Question

Which of the following does NOT give yellow solid on treatment with sodium hypoiodite?

• A. Ethanal
• B. Pentan-3-one
• C. Acetophenone
• D. Pentan-2-one

Answer 1

Answer: (B)

Pentan-3-one

Answer 2

The iodoform test is positive if the compound has a CH₃CO– group or can be oxidized to form one.

• Ethanol (acetaldehyde, CH₃CHO): Contains the CH₃CO unit; positive test.
• Pentan-2-one (CH₃COCH₂CH₂CH₃): Contains the CH₃CO group; positive test.
• Acetophenone (C₆H₅COCH₃): Contains the CH₃CO group; positive test.
• Pentan-3-one (CH₃CH₂COCH₂CH₃): Lacks the CH₃CO group (both sides of the carbonyl are CH₂CH₃); negative test.

Thus, Pentan-3-one does NOT give a yellow solid (iodoform).

Only compounds with a CH₃CO– group (or that can form one) give the yellow iodoform precipitate. Pentan-3-one lacks this CH₃ group adjacent to the carbonyl.