Question

Give two tests for aldehydes.

Answer 1

(i) Aldehydes reduce ammonical slyer nitirate (Tollen's reagent) to metallic silver and Fehling's solution (copper sulphate, sodium potassium tartrate)to red cuprous oxide.
$\underset{\left(Acetaldehyde\right)}{C{H}_{3}CHO+2A{g}^{+}3O{H}^{-}}\to \underset{\left(Acetateion\right)}{C{H}_{3}CO{O}^{-}}+\underset{\left(silvermirror\right)}{2Ag+2H_{2}O}$
$C{H}_{3}CHO+2A{g}^{+}\underset{\left(Blue\right)}{2C{u}^{2+}+5O{H}^{-}}\to C{H}_{3}CO{O}^{-}+\underset{\left(Red\right)}{C{u}_{2}O+3H_{2}O}$
(ii) Alipha'tic aldehydes restore the original colour (red-pink) of the Schiffs reagent (When SO is passed through intensely pink coloured Schiff's reagent in water it fonts a colourless Solution. This colourless solution is used to for this test). This is characteristic test for aldehydes.