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

Step 1: Understanding the Concept:
The nucleophilic addition of \(HCN\) to carbonyl compounds depends on two factors: 
1. Steric hindrance: Bulkier groups around the carbonyl carbon slow down the attack of the nucleophile. 
2. Electronic factors: Electron-releasing alkyl groups reduce the electrophilicity (positive charge) of the carbonyl carbon. 
Step 2: Detailed Explanation: 
- Acetaldehyde (\(CH_3CHO\)): It is an aldehyde with only one methyl group and one hydrogen. It has the least steric hindrance and highest electrophilicity. 
- Acetone (\(CH_3COCH_3\)): It is a ketone with two methyl groups. It has more steric hindrance and lower electrophilicity than acetaldehyde due to two \(+I\) groups. 
- Di-tert. butyl ketone (\(((CH_3)_3C)_2CO\)): It has two extremely bulky tert-butyl groups. Steric hindrance is maximum here, making it the least reactive. 
Step 3: Final Answer: 
Order: Di-tert. butyl ketone \(<\) Acetone \(<\) Acetaldehyde.