating it with alkali in the high-boiling ethylene glycol removes the nitrogen as N2 and leaves CH2. This is the Wolff-Kishner reduction:
\( \displaystyle \ce{R2C=O} \xrightarrow{\ce{NH2NH2}} \ce{R2C=NNH2} \xrightarrow[\text{heat}]{\ce{KOH}/\text{ethylene glycol}} \ce{R2CH2 + N2} \)
(D) 2,4-Dinitrophenylhydrazine — This matches (II) a yellow, orange or red solid. The 2,4-DNP derivatives of aldehydes and ketones are coloured solids, so the reagent is used to detect and characterise carbonyl compounds.
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. … Cyanohydrins are useful synthetic intermediates.”
“The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for separation and purification of aldehydes.”
“(ii) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).”
“* 2,4-DNP-derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones.”