Question

What is a silver mirror test? Explain with reaction.

Answer 1

Hint: The silver mirror test is a qualitative test used to distinguish between carbonyl groups i.e. aldehydes and ketones. It is also called Tollens' test after the name of the scientist.

Complete answer:
It is a qualitative laboratory test which is used to distinguish between an aldehyde and a ketone. The reagent used for this test is Tollens’ reagent which has silver ions coordinated to ammonia ions. The solution is colourless initially.
Step 1:- The aqueous silver nitrate is taken in the test tube. Added aqueous sodium hydroxide solution to it. The reaction for this step is :

$AgN{O_3} + NaOH \to AgOH + NaH{O_3}$

$2AgOH \to A{g_2}O + {H_2}O$

Step 2 :- To this precipitate of $A{g_2}O$ (silver oxide), aqueous ammonia is added drop wise till we get these ppt completely dissolved.

$A{g_2}O + 4N{H_3} + {H_2}O \to 2Ag{(N{H_3})_2}^ + + 2O{H^ - }$

Step 3 :- These hydroxide ions produced in step 2 are used to oxidize the group. If the compound given is aldehyde; it readily gets oxidized while if the compound contains a ketonic group; then it is not hydrolyzed.

$C{H_3}CHO + 2A{g^ + } + 2O{H^ - } \to C{H_3}COOH + 2Ag + {H_2}O$

Further, the silver ions are reduced into metallic silver. This metallic silver forms a mirror in a clean test tube. Thus, the test is named the silver mirror test.

Note: If the ketonic group is present in the solution; then no silver mirror is formed. We get a clear solution.
This test is based on the principle that aldehydes are easily oxidized while ketones are not.
If there are alpha-hydroxy ketones in the test tube then also the test shows positive. This is because these can tautomerize into an aldehyde.