Chemistry · Ch 12 — Aldehydes, Ketones and Carboxylic Acids
Physical state and boiling points of aldehydes and ketones
Physical state and boiling points of aldehydes and ketones
Formaldehyde is a GAS at ordinary room temperature, with an irritating odour. Acetaldehyde is an extremely volatile, colourless LIQUID. Higher aldehydes, further up the homologous series, generally have a distinctly pleasant odour instead. Acetone, the simplest ketone, is a liquid at room temperature with a pleasant odour of its own, though most of the higher ketones tend to have comparatively bland odours. Across both the aldehyde and the ketone homologous series, boiling points rise steadily as chain length increases -- Table 12.4 tabulates this rise carbon-by-carbon, from methanal (252 K) up through hexanal (392 K) on the aldehyde side, and from propanone (329 K) up through hexan-2-one (400 K) on the ke …
Table 12.4 lists, by carbon-atom count, the aldehyde/ketone name and boiling point at each: 1 carbon -- Methanal 252 K (no ketone possible with 1 carbon). 2 carbons -- Ethanal 294 K (no ketone possible with 2 carbons). 3 carbons -- Propanal 319 K / Propanone 329 K. 4 carbons -- Butanal 348 K / Butan-2-one 353 K. 5 carbons -- Pentanal 376 K / Pentan-2-one 375 K. 6 carbons -- Hexanal 392 K / Hexan-2-one 400 K. From 3 carbons onward, at matched chain length the ketone's boiling point is consistently at or slightly above its isomeric aldehyde's (the one exception being 5 carbons, where pentanal at 37 …