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JEE Main · 2023 · 24 Jan 2023 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

Compound (X) undergoes the following sequence of reactions to give the Lactone (Y): (i) HCHO, KOH (ii) KCN (alc.) (iii) H3O+. The Lactone (Y) is a five-membered gamma-butyrolactone bearing a gem-dimethyl group and an alpha-hydroxy group (alpha-hydroxy-beta,beta-dimethyl-gamma-butyrolactone / pantolactone). Compound (X) is:

Structure Lactone (Y) — Lactone (Y)

(A) $HOC H_{2} - C (C H_{3})_{2} - CHO$ (3-hydroxy-2,2-dimethylpropanal)
(B) $HOC H_{2} - C H_{2} - C H_{2} - C H_{2} - CHO$ (5-hydroxypentanal)
(C) $C H_{3} - C H_{2} - C H_{2} - CHO$ (butanal)
(D) $(C H_{3})_{2} CH - CHO$ (isobutyraldehyde / 2-methylpropanal)
✓ Correct

Answer: (D)

Solution

The lactone is a five-membered

γ

-butyrolactone carrying a gem-dimethyl group and an

α

-hydroxy group, so the carbon skeleton is built from isobutyraldehyde. Its single

α

-hydrogen undergoes a crossed aldol with formaldehyde in base to install a

- C H_{2} OH

group, giving hydroxypivaldehyde

HOC H_{2} - C (C H_{3})_{2} - CHO

. Addition of

KCN

to the aldehyde forms the cyanohydrin, and acidic hydrolysis of the nitrile converts it to the

α

-hydroxy acid

HOC H_{2} - C (C H_{3})_{2} - CH (OH) - COOH

. Intramolecular esterification of the primary

- C H_{2} OH

with the

- COOH

then closes the

γ

-lactone. The required compound X is isobutyraldehyde,

(C H_{3})_{2} CHCHO

.

JEE Main · 2023 · 24 Jan 2023 · Shift 1 · Single correct option

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