Describe the structure of the carbonyl group. Compare the boiling points and water solubility of aldehydes and ketones with those of hydrocarbons, ethers and alcohols.

1. 1 · Describe the carbonyl carbon. It is sp² hybridised: three σ bonds in one plane at about 120°, and a π bond with oxygen.
2. 2 · Explain the polarity. O is more electronegative, so C=O is polar: C is δ+ (electrophilic), O is δ− (nucleophilic). Resonance: >C=O ↔ >C⁺-O⁻.
3. 3 · Draw the group. Draw C with two groups and a double bond to O, all in one plane. Mark δ+ on C, δ− on O and 120° between the bonds.
4. 4 · Compare boiling points. Higher than hydrocarbons and ethers (dipole-dipole attraction); lower than alcohols (no intermolecular hydrogen bonding). Example: propanal 322 K, propan-1-ol 370 K.
5. 5 · Explain solubility. Lower members form hydrogen bonds with water and dissolve; solubility falls as the alkyl chain grows.
6. 6 · Link to real life. Acetone mixes with water and also dissolves nail polish, so it is a useful solvent.
7. 7 · Close in one line. So the polar C=O decides both the reactions and the physical properties of these compounds.

Scoring tip: Make both comparisons with a reason each: higher than ethers because of dipole-dipole attraction, lower than alcohols because there is no hydrogen bonding.