17.1.6—Iodoform test for CH3CO- group

An aldehyde or ketone containing CH₃CO– gives a yellow CHI₃ precipitate with alkaline iodine. The methyl group is oxidised and the carbonyl fragment becomes a carboxylate ion.

The test is positive for ethanal and methyl ketones, not for every aldehyde or ketone. Use the structural motif rather than memorising a list of names.

Propanone, CH₃COCH₃, gives the yellow precipitate and propanoate ion. Propanal, CH₃CH₂CHO, has no CH₃CO– group and is negative.

This is a different use of the iodoform test from identifying CH₃CH(OH)– alcohols: oxidation can make both routes converge on the same motif.