Iodoform reaction with sodium hypoiodite is used for detection of .
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH3CO and CH3CH(OH) groups
Aldehydes and ketones with a CH3CO– group (methyl ketones and ethanal) react with sodium hypohalite (NaOX, made from \( \ce{X2} \) and NaOH). The three hydrogens of the methyl group are replaced by halogen, and the trihalomethyl group then breaks off as haloform, leaving a carboxylate salt with one carbon fewer:
\( \displaystyle \ce{R-CO-CH3 + 3NaOI -> R-CO-CI3 + 3NaOH} \)
\( \displaystyle \ce{R-CO-CI3 + NaOH -> R-COONa + CHI3 v} \)
With iodine, the haloform is iodoform (CHI3), a yellow precipitate with a characteristic smell. This is the iodoform test.
Alcohols with a CH3CH(OH)– group also give the test, because hypoiodite first oxidises them to compounds containing \( \ce{CH3CO} \)–:
\( \displaystyle \ce{CH3-CH(OH)-R + NaOI -> CH3-CO-R + NaI + H2O} \)
Why the other options are incorrect
Option (1) CH3CO group — Incorrect. This is true but incomplete; compounds with a \( \ce{CH3CH(OH)} \)– group, such as ethanol and propan-2-ol, also give the test.
Option (3) CH3O group — Incorrect. In a methoxy group the \( \ce{CH3} \) is attached to oxygen, not to a carbonyl carbon, so its hydrogens are not replaced by iodine.
Option (4) (CH3)3CO group — Incorrect. Like \( \ce{CH3O} \)– in Option (3), this is an alkoxy (tert-butoxy) group: it has no C=O, and the carbon bearing the oxygen carries no hydrogen, so it is neither a \( \ce{CH3CO} \)– group nor oxidised to one.
NCERT (2026-27 edition):
Oxidation of Methyl Ketones by Haloform Reaction – “Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform. This oxidation does not affect a carbon-carbon double bond, if present in the molecule. Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.”