JEE Main 2024 · 6 April · morning · Single correct · +4 / −1
Which among the following aldehydes is most reactive towards nucleophilic addition reactions?
- (a) C₂H₅-C(∥(O))-H
- (b) O; ∥; H -, C, - H
- (c) C₃H₇-C(∥(O))-H
- (d) CH₃-C(∥(O))-H
Answer and solution
Answer (b) O; ∥; H -, C, - H
Solution
Chemistry ma’am · 3 steps
Fewer and smaller groups on the carbonyl carbon mean more positive charge and more room. HCHO has none at all.
- What the nucleophile needs. A nucleophile attacks the carbonyl carbon. The attack is fastest when that carbon carries the most partial positive charge and is the least crowded.
- Effect of alkyl groups. An alkyl group pushes electrons towards the carbonyl carbon (+I effect), so the carbon becomes less positive. A bigger alkyl group also blocks the path of the nucleophile more (steric hindrance).
Aldehyde Group on carbonyl carbon +I push and crowding HCHO H only none CH₃CHO CH₃ small C₂H₅CHO C₂H₅ more C₃H₇CHO C₃H₇ most - So. Reactivity order is HCHO > CH₃CHO > C₂H₅CHO > C₃H₇CHO. Formaldehyde, option (b), is the most reactive.
The idea to keep. For nucleophilic addition, every alkyl group on the carbonyl carbon lowers reactivity by +I push and by steric crowding. So HCHO comes first, then other aldehydes, then ketones.
Answer from the NTA final answer key. Solution by PrepWiser.