Animated Solution for Chemistry - Organic Chemistry: Positive Tollen's test is observed for

Beyond Just Aldehydes

When we first learn about Tollens' reagent, it is almost always introduced as the classic test for aldehydes. The beautiful silver mirror that forms on the inside of the test tube is a hallmark of organic chemistry labs. But in competitive exams like JEE, knowing just the basics is never enough. You have to know the exceptions, the edge cases, and the hidden tricks.

Tollens' reagent is an ammoniacal silver nitrate solution, containing the diamminesilver(I) complex, $[Ag(N{H}_3{)}_2{]}^{+}$. It acts as a mild oxidizing agent. Because aldehydes have a hydrogen atom attached directly to the carbonyl carbon, they are easily oxidized to carboxylic acids (or carboxylate ions in the basic medium). In the process, the silver ions are reduced to elemental silver ($Ag$), which precipitates out and coats the glass, creating the famous silver mirror.

Here is where the trick lies. While regular ketones do not react with Tollens' reagent because they lack that easily oxidizable hydrogen, $\alpha$-hydroxy ketones are a major exception.

Why does this happen? The Tollens' test is conducted in a basic medium. Under these basic conditions, an $\alpha$-hydroxy ketone can undergo tautomerization. It first forms an enediol intermediate, which can then rearrange itself into an aldehyde. Once the aldehyde is formed in the solution, Tollens' reagent immediately oxidizes it, resulting in a positive test! This is a favorite concept for examiners to test your depth of understanding.

Let's break down the molecules given in the problem one by one:

Molecule (A): Acraldehyde ($H_{2}C=CH-CHO$) This molecule contains a clear aldehyde group ($-CHO$). The presence of the double bond does not interfere with the oxidation of the aldehyde. Therefore, it will definitely give a positive Tollens' test.

Molecule (B): Benzaldehyde ($Ph-CHO$) Here, the aldehyde group is directly attached to a benzene ring. While aromatic aldehydes are less reactive and fail Fehling's test, Tollens' reagent is a slightly stronger oxidizing agent and successfully oxidizes them. Thus, benzaldehyde gives a positive test.

Molecule (C): Benzoin ($Ph-CH(OH)-CO-Ph$) Look closely at the functional groups here. We have a ketone, but right next to it, on the alpha carbon, there is a hydroxyl ($-OH$) group. This makes it an $\alpha$-hydroxy ketone. As we discussed, this will tautomerize in the basic medium and give a positive Tollens' test.

Molecule (D): Chalcone ($Ph-CH=CH-CO-Ph$) This molecule is an $\alpha ,\beta$-unsaturated ketone. It has a ketone group, but there is no aldehyde, and crucially, there is no hydroxyl group on the alpha carbon. Because it lacks these oxidizable features, Tollens' reagent will not react with it, resulting in a negative test.

By carefully analyzing the functional groups and remembering the special case of $\alpha$-hydroxy ketones, we can confidently conclude that Acraldehyde, Benzaldehyde, and Benzoin will all give a positive Tollens' test. Always keep an eye out for those sneaky $\alpha$-hydroxy groups!