12.6 Test for Aldehydes#
i) Tollen’s Reagent Test
Tollen’s reagent is an ammonical silver nitrate solution. When an aldehyde is warmed with Tollen’s reagent a bright silver mirror is produced due to the formation of silver metal. This reaction is also called silver mirror test for aldehydes.
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.
iii) Benedict’s solution Test:
Benedict’s solution is a mixture of \(CuSO_4\) + sodium citrate + NaOH. \(Cu^{2+}\) is reduced by aldehyde to give red precipitate of cuprous oxide.
iv) Schiff’s reagent Test
Dilute solution of aldehydes when added to Schiff’s reagent (Rosaniline hydrochloride dissolved in water and its red colour decolourised by passing \(SO_2\)) yields its red colour. This is known as Schiff’s test for aldehydes. Ketones do not give this test. Acetone however gives a positive test but slowly.