Oxidation and the tests

Aldehydes are oxidised easily to carboxylic acids by nitric acid, potassium permanganate, potassium dichromate and similar reagents, and even by mild oxidising agents such as Tollens' and Fehling's reagents.

Ketones are oxidised only under vigorous conditions (strong oxidants, high temperature). C–C bonds break, giving a mixture of carboxylic acids with fewer carbons than the ketone.

Tollens' test: an aldehyde warmed with freshly prepared ammoniacal silver nitrate gives a bright silver mirror. The aldehyde becomes the carboxylate anion; the medium is alkaline. RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃.

Fehling's test: Fehling solution A (aqueous copper sulphate) and solution B (alkaline sodium potassium tartrate, Rochelle salt) are mixed in equal amounts. Heating with an aldehyde gives a reddish-brown precipitate of Cu₂O. Aromatic aldehydes do not give this test.

Haloform reaction: aldehydes and ketones with at least one CH₃ on the carbonyl carbon (methyl ketones) are oxidised by sodium hypohalite to the sodium salt of an acid with one carbon fewer, and the CH₃ becomes haloform. A C=C in the molecule is not attacked.

The iodoform test (sodium hypoiodite, yellow CHI₃) detects a CH₃CO group, or a CH₃CH(OH) group that is oxidised to CH₃CO under the test conditions.

Worked example: C₈H₈O gives an orange-red 2,4-DNP precipitate and a positive iodoform test, reduces neither Tollens' nor Fehling's reagent, does not decolourise bromine water or Baeyer's reagent, and on drastic oxidation gives C₇H₆O₂. It is acetophenone; the acid is benzoic acid.