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.

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

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.

CH₃ CHO + 2Cu²⁺ (blue) + 5OH^-  → CH₃COO^- + Cu₂O↓ (red)+ 3H₂O

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⁺ (blue) + 5OH^-  → CH₃COO^- + Cu₂O↓ (red)+ 3H₂O

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.