Physical properties and nucleophilic addition

Boiling points. Aldehydes and ketones have polar C=O groups, so their molecules attract each other by dipole-dipole forces. They boil higher than alkanes and ethers of similar mass, but lower than alcohols, because they cannot hydrogen-bond to each other. Small ones (methanal, ethanal, propanone) mix with water, since water can hydrogen-bond to their oxygen.

Nucleophilic addition is their typical reaction. A nucleophile (an electron-rich species) attacks the positive carbonyl carbon; the C=O opens to C–O⁻, which then picks up an H⁺.

How fast? The rule: the fewer electrons on the carbonyl carbon, and the less crowded it is, the faster the attack.

• Aldehydes react faster than ketones. A ketone has two alkyl groups, which push electrons towards the carbon (making it less positive) and block the nucleophile's path.
• Aromatic aldehydes react more slowly than aliphatic ones: resonance with the ring makes the carbonyl carbon less positive.

The main additions:

• HCN gives a cyanohydrin. HCN alone is slow; a little base pulls off the H⁺ and frees the CN⁻ nucleophile.
• Sodium hydrogensulphite (NaHSO₃) gives a crystalline, water-soluble addition product. Acid or alkali turns it back into the carbonyl compound, so this is used to separate and purify aldehydes.
• Alcohols, with dry HCl, add once to an aldehyde to give a hemiacetal, and a second time (losing water) to give an acetal. Ketones with ethylene glycol give a cyclic ketal, which protects the C=O during other reactions.
• Grignard reagents add to give alcohols (see chapter 7).

Ammonia derivatives (Z–NH₂) add and then lose water, giving a C=N product:

• Hydroxylamine (NH₂OH) gives an oxime.
• Hydrazine (NH₂NH₂) gives a hydrazone; 2,4-dinitrophenylhydrazine (2,4-DNP) gives a coloured 2,4-DNP derivative, a common test for C=O.
• Semicarbazide gives a semicarbazone: only the NH₂ that is not next to the C=O reacts, because the other one's lone pair is tied up by resonance.
• A primary amine gives an imine (Schiff's base).