Physical Properties of Aldehydes and Ketones

Four physical trends recur across exam questions on this topic. (1) Physical state: formaldehyde is a gas at room temperature and acetaldehyde is a volatile liquid; every other aldehyde and ketone up to about C11 is a colourless liquid, while the higher members are solids. (2) Boiling points: aldehydes and ketones boil distinctly higher than hydrocarbons or ethers of comparable molecular mass, because of weak dipole-dipole association between neighbouring polar C=O groups -- but they boil LOWER than the corresponding alcohols and carboxylic acids, since those families additionally have genuine intermolecular hydrogen bonding (via O-H), a stronger interaction than the plain dipole-dipole attraction available to a carbonyl compound. (3) Solubility: the lower members -- formaldehyde, acetaldehyde, acetone -- are miscible with water in all proportions, since their own carbonyl oxygen can hydrogen-bond with water molecules; solubility then falls off rapidly as the alkyl chain lengthens and the molecule becomes progressively more hydrocarbon-like. (4) Dipole moment: because oxyge …