The increasing order of the rate of HCN addition to compounds A-D is (A) HCHO (B) $\mathrm{CH}_{3} \mathrm{COCH}_{3}$ (C) $\mathrm{PhCOCH}_{3}$ (D) PhCOPh

(A) $\mathrm{A}<\mathrm{B}<\mathrm{C}<\mathrm{D}$

(D) $\mathrm{C}<\mathrm{D}<\mathrm{B}<\mathrm{A}$

JEE Main · 2006 · 30 Apr 2006 · Offline · Aldehydes Ketones and Carboxylic Acids

The increasing order of the rate of HCN addition to compounds A-D is
(A) HCHO
(B)

CH_{3} COCH_{3}

(C)

PhCOCH_{3}

(D) PhCOPh

(A) $A < B < C < D$
(B)D $<$ B $<$ C $<$ A
(C)D $<$ C $<$ B $<$ A✓ Correct
(D) $C < D < B < A$

Answer: (C)

Solution

Addition of HCN to a carbonyl is a nucleophilic addition, so the rate is governed by how electrophilic the carbonyl carbon is and how crowded its surroundings are. Formaldehyde,

HCHO

, carries only hydrogens on the carbonyl and is both the most electrophilic and the least hindered, so it reacts fastest. Acetone,

C H_{3} COC H_{3}

, bears two electron-donating, bulky methyl groups and is slower; acetophenone,

PhCOC H_{3}

, is slower still because the phenyl ring delocalises the carbonyl and adds bulk; and benzophenone,

PhCOPh

, with two phenyl groups is the most hindered and least reactive. The rate therefore rises in the order

PhCOPh < PhCOC H_{3} < C H_{3} COC H_{3} < HCHO

JEE Main · 2006 · 30 Apr 2006 · Offline · Single correct option

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JEE Main · 2006 · 30 Apr 2006 · Offline · Aldehydes Ketones and Carboxylic Acids

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