Imine -R Amine —OH Hydroxylamine Oxime —NH Hydrazine Hydrazone Phenylhydrazine Phenylhydrazone Z Reagent name Carbonyl derivative Product name Substituted imine (Schiff’s base) * , -DNP-derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones. , -Dinitrophenyl- , Dinitrophenyl- Semicarbazide Semicarbazone . Reduction (i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH ) or lithium aluminium hydride (LiAlH ) as well as by catalytic hydrogenation (Unit , Class XII). ( ii ) Reduction to hydrocarbons: The carbonyl group of aldehydes and ketones is reduced to CH group on treatment with zinc- amalgam and concentrated hydrochloric acid [ Clemmensen hydrazone hydrazine reduction ] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol ( Wolff-Kishner reduction ).
. Oxidation Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.
Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic acids having lesser number of carbon atoms than the parent ketone. The mild oxidising agents given below are used to distinguish aldehydes from ketones: (i) Tollens’ test: On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion.
The reaction occurs in alkaline medium. (ii) Fehling’s test: Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test.
On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this