JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemMedAdv 2012 • Paper 2

Identify Major Product in Cyanohydrin Formation and Hydrolysis Sequence

Find the major product H\mathbf{H} formed in the specified sequence of reactions:

CH3−CH2−CO−CH3→⊖CNG→95% H2SO4, HeatH\text{CH}_3-\text{CH}_2-\text{CO}-\text{CH}_3 \xrightarrow{\ominus\text{CN}} \mathbf{G} \xrightarrow{\text{95\% H}_2\text{SO}_4, \text{ Heat}} \mathbf{H}

Options

A

CH3−CH=C(CH3)−COOH\text{CH}_3-\text{CH}=\text{C}(\text{CH}_3)-\text{COOH}

Correct
B

CH3−CH=C(CH3)−CN\text{CH}_3-\text{CH}=\text{C}(\text{CH}_3)-\text{CN}

C

CH3−CH2−C(OH)(CH3)−COOH\text{CH}_3-\text{CH}_2-\text{C}(\text{OH})(\text{CH}_3)-\text{COOH}

D

CH3−CH=C(CH3)−CO−NH2\text{CH}_3-\text{CH}=\text{C}(\text{CH}_3)-\text{CO}-\text{NH}_2

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAldehydes and KetonesElimination Reactions

Step-by-Step Solution

To determine the major product H\mathbf{H} formed in the given reaction sequence, we analyze each step sequentially:

Step 1: Nucleophilic Addition (Formation of Cyanohydrin G\mathbf{G})

When butan-2-one (CH3−CH2−CO−CH3\text{CH}_3-\text{CH}_2-\text{CO}-\text{CH}_3) is treated with cyanide ion (CN−\text{CN}^-), a nucleophilic addition reaction occurs at the electrophilic carbonyl carbon:

CH3−CH2−CO−CH3+CN−⟶CH3−CH2−C(OH)(CN)−CH3(G)\text{CH}_3-\text{CH}_2-\text{CO}-\text{CH}_3 + \text{CN}^- \longrightarrow \text{CH}_3-\text{CH}_2-\text{C}(\text{OH})(\text{CN})-\text{CH}_3 \quad (\mathbf{G})

Thus, intermediate G\mathbf{G} is 2-hydroxy-2-methylbutanenitrile (a cyanohydrin).

Step 2: Acid-Catalyzed Dehydration and Nitrile Hydrolysis (Formation of Product H\mathbf{H})

When cyanohydrin G\mathbf{G} is heated with concentrated acid (95% H2SO495\%\ \text{H}_2\text{SO}_4), two major chemical transformations occur simultaneously:

  1. Dehydration of Alcohol:

    • The tertiary hydroxyl group (−OH-\text{OH}) is protonated by H2SO4\text{H}_2\text{SO}_4 to form −OH2+-\text{OH}_2^+, which leaves as a water molecule to generate a stable carbocation intermediate: CH3−CH2−C+(CN)−CH3\text{CH}_3-\text{CH}_2-\overset{+}{\text{C}}(\text{CN})-\text{CH}_3
    • Elimination of a proton (H+\text{H}^+) occurs preferentially from the C-3 methylene (CH2\text{CH}_2) carbon to yield the more substituted, thermodynamic alkene (Saytzeff's rule): CH3−CH=C(CN)−CH3\text{CH}_3-\text{CH}=\text{C}(\text{CN})-\text{CH}_3

  2. Hydrolysis of Nitrile Group:

    • Under strong acidic conditions (95% H2SO495\%\ \text{H}_2\text{SO}_4) with heating, the nitrile group (−CN-\text{CN}) undergoes complete acidic hydrolysis to form a carboxylic acid group (−COOH-\text{COOH}): CH3−CH=C(CN)−CH3→95% H2SO4,ΔCH3−CH=C(CH3)−COOH\text{CH}_3-\text{CH}=\text{C}(\text{CN})-\text{CH}_3 \xrightarrow{95\%\ \text{H}_2\text{SO}_4, \Delta} \text{CH}_3-\text{CH}=\text{C}(\text{CH}_3)-\text{COOH}

Final Product:

The major product H\mathbf{H} is 2-methylbut-2-enoic acid: H=CH3−CH=C(CH3)−COOH\mathbf{H} = \text{CH}_3-\text{CH}=\text{C}(\text{CH}_3)-\text{COOH}

This matches Option (A).

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