Iodoform test distinguishes alcohols with CH3CH(OH)- structure

Steps:

• Ethanol (CH3CH2OH) and propanol (CH3CH2CH2OH) are both primary alcohols but differ in carbon chain.
• Iodoform test (I2/NaOH) requires CH3CH(OH)R or CH3COR for yellow precipitate formation.
• Ethanol fits CH3CH(OH)- and yields iodoform; propanol (CH3CH2CH2OH) does not.
• Other tests react similarly for both or not at all.

Why B is correct:

• I2/NaOH performs iodoform reaction; ethanol oxidizes to acetaldehyde (CH3CHO), forming CHI3 precipitate, while propanol forms propanal (CH3CH2CHO) without methyl carbonyl group.

Why the others are wrong:

• A: 2,4-Dinitrophenylhydrazine tests for carbonyls; neither alcohol has one.
• C: K2Cr2O7/H2SO4 oxidizes both primary alcohols to carboxylic acids with color change.
• D: Tollen's reagent tests for aldehydes; neither is an aldehyde.

Final answer: B