What does Tollens' test do?

Question

Ava Andrews · Accepted Answer

Differentiates aldehydes from ketones; at the very least it indicates the presence of a #C(=O)H# group.

Ketones have a general formula of #RC(=O)R'#; while maintaining the carbon chain, the carbonyl group cannot be oxidized any further. On the other hand, an aldehyde #R(C=O)H# can be oxidized up to a carboxylic acid, #R(C=O)OH#. Since silver ion, #Ag^+#, is the oxidant, macroscopic evidence of such oxidation is given by the appearance of a beautiful silver mirror coating the interior of the test tube. In fact, the best mirrors (i.e. sheet glass that you paint your face in) are precisely made by this reaction: #RC(=O)H +H_2O rarr R(C=O)OH +2H^+ +2e^-# #"Oxidation (i)"# #Ag^+ + e^(-) rarr Ag(s)darr# #"Reduction (ii)"# #(i) + 2xx(ii):# #RC(=O)H +H_2O + 2Ag^(+) rarr R(C=O)OH +2H^+ + 2Ag^0darr# While acetone WILL not undergo this reaction, cyclohexanone WILL, and sometimes you get the reduction to silver metal without the silver mirror.

Isabella Acosta · Answer

undefined Tollens' test is used to distinguish between aldehydes and ketones. It involves the reaction of an aldehyde with Tollens' reagent, which is a solution of silver nitrate in aqueous ammonia. In the presence of an aldehyde, Tollens' reagent oxidizes the aldehyde to a carboxylic acid, while the silver ions in the reagent are reduced to metallic silver, forming a silver mirror on the inner surface of the reaction vessel. Ketones do not react with Tollens' reagent under normal conditions.