Question

Arrange the following in increasing order of their reactivity towards HCN:
Di-tert. butyl ketone, Acetaldehyde, Acetone

Hint:

Aldehydes are generally much more reactive than ketones towards nucleophilic addition reactions due to both steric and inductive effects.

Answer 1

Arrange the following in increasing order of their reactivity towards HCN:
Di-tert. butyl ketone, Acetaldehyde, Acetone
Solution:
The reactivity of carbonyl compounds (such as aldehydes and ketones) towards HCN depends on the stability of the carbonyl carbon and the electron-donating or electron-withdrawing effects of the substituents attached to it.
1. Aldehydes are generally more reactive towards HCN than ketones because the carbonyl carbon in aldehydes is less sterically hindered and more electrophilic.
2. Acetone (\(CH_3COCH_3\)) is a ketone where two methyl groups are attached to the carbonyl group. The methyl groups donate electron density through the inductive effect, making the carbonyl carbon less electrophilic and thus less reactive towards nucleophiles like HCN compared to an aldehyde.
3. Di-tert. butyl ketone is a bulky ketone with two bulky tert-butyl groups attached to the carbonyl carbon. These groups create significant steric hindrance around the carbonyl group, making the carbonyl carbon less accessible for nucleophilic attack, resulting in lower reactivity towards HCN.
Based on this reasoning, the increasing order of reactivity towards HCN is:
Di-tert. butyl ketone < Acetone < Acetaldehyde