Question

1. Why do ketones not yield to a silver mirror?

2. Why is HNO3 (nitric acid)

suitable for removing the silver mirror? Write a chemical equation to support your answer.

Answer 1

1) Aldehyde having Hydrogen atom attached to the functional group carbon (C=O). So, it is oxidised even by mild oxidising reagent like Tollen's reagent (alkalined ammoniacal silver nitrate solution) which is silver mirror test for aldehyde. Ketones does not have such hydrogen atom. So, Ketones oxidised only by strong oxidising agent like Potassium permanganate solution.

Tollen's reagent has [Ag(NH3)2]+ complex ion it react with aldehyde as follows

2Ag(NH3)2+ + RCHO + 3OH- - - - - - - - -> 2Ag + RCOO- + 4NH3 + 2H2O

in this reaction Siver is reduced metallic silver which is called silver mirror.

The Hydrogen atom attached to functional group carbon is fecilitate the attachment of OH- group to the carbonyl group.

2) Siver is noble metal, present in lower side of activity series and it will not attacked by even by strong acids HCl, H2SO4 and HClO4. However, Siver is attacked by oxidative mechanism by HNO3,

3Ag(s) + 4HNO3 - - - - - > 3AgNO3(aq) + NO(g) + 2H2O(l)