Chapter 8 · Chemistry
The Carbonyl Group: Structure and Polarity
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Why the carbon of C=O is every nucleophile's target: an sp², planar carbon, a π bond from sideways p–p overlap, and a polar C=O with δ⁺ carbon and δ⁻ oxygen, explained by resonance.
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Aldehyde or Ketone? Structure and IUPAC NamesMethanal and propanone both have C=O, yet one is an aldehyde and one a ketone. Where the C=O sits decides it, and the names follow: -al for aldehydes, -one for ketones, plus the common names.Chemistry
Key points
- The carbonyl carbon is sp²: three σ bonds in one plane, about 120° apart (trigonal planar).
- The π bond comes from sideways overlap of C and O p orbitals; the π cloud lies above and below the plane.
- O is more electronegative: O δ⁻ (nucleophilic), C δ⁺ (electrophilic); carbonyl compounds are more polar than ethers.
- Resonance C=O ⟷ C⁺–O⁻: the real molecule is a hybrid, so nucleophiles attack the carbon.
Structure
| Feature | Carbonyl group |
|---|---|
| Carbon hybridisation | sp² |
| σ bonds on carbon | three, in one plane |
| Bond angles | about 120° (trigonal planar) |
| π bond | sideways overlap of C and O p orbitals; cloud above and below the plane |
| Oxygen | two lone pairs |
Polarity and resonance
Oxygen pulls the bonding electrons, so carbon is δ⁺ (electrophilic, a Lewis acid) and oxygen is δ⁻ (nucleophilic, a Lewis base). Resonance between R₂C=O and R₂C⁺–O⁻ explains the large dipole moment; the real molecule is a hybrid, and a nucleophile attacks the carbon.
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