12.6  Test for Aldehydes

. Test for Aldehydes i) Tollens Reagent Test Tollens reagent is an ammonical silver nitrate solution. When an aldehyde is warmed with Tollens reagent a bright silver mirror is produced due to the formation of silver metal. This reaction is also called silver mirror test for aldehydes. CH CHO + [Ag(NH ) ] + + 3OH - CH COO - + 4NH + 2Ag + 2H O Silver ii) Fehling's solution Test Fehling's solution is prepared by mixing equal volumes of Fehling's solution ‘A’ containing aqueous copper sulphate and Fehling's solution ‘B’ containing alkaline solution of sodium potassium tartarate (Rochelle salt) When aldehyde is warmed with Fehling's solution deep blue colour solution is changed to red precipitate of cuprous oxide. Acetophenone Con H SO m - nitro acetophenone COCH NO Con HNO COCH XII U12-Carbonyl XII U12-Carbonyl - - - - CH CHO + 2Cu + + 5OH − CH COO + Cu O + 3H O (blue) (red) iii) Benedict's solution Test: Benedicts solution is a mixture of CuSO + sodium citrate + NaOH. Cu + is reduced by aldehyde to give red precipitate of cuprous oxide. CH CHO + 2Cu + + 5OH − CH COO + Cu O + 3H O (blue) (red) iv) Schiffs’ reagent Test Dilute solution of aldehydes when added to schiffs’ reagent (Rosaniline hydrochloride dissolved in water and its red colour decolourised by passing SO ) yields its red colour. This is known as Schiffs’ test for aldehydes . Ketones do not give this test. Acetone however gives a positive test but slowly.