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.

CH3 CHO + 2 [Ag(NH3)2]+ + 3OH**-** CH3COO**-** + 4NH3 + 2Ag + 2H2O

Silver

ii) Fehlings solution Test Fehlings solution is prepared by mixing equal volumes of Fehlings solution ‘A’ containing

aqueous copper sulphate and Fehlings solution ‘B’ containing alkaline solution of sodium potassium tartarate (Rochelle salt)

When aldehyde is warmed with Fehlings solution deep blue colour solution is changed to red precipitate of cuprous oxide.

Acetophenone

Con H2SO4

m - nitro acetophenone

COCH3

NO2

Con HNO3

COCH3

XII U12-Carbonyl Compounds.indd 166 2/19/2020 5:15:10 PM

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CH3 CHO + 2Cu2+ + 5OH**-** CH3COO**-** + Cu2O + 3H2O

(blue) (red)

iii) Benedict’s solution Test: Benedicts solution is a mixture of CuSO4 + sodium citrate + NaOH. Cu2+ is reduced by

aldehyde to give red precipitate of cuprous oxide.

CH3 CHO + 2Cu2+ + 5OH**-** CH3COO**-** + Cu2O + 3H2O

(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 SO2) 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.