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Chemistry Question 68 – JEE-MAIN 2025

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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.

Recall the structural requirements for a compound to give a positive iodoform test.

Step 1: Identify Iodoform Test Requirements✦ Active

The iodoform reaction is a characteristic test for compounds containing either a methyl ketone (CH3CO−) group or a methyl carbinol (CH3CH(OH)−) group. The methyl carbinol group is oxidized to a methyl ketone under the reaction conditions.

Step 2: Analyze Each Compound for Reactivity○ Expand

Let's examine each given molecule: 1. Ethanol (CH3CH2OH): Has CH3CH(OH)H (methyl carbinol). Gives test. 2. Isopropyl alcohol (CH3CH(OH)CH3): Has CH3CH(OH)CH3 (methyl carbinol). Gives test. 3. Bromoacetone (CH3COCH2Br): Has CH3CO− (methyl ketone). Gives test. 4. 2-Butanol (CH3CH(OH)CH2CH3): Has CH3CH(OH)CH2CH3 (methyl carbinol). Gives test. 5. 2-Butanone (CH3COCH2CH3): Has CH3CO− (methyl ketone). Gives test. 6. Butanal (CH3CH2CH2CHO): No CH3CO− or CH3CH(OH)−. Does not give test. 7. 2-Pentanone (CH3COCH2CH2CH3): Has CH3CO− (methyl ketone). Gives test. 8. 3-Pentanone (CH3CH2COCH2CH3): No CH3CO−. Does not give test. 9. Pentanal (CH3CH2CH2CH2CHO): No CH3CO− or CH3CH(OH)−. Does not give test. 10. 3-Pentanol (CH3CH2CH(OH)CH2CH3): No CH3CH(OH)−. Does not give test.

💡 Teacher's Secret Hint

Pay close attention to the position of the functional group and the presence of a methyl group adjacent to it.

Step 3: Count Non-Reacting Molecules○ Expand

The molecules that cannot give the iodoform reaction are Butanal, 3-Pentanone, Pentanal, and 3-Pentanol. There are 4 such molecules.

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