Question

Consider the following aldehydes and ketones. Which is the correct ranking in order of increasing reactivity towards nucleophilic addition to the carbonyl group?

A) 2-methylcyclohexanone < cyclohexanone < CH3CHO < H2CO

B) CH3CHO < 2-methylcyclohexanone < cyclohexanone < H2CO

C) cyclohexanone < 2-methylcyclohexanone < H2CO < CH3CHO

D) H2CO < cyclohexanone < CH3CHO < 2-methylcyclohexanone

E) cyclohexanone < 2-methylcyclohexanone < CH3CHO < H2CO

Answer 1

Answer:-

Option A) 2-methylcyclohexanone < cyclohexanone < CH3CHO < H2CO is correct answer.

The reactivity of the carbonyl group depends upon the nature of groups attached to the carbonyl group. Any group that increases the positive charge over carbonyl carbon atom increases the reactivity of the carbonyl group.

Electron releasing group/groups (alkyl groups) when attached to the carbonyl carbon tend to neutralise the positive charge of carbonyl carbon and thereby decrease the reactivity of the carbonyl group.

Furthermore, greater the size of the alkyl substituents attached to the carbonyl carbon, the less is the reactivity of the carbonyl group because the larger groups around the carbonyl group hinder the attacking nucleophile from reaching the positively charged carbon (steric hindrance).