Question

Arrange the following compounds in increasing order of their reactivity towards HCN : \[ (CH_3)_3C-CO-CH_3,\qquad CH_3COCH_3,\qquad CH_3CHO \]

Hint:

Aldehydes are generally more reactive than ketones towards nucleophilic addition because they have less steric hindrance and a more electrophilic carbonyl carbon.

Answer 1

Concept: The addition of HCN to carbonyl compounds is a nucleophilic addition reaction. The reactivity depends mainly upon:

• Steric hindrance around the carbonyl carbon.

• Electron donating effect of alkyl groups.

• Electrophilicity of the carbonyl carbon.

Step 1: Compare aldehydes and ketones Aldehydes are generally more reactive than ketones because:

• They possess only one alkyl group.

• They have less steric hindrance.

• Their carbonyl carbon is more electrophilic.
Therefore: \[ CH_3CHO \] is more reactive than acetone.

Step 2: Compare the ketones \[ (CH_3)_3C-CO-CH_3 \] contains a bulky tert-butyl group. The large tert-butyl group causes severe steric hindrance and decreases the accessibility of the carbonyl carbon. Hence it is less reactive than acetone.

Step 3: Increasing order \[ \boxed{ (CH_3)_3C-CO-CH_3 \lt CH_3COCH_3 \lt CH_3CHO } \]

Final Answer \[ \boxed{ (CH_3)_3C-CO-CH_3 \lt CH_3COCH_3 \lt CH_3CHO } \]