Question

Why is CH\(_3\)CHO more reactive than acetone towards reaction with HCN?

Hint:

Aldehydes are more reactive than ketones due to less steric hindrance and weaker +I effect.

Answer 1

Step 1: Structure of CH₃CHO (Acetaldehyde) and Acetone (CH₃COCH₃).
CH₃CHO (acetaldehyde) has the structure \( \text{CH}_3\text{CHO} \), where the carbonyl group (\( \text{C=O} \)) is attached to a methyl group (\( \text{CH}_3 \)) and a hydrogen atom. Acetone (CH₃COCH₃), on the other hand, has the structure \( \text{CH}_3\text{COCH}_3 \), where the carbonyl group is attached to two methyl groups.

Step 2: Electron Donating vs. Electron Withdrawing Effect.
The reactivity of carbonyl compounds towards nucleophiles like HCN (hydrogen cyanide) depends on the electron density on the carbonyl carbon. The more electrophilic (electron-deficient) the carbonyl carbon is, the more reactive it is towards nucleophilic attack.
- In acetaldehyde (CH₃CHO), the carbonyl carbon is attached to a hydrogen atom, which does not donate electron density to the carbonyl group. This makes the carbonyl carbon more electrophilic and susceptible to nucleophilic attack by HCN.
- In acetone (CH₃COCH₃), both carbonyl carbon atoms are attached to methyl groups (\( \text{CH}_3 \)), which are electron-donating groups via their inductive effect. This electron donation decreases the partial positive charge on the carbonyl carbon, making it less electrophilic and thus less reactive towards nucleophiles like HCN.

Step 3: Conclusion.
Acetaldehyde (CH₃CHO) is more reactive than acetone (CH₃COCH₃) towards reactions with HCN because the hydrogen atom in acetaldehyde does not donate electrons to the carbonyl group, making the carbonyl carbon more electrophilic and more prone to nucleophilic attack. In contrast, the methyl groups in acetone donate electrons, reducing the electrophilicity of the carbonyl carbon and thus its reactivity.