The question is about distinguishing between formaldehyde and acetaldehyde using specific reagents. We are given several options to choose from, each of which is a reagent used to test for the presence of specific functional groups or types of aldehydes.
Let's analyze the reagents one by one:
Schiff's reagent: This reagent is used to test for the presence of aldehydes. Both formaldehyde and acetaldehyde react with Schiff’s reagent to give a pink color; therefore, it cannot distinguish between the two.
Tollen's reagent: Tollen's reagent is an ammoniacal silver nitrate solution. It is a mild oxidizing agent and reacts with aldehydes to give a silver mirror. Both formaldehyde and acetaldehyde can react with Tollen's reagent and hence cannot be used to distinguish them.
I_2/Alkali (Iodoform test): This is used to test for methyl ketones and compounds that can be converted to methyl ketones, such as acetaldehyde. Acetaldehyde gives a positive iodoform test because it has a CH_3C=O group which is a prerequisite for this reaction. Formaldehyde does not have such a group and does not respond to this test. Therefore, I_2/Alkali can distinguish between formaldehyde and acetaldehyde, giving a positive result only with acetaldehyde.
Fehling's solution: Fehling’s solution is another reagent used to detect aldehydes, specifically those which are aliphatic. Acetaldehyde will react with Fehling's solution to give a red precipitate, whereas formaldehyde also reacts, and thus it cannot be used effectively to distinguish the two.
Based on the above analysis, the reagent that can be used to distinguish formaldehyde from acetaldehyde is I_2/Alkali. This reagent utilizes the iodoform test, which is positive for acetaldehyde but not for formaldehyde.
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