Physical properties of aldehydes and ketones

Methanal is a gas at room temperature and ethanal a volatile liquid. The other aldehydes and ketones are liquids or solids.

Dipole–dipole attraction between C=O groups gives weak association, so aldehydes and ketones boil above hydrocarbons and ethers of similar mass. With no O–H they cannot hydrogen-bond to each other, so they boil below alcohols of similar mass.

Boiling points for masses 58–60: n-butane 273 K (58), methoxyethane 281 K (60), propanal 322 K (58), acetone 329 K (58), propan-1-ol 370 K (60).

Worked order for masses 72–74, lowest first: n-butane < ethoxyethane < butanal < butan-1-ol. Butanal is more polar than the ether; only the alcohol is hydrogen-bonded.

Methanal, ethanal and propanone mix with water in all proportions because their oxygen accepts hydrogen bonds from water. Solubility falls quickly as the alkyl chain grows; all of them dissolve well in benzene, ether, methanol and chloroform.

The lower aldehydes smell sharp and pungent. Larger molecules smell less pungent and more fragrant, and many natural aldehydes and ketones are blended into perfumes and flavours.