Arrange the following carbonyl compounds in decreasing order of their reactivity toward hydrogen cyanide.
A. Benzaldehyde
B. Acetone
C. Formaldehyde
D. Benzophenone
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C, A, B, D
Addition of HCN is a nucleophilic addition to the C=O group: the cyanide ion attacks the carbonyl carbon. Two factors decide how fast this happens:
Electronic factor: groups that push electrons towards the carbonyl carbon (the +I effect of alkyl groups, the resonance of a phenyl ring) reduce its positive charge and make it less reactive. More such groups mean lower reactivity, so on this count: HCHO (no group) > aldehydes (one group) > ketones (two groups).
Steric factor: groups around the carbonyl carbon crowd the approach of the nucleophile. Two groups hinder more than one, and two bulky phenyl groups hinder the most, so on this count too: HCHO > aldehydes > ketones, with benzophenone last.
Both factors act in the same direction, so the combined order is:
\( \displaystyle \text{methanal}>\text{aldehyde}>\text{ketone with small groups}>\text{ketone with two aryl groups} \)
\( \displaystyle \ce{HCHO}>\ce{C6H5CHO}>\ce{CH3COCH3}>\ce{(C6H5)2CO} \)
Matching the given compounds:
(C) HCHO: formaldehyde (no group on C=O)
(A) \( \ce{C6H5CHO} \): benzaldehyde (aldehyde, one group)
(B) \( \ce{CH3COCH3} \): acetone (ketone, two methyl groups)
(D) \( \ce{(C6H5)2CO} \): benzophenone (ketone, two phenyl groups)
Shortcut: Options (3) and (4) start with C (formaldehyde) and end with D (benzophenone). An aldehyde (A) must come before a ketone (B), which gives Option (4).
NCERT (2026-27 edition):
Chemical Reactions – “… Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.”