Reduction of Aldehydes and Ketones

Aldehydes are readily oxidised even by comparatively mild oxidising agents, all the way to carboxylic acids, because the hydrogen sitting directly on the carbonyl carbon of an aldehyde can be abstracted as part of the oxidation. A ketone's carbonyl carbon, by contrast, has no such hydrogen -- both its remaining bonds go to carbon -- so oxidising a ketone would require breaking a $\text{C-C}$ bond, which mild oxidants cannot do. This single structural difference is exploited by two classic named tests that distinguish an aldehyde from a ketone by simple observation.

Tollens' test. Tollens' reagent is freshly prepared by adding aqueous ammonia to silver nitrate until the initially formed brown precipitate of $\text{AgOH}$ (or $\text{Ag}_2\text{O}$) just redissolves, giving the soluble diammine-silver(I) complex ion, $\left[\text{Ag}(\text{NH}_3)_2\right]^+$. When an aldehyde is warmed gently with this reagent, the aldehyde is oxidised to the corresponding carboxylate (as its ammonium salt, since the medium is basic), while the silver(I) in the complex is reduced all the way to metallic silver, $\text{Ag}(0)$, which deposits as a bright, adherent film on the clean inner surface of the test tube -- the familiar silver mirror. A ketone, lacking the required $\alpha$-hydrogen on its carbonyl carbon, gives no reaction and no mirror.

Fehling's test. Fehling's reagent is made freshly by mixing Fehling's solution A (aqueous copper(II) sulphate, giving the deep blue $\text{Cu}^{2+}$ colour) with Fehling's solution B (an alkaline solution of sodium potassium tartrate, which complexes the $\text{Cu}^{2+}$ and keeps it in solution at high $\text{pH}$). When an aliphatic aldehyde such as acetaldehyde is heated with this mixture, the aldehyde reduces the complexed $\text{Cu}^{2+}$ to $\text{Cu}^{+}$, which precipitates as a distinctive brick-red precipitate of cuprous oxide, $\text{Cu}_2\text{O}$, while the aldehyde is itself oxidised to the carboxylate. A ketone again gives no reaction. …