Which of the following will not give iodoform reaction?

The correct answer is Option (4) → Propionaldehyde

The iodoform test (iodine and sodium hydroxide, that is, sodium hypoiodite) is given by compounds with a \( \ce{CH3CO-} \) group, a methyl group on the carbonyl carbon. Alcohols with a \( \ce{CH3CH(OH)-} \) group also respond, because the hypoiodite first oxidises them to such a carbonyl compound. The three hydrogens of the methyl group are replaced by iodine and the yellow solid iodoform, \( \ce{CHI3} \), separates:

    \( \displaystyle \ce{RCOCH3 + 3I2 + 4NaOH -> CHI3 v + RCOONa + 3NaI + 3H2O} \)

Propionaldehyde (propanal) is \( \ce{CH3CH2CHO} \). Its carbonyl carbon carries an ethyl group and a hydrogen atom, not a methyl group, so it has no \( \ce{CH3CO-} \) unit.

• Option (1) Acetaldehyde — Gives the iodoform test. \( \ce{CH3CHO} \) is the only aldehyde with a \( \ce{CH3CO-} \) group:

      \( \displaystyle \ce{CH3CHO + 3I2 + 4NaOH -> CHI3 v + HCOONa + 3NaI + 3H2O} \)

• Option (2) Isopropyl alcohol — Gives the iodoform test. \( \ce{CH3CH(OH)CH3} \) is first oxidised by the hypoiodite to acetone, which then gives iodoform:

      \( \displaystyle \ce{CH3CH(OH)CH3 + NaOI -> CH3COCH3 + NaI + H2O} \)

• Option (3) Butanone — Gives the iodoform test. \( \ce{CH3COCH2CH3} \) is a methyl ketone:

      \( \displaystyle \ce{CH3COCH2CH3 + 3I2 + 4NaOH -> CHI3 v + CH3CH2COONa + 3NaI + 3H2O} \)

• Option (4) Propionaldehyde — Does not give the iodoform test. Propanal has an ethyl group, not a methyl group, on its carbonyl carbon.

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.”