Which of the following compounds will give positive iodoform test?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hexan-2-one
The iodoform test is given by methyl ketones (the CH3CO– group): with sodium hypoiodite the methyl group ends up as a yellow precipitate of iodoform, CHI3:
\( \displaystyle \ce{I2 + 2NaOH -> NaOI + NaI + H2O} \)
\( \displaystyle \ce{R-CO-CH3 + 3NaOI -> R-COONa + CHI3 v + 2NaOH} \)
Hexan-2-one, \( \ce{CH3-CO-CH2CH2CH2CH3} \), is a methyl ketone, so it gives a positive iodoform test:
\( \displaystyle \ce{CH3CO(CH2)3CH3 + 3NaOI -> CH3(CH2)3COONa + CHI3 v + 2NaOH} \)
Shortcut: The carbonyl of an “alkan-2-one” is always next to a \( \ce{CH3} \) group, so any -2-one gives the test; only Option (2) is a -2-one.
Why the other options are incorrect
Option (1) Hexan-3-one — Incorrect. \( \ce{CH3CH2-CO-CH2CH2CH3} \) has ethyl and propyl groups on the carbonyl, no \( \ce{CH3CO} \)– group.
Option (3) Pentan-3-one — Incorrect. \( \ce{CH3CH2-CO-CH2CH3} \) has two ethyl groups on the carbonyl, no \( \ce{CH3CO} \)– group.
Option (4) Heptan-4-one — Incorrect. \( \ce{CH3CH2CH2-CO-CH2CH2CH3} \) has two propyl groups on the carbonyl, no \( \ce{CH3CO} \)– group.
NCERT (2026-27 edition):
Oxidation of Methyl Ketones by Haloform Reaction
