Question
Arrange the following in order of increasing reactivity towards nucleophilic addition: acetaldehyde, acetone, formaldehyde, benzaldehyde.
Solution — Step by Step
Nucleophilic addition reactivity depends on:
- Electrophilicity of the carbonyl carbon: more → more reactive.
- Steric hindrance: bulkier groups slow down nucleophile approach.
Both effects work together: more alkyl groups → more electron donation (less ) AND more steric bulk → less reactive.
- Formaldehyde (HCHO): no alkyl groups. Most electrophilic, least hindered. Most reactive.
- Acetaldehyde (CH₃CHO): one methyl group. Some donation, mild hindrance.
- Acetone ((CH₃)₂CO): two methyl groups. More donation, more hindrance.
- Benzaldehyde (C₆H₅CHO): phenyl group is a +M donor (resonance into the ring), strongly stabilising the carbonyl. Least reactive among aldehydes.
Wait — acetone vs benzaldehyde: benzaldehyde has only one phenyl (one resonance donor) while acetone has two methyls. Resonance donation typically beats two inductive donations, so benzaldehyde is less reactive than acetone? Actually for nucleophilic addition, acetone > benzaldehyde in reactivity is wrong — benzaldehyde is more reactive than acetone because aldehydes (one H) are generally more reactive than ketones (no H) due to less hindrance.
Corrected order:
Actually for nucleophilic addition, the trend is: aldehydes > ketones (due to one fewer alkyl group and less hindrance). Among aldehydes: HCHO > CH₃CHO > C₆H₅CHO.
So: Acetone < Benzaldehyde < Acetaldehyde < Formaldehyde is the standard NCERT answer.
Final answer: Acetone < Benzaldehyde < Acetaldehyde < Formaldehyde (in increasing reactivity)
Why This Works
Two methyl groups in acetone donate more electron density than one phenyl group + one H in benzaldehyde, even though phenyl donates by resonance. Plus acetone has two alkyl groups blocking nucleophile approach, while benzaldehyde has just the planar phenyl.
Result: ketones are less reactive than aldehydes in nucleophilic addition. Always.
Alternative Method
Compare the equilibrium constants for cyanohydrin formation (HCN addition) — directly proportional to reactivity. NCERT lists values that confirm this order.
Common Mistake
Students put benzaldehyde at the bottom (least reactive), thinking phenyl is a strong donor. But the +M effect of phenyl is moderate, and benzaldehyde still has only one alkyl group on the carbonyl. Acetone has two, so acetone is more deactivated.
The general rule: ketones < aldehydes in nucleophilic addition reactivity.
JEE/NEET ask this exact ordering question almost every year. Memorise: HCHO > CH₃CHO > PhCHO > (CH₃)₂CO for nucleophilic addition reactivity.