Question

Benzaldehyde (aromatic aldehydes) although reduces Tollen's reagent, it does not reduce Fehling and Benedict solutions.

Answer 1

On the other hand, ketones¹ are not oxidised by milder oxidising agents and thus they do not reduce Tollen's reagent, Fehling and Benedict solutions (difference from aldehydes). However, stronger oxidising agents like acid dichromate², alk. KMnO₄ and hot conc. HNO₃ oxidise ketones to carboxylic acids with lesser number of carbon atoms.

\(CH_3.CO.CH_3 \xrightarrow [H_2SO_4]{K_2Cr_2O_7}\)

\(CH_3COOH +HCOOH\)

In case, ketone is unsymmetrical, cleavage takes place in such a way that carbonyl group is retained by smaller alkyl group (Popoff's rule). 

For example,

Aldehydes or ketones containing a methyl or methylene group adjacent to the carboxyl roup are oxidised by SeO₂ to give dicarbonyl compounds.

For example,

\(CH_3CHO \xrightarrow {SeO_2} \underset {Glyoxal}{CHO}.CHO+Se +H_2O\)