JEE Chemistry 2023 Question with Solution

Given: We need to count how many of the listed compounds give a positive iodoform reaction.

Find: The number of compounds showing a positive iodoform test.

The iodoform test is a qualitative test for the presence of methyl ketones with the group $\text{R-CO-CH}_3$ and certain secondary alcohols with the group $\text{R-CH(OH)-CH}_3$. A positive iodoform test gives yellow precipitate of $\text{CHI}_3$.

The necessary structural feature is the presence of a methyl ketone group $\text{CH}_3\text{C=O}$ or a methyl carbinol group $\text{CH}_3\text{CH(OH)-}$.

Now examine each compound:

• (a) 1-Phenylbutan-2-one: This is a methyl ketone, so it gives a positive iodoform test.
• (b) 2-Methylbutan-2-ol: This is a tertiary alcohol, so it does not give a positive iodoform test.
• (c) 3-Methylbutan-2-ol: This is a secondary alcohol containing the required $\text{CH}_3\text{CH(OH)-}$ group, so it gives a positive iodoform test.
• (d) 1-Phenylethanol: This is a secondary alcohol containing the required $\text{CH}_3\text{CH(OH)-}$ group, so it gives a positive iodoform test.
• (e) 3,3-dimethylbutan-2-one: This is a methyl ketone, so it gives a positive iodoform test.
• (f) 1-Phenylpropan-2-ol: This is a secondary alcohol containing the required $\text{CH}_3\text{CH(OH)-}$ group, so it gives a positive iodoform test.

Thus, the compounds giving positive iodoform reaction are (a), (c), (d), (e), and (f).

Therefore, the number of compounds giving a positive iodoform reaction is $5$.

Given: The listed compounds must be checked for the functional groups required in the iodoform test.

Find: Which structures contain either $\text{R-CO-CH}_3$ or oxidisable $\text{R-CH(OH)-CH}_3$.

Use the criterion:

Checking one by one:

1. 1-Phenylbutan-2-one contains a methyl ketone unit, so it is positive.
2. 2-Methylbutan-2-ol is tertiary, so it is negative.
3. 3-Methylbutan-2-ol is a secondary alcohol with a methyl group on the carbon bearing $\text{OH}$, so it is positive.
4. 1-Phenylethanol has the form $\text{Ph-CH(OH)-CH}_3$, so it is positive.
5. 3,3-dimethylbutan-2-one has the form $\text{CH}_3\text{-CO-C(CH}_3\text{)}_3$, which is a methyl ketone, so it is positive.
6. 1-Phenylpropan-2-ol has the form $\text{Ph-CH}_2\text{-CH(OH)-CH}_3$, so it is positive.

Hence total positive compounds:

Therefore, the answer is $5$.