How do aldehydes react with Tollens' reagent?

Aldehydes react with Tollens' reagent to form a silver mirror and a carboxylic acid.

Tollens' reagent, also known as silver mirror test, is a chemical reagent used to determine the presence of aldehydes. The reagent is a solution of silver nitrate (AgNO3) and ammonia (NH3). When an aldehyde is added to this solution, it undergoes an oxidation reaction. This reaction is facilitated by the Tollens' reagent, which acts as an oxidising agent.

The aldehyde is oxidised to a carboxylic acid, while the silver ions in the Tollens' reagent are reduced to metallic silver. This reduction of silver ions to silver metal is what forms the characteristic 'silver mirror' on the inside of the test tube. This is a clear visual indication of the presence of an aldehyde. The reaction can be summarised as follows:

RCHO + 2[Ag(NH3)2]+ + 3OH- → RCOO- + 2Ag + 4NH3 + 2H2O

Where RCHO represents the aldehyde, and RCOO- represents the carboxylic acid.

It's important to note that this reaction is specific to aldehydes. Ketones, another type of carbonyl compound, do not react with Tollens' reagent. This is because ketones are not easily oxidised. Therefore, the Tollens' reagent test can also be used to distinguish between aldehydes and ketones.

In terms of practical application, the test is quite straightforward. A small amount of the aldehyde is added to the Tollens' reagent in a test tube. The test tube is then gently heated in a water bath. If an aldehyde is present, a silver mirror will form on the inside of the test tube within a few minutes.

In conclusion, the reaction of aldehydes with Tollens' reagent is a key concept in organic chemistry. It not only helps in the identification of aldehydes but also provides a clear visual demonstration of an oxidation-reduction reaction.