Chapter 8 · Chemistry
Aldehyde vs Ketone: Why Aldehydes React Faster
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Both have the same C=O, yet a nucleophile attacks an aldehyde faster. Two reasons: alkyl groups push electrons (+I) and lower the carbonyl carbon's δ+, and a ketone's two groups crowd the path to it. So HCHO > CH₃CHO > CH₃COCH₃.
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Nucleophilic Addition Reactions: One Carbonyl, Five PathwaysEvery nucleophilic addition reaction of aldehydes and ketones in one place: HCN, NaHSO₃, Grignard reagents, alcohols, and the addition–elimination of ammonia derivatives, with the product each one gives.Chemistry · simulation
Key points
- A nucleophile attacks the δ+ carbonyl carbon: the more positive and the more open it is, the faster the attack.
- Electronic: alkyl groups release electrons (+I) and lower the δ+; an aldehyde has one, a ketone two.
- Steric: an aldehyde's carbonyl carbon carries a small H; a ketone's carries two larger groups that block the approach.
- Order (nucleophilic addition): HCHO > CH₃CHO > CH₃COCH₃.
Two reasons at one carbon
| Reason | Aldehyde (CH₃CHO) | Ketone (CH₃COCH₃) |
|---|---|---|
| Alkyl groups on C=O | one CH₃ (+ one H) | two CH₃ |
| δ+ on the carbonyl carbon | higher | lower (more +I) |
| Path for the nucleophile | open (small H) | crowded (two CH₃) |
| Reactivity | faster | slower |
Order of reactivity
| Compound | Groups on C=O | Rank |
|---|---|---|
| methanal, HCHO | H, H | 1 (most reactive) |
| ethanal, CH₃CHO | CH₃, H | 2 |
| propanone, CH₃COCH₃ | CH₃, CH₃ | 3 |
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