Chapter 8 · Chemistry
Nucleophilic Addition to the Carbonyl Group
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Most reactions of aldehydes and ketones begin with one move. How a nucleophile attacks the polar C=O, and why aldehydes react faster than ketones.
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Aldehyde vs Ketone: Why Aldehydes React FasterBoth 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.Chemistry
Key points
- The C=O bond is polar: carbon is delta plus, oxygen delta minus.
- A nucleophile attacks the electrophilic carbon; the pi electrons move onto oxygen.
- The carbon changes from sp² (flat) to sp³ (tetrahedral), giving an alkoxide intermediate.
- The intermediate takes a proton to give the product: HCN gives a cyanohydrin.
- Aldehydes react faster than ketones: less crowding, and a more positive carbon.
Why aldehydes react faster
| Reason | Aldehyde | Ketone |
|---|---|---|
| Steric | One alkyl group and a small H: easy to reach | Two alkyl groups: crowded |
| Electronic | One electron-releasing group: carbon more positive | Two electron-releasing groups: carbon less positive |
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