Number of molecules from below which cannot give iodoform reaction is :
Ethanol, Isopropyl alcohol, Bromoacetone, 2-Butanol, 2-Butanone, Butanal, 2-Pentanone, 3-Pentanone, Pentanal and 3-Pentanol.

Solution

Chemistry ma’am · 3 steps

Look for a CH₃ right next to the carbonyl carbon, or next to a CH–OH that iodine can oxidise into one.

1. The test. A compound gives iodoform (CHI₃) if it has a CH₃CO⁻ group. It also works if it has a CH₃CH(OH)⁻ group, because I₂/NaOH first oxidises that to CH₃CO⁻.

   Compounds	Key group	Iodoform?
   Ethanol, isopropyl alcohol, 2-butanol	CH₃CH(OH)⁻, oxidised to CH₃CO⁻	yes
   2-Butanone, 2-pentanone	CH₃CO⁻	yes
   Bromoacetone, CH₃COCH₂Br	CH₃CO⁻ on one side	yes
   Butanal, pentanal	-CH₂CHO, no CH₃ on the carbonyl carbon	no
   3-Pentanone, 3-pentanol	ethyl groups on both sides of C=O or CH-OH	no

2. Count the failures. Six compounds give the test. Butanal, pentanal, 3-pentanone and 3-pentanol do not.
3. So. 4 molecules cannot give the iodoform reaction, which is option (a).

The idea to keep. Iodoform needs a methyl group on the carbonyl carbon, or on a CH–OH carbon. Ethanol is the only primary alcohol and acetaldehyde the only aldehyde that pass.

Answer from the NTA final answer key. Solution by PrepWiser.