Question

What is a Tollens' test?

Answer 1

This is the classic test to detect the presence of aldehydes, and (sometimes) to differentiate ketones from aldehydes.

Explanation:

An aldehyde is easily oxidizable to the carboxylic acid. When an aldehyde is treated with ammoniacal silver nitrate the following redox reaction occurs:

`2[Ag(NH_3)_2]^(+)(aq) + RC(=O)H(aq) + H_2O(l) → 2 Ag(s)darr + 2NH_4^+(aq) + RCO_2H(aq) + 2NH_3(aq)`

In this reaction, often the inside of the test-tube is coated with a beautiful silver mirror. In fact, the best mirrors (i.e. the ones you paint your face in or shave) are made precisely by this reaction.