Which test allows us to distinguish between benzophenone and acetophenone? 

Iodoform test

The correct answer is option 2. Iodoform test.

Here is a detailed explanation of why the Iodoform test is the most suitable for distinguishing between benzophenone and acetophenone:

Iodoform Test

Principle: The iodoform test identifies compounds that have a methyl ketone group \((CH_3-CO^-)\) or compounds that can be oxidized to form a methyl ketone. The test involves reacting the compound with iodine \((I_2)\) and a base (like sodium hydroxide). A positive result is indicated by the formation of a yellow precipitate of iodoform \((CHI_3)\).

Reaction Mechanism: 

For a methyl ketone, the reaction involves the formation of iodoform \((CHI_3)\) as follows:

\(\text{R-CO-CH}_3 + 3\text{I}_2 + 4\text{NaOH} \rightarrow \text{R-COONa} + 3\text{CHI}_3 + 3\text{NaI} + 3\text{H}_2\text{O}\)

The yellow precipitate of iodoform \((CHI_3)\) is observed in a positive test.

Differentiation between Benzophenone and Acetophenone

Acetophenone:

Structure:

Acetophenone contains a methyl group attached to the carbonyl group, which is a methyl ketone. This structure allows acetophenone to undergo the iodoform reaction, giving a positive result with the formation of yellow iodoform precipitate.

Benzophenone:

Structure:

Benzophenone has two phenyl groups attached to the carbonyl group. It does not contain a methyl group attached directly to the carbonyl group, so it does not form iodoform. Consequently, it gives a negative result in the iodoform test.

Conclusion: The iodoform test is used to detect the presence of a methyl ketone group, which is present in acetophenone but absent in benzophenone. This ability to form a yellow precipitate of iodoform in the presence of a methyl ketone group allows this test to differentiate between acetophenone and benzophenone effectively.

Thus, the correct test to distinguish between these two compounds is: Iodoform test.