Aldehydes And Ketones - Iodoform Test

Question

Four organic liquids are tested with iodine and sodium hydroxide, and only some give a yellow precipitate, so which compound gives a positive iodoform test?

RankGuru · Accepted Answer

The iodoform test gives a yellow precipitate of triiodomethane only for compounds that contain a CH3CO group or can be oxidised by the reagent to such a group, namely methyl ketones and alcohols of the form CH3CH(OH)R. The alkaline iodine first oxidises a suitable secondary alcohol to a methyl ketone, then iodinates and cleaves it to give iodoform. Propan-2-ol is CH3CH(OH)CH3, which oxidises to propanone bearing the required acetyl methyl group, so it gives a positive test and is the answer. Propan-1-ol oxidises to propanal and then propanoic acid, neither of which has the CH3CO unit, so it is negative. Methanol cannot form a methyl ketone or acetaldehyde framework and tests negative. Diethyl ether is unreactive toward this reagent and gives no precipitate. This diagnostic reaction is described in NCERT and is a standard JEE practical-identification question. As a plausibility check, only structures carrying or yielding a CH3CO or CH3CH(OH) fragment respond, which singles out propan-2-ol.

Aldehydes And Ketones - Iodoform Test

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