The iodoform reaction (triiodomethane reaction) is a test for the methyl carbonyl group, CH3C=O, in which the carbonyl carbon is bonded to a methyl group. Iodine is added to the compound in the presence of alkali (sodium hydroxide solution) and the mixture is warmed gently.A positive result is a pale yellow precipitate of triiodomethane (iodoform), CHI3, which has a characteristic antiseptic smell.For a methyl ketone, RCOCH3:RCOCH3 + 3I2 + 4NaOH → CHI3 + RCOONa + 3NaI + 3H2O The three hydrogen atoms of the methyl group are replaced by iodine atoms. The CI3 group is then split off as CHI3, leaving the sodium salt of a carboxylic acid with one carbon atom fewer.Compounds that give a positive test:
• all methyl ketones, e.g. propanone and butanone
• ethanal, the only aldehyde with a CH3CO group
• alcohols containing the CH3CH(OH) group, e.g. propan-2-ol and ethanol, because alkaline iodine first oxidises them to a methyl carbonyl compoundWorked example: which of pentan-2-one and pentan-3-one gives a yellow precipitate? Pentan-2-one is CH3COCH2CH2CH3, a methyl ketone, so it is positive. Pentan-3-one is CH3CH2COCH2CH3; its carbonyl carbon carries two ethyl groups and no methyl group, so it is negative.
• all methyl ketones, e.g. propanone and butanone
• ethanal, the only aldehyde with a CH3CO group
• alcohols containing the CH3CH(OH) group, e.g. propan-2-ol and ethanol, because alkaline iodine first oxidises them to a methyl carbonyl compoundWorked example: which of pentan-2-one and pentan-3-one gives a yellow precipitate? Pentan-2-one is CH3COCH2CH2CH3, a methyl ketone, so it is positive. Pentan-3-one is CH3CH2COCH2CH3; its carbonyl carbon carries two ethyl groups and no methyl group, so it is negative.