How can aldehydes and ketones be distinguished from one another?

Question

Ellie Adams · Accepted Answer

The Tollens test is classically the usual means to distinguish between aldehyde and ketone.

The Tollens test utilizes the redox chemisty of #Ag^+#. An aldehyde can be oxidized to a carboxylic acid, and such oxidation must be accompanied by a corresponding reduction of #Ag^+# to #Ag^0#, which usually manifests as a beautiful silver mirror lining your test tube (I save the good ones, they are quite pretty - in fact, high quality mirrors are made by this reaction). A ketone cannot normally be oxidized by these means, and thus would fail the test. The chemistry of the reaction is: #2Ag(NH_3)_2^(+) + RC(=O)H + H_2O rarr 2Ag(s)darr + 4NH_3 + RC(=O)OH + 2H^+# (I think this is balanced!) As a result, the aldehyde is oxidized to carboxylic acid and silver nitrate is reduced to atomic silver. So the Tollens reagent is silver nitrate in ammoniacal solution (which forms the #Ag(NH_3)_2^+# ion). A note of caution (unnecessary!): cyclohexanone, a ketone, does, in fact, produce a beautiful silver mirror when combined with the Tollens reagent; therefore, the Tollens reagent is not discriminating in this specific instance.

Isabella Acosta · Answer

undefined Aldehydes and ketones can be distinguished from one another using chemical tests. One common test is the Tollens' test, which detects aldehydes but not ketones. In this test, an aldehyde reacts with Tollens' reagent (ammoniacal silver nitrate), forming a silver mirror on the inner surface of the test tube. Ketones do not react in this manner. Another test is the Fehling's test, which also distinguishes aldehydes from ketones based on their reactivity with copper(II) ions. Aldehydes reduce Cu(II) to Cu(I), resulting in the formation of a reddish-brown precipitate of Cu2O, while ketones do not undergo this reaction.