Question

Propanal is more reactive than propanone towards nucleophilic addition reaction. Give reason.

Hint:

Propanal has one alkyl group and one H on C=O; propanone has two alkyl groups. Less +I effect and less steric hindrance make propanal more reactive.

Answer 1

Step 1: Understanding the Concept:
In a nucleophilic addition reaction the nucleophile attacks the carbonyl carbon, which carries a partial positive charge. The reaction is faster when this carbon is more positive and when it is less crowded.

Step 2: Compare the two compounds.
Propanal, \( \mathrm{CH_3CH_2CHO} \), has one ethyl group and one hydrogen on the carbonyl carbon. Propanone, \( \mathrm{CH_3COCH_3} \), has two methyl groups on the carbonyl carbon.

Step 3: Electronic effect.
An alkyl group pushes electrons towards the carbonyl carbon (+I effect) and lowers its positive charge. Propanone has two such groups and propanal has only one. So the carbonyl carbon of propanal is more electrophilic.

Step 4: Steric effect.
The small hydrogen atom in propanal leaves the carbonyl carbon open to attack. The two methyl groups in propanone crowd the carbon, and the transition state of the addition is also more crowded.

Final Answer:
Propanal has only one electron-releasing alkyl group and a small hydrogen atom, so its carbonyl carbon is more positive and less hindered than that of propanone. So propanal is more reactive.