Which carbonyl compound passes which test?

• 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.

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