Question

Tollen’s reagent form a silver mirror when reacted with:
A.Ketones
B.Alcohols
C.Formaldehyde
D.Carbonyl

Answer 1

Hint: The silver mirror is an identification test to distinguish the ketones from the aldehydes. In the reaction, the compound is oxidised to carboxylic acid and a layer of silver is formed in the test tube.

Complete step by step answer:
The Tollen’s reagent is a colourless, basic aqueous solution of ammoniacal silver nitrate. This test uses the oxidative ability of the substance to distinguish between aldehydes and ketones. The aldehydes that respond positively to the Tollen’s test show the “keto-enol” tautomerism and that is why when the aldehyde is in the “enol” form, it is easily oxidised to form carboxylic acid.
The silver ions from, silver nitrate, in the presence of the hydroxide ions form a brown precipitate of Ag(I) oxide,$\text{A}{\text{g}}_{\text{2}}\text{O}$. This precipitate dissolves in excess of ammonia, forming the diamminesilver (I) ion, ${\left[\text{Ag}{\left(\text{N}{\text{H}}_{\text{3}}\right)}_2\right]}^{+}$
Ketones on the other hand do not react with the Tollen’s reagent due to the inability to form “keto-enol” tautomers. Similarly, alcohols also do not react with the Tollen’s Reagent due to the inability to show “keto-enol” tautomerism. So the carbonyl group of the aldehyde is oxidised by the silver ions.
As per the following reaction:
$\text{RCHO + 2A}{\text{g}}^{\text{+}}\text{+2O}{\text{H}}^{\text{-}}\to \text{RCOOH + 2Ag + }{\text{H}}_{\text{2}}\text{O}$

Hence, the correct answer to this question is C.

Note:
Not all aldehydes respond to the Tollen’s test. Only those that can form “Keto-enol” tautomers show response to this test. Exceptions being formaldehyde and benzaldehyde.
In organic chemistry “Keto-enol” tautomerism refers to a chemical equilibrium between the “keto” form and the “enol” form of a compound. Where the “enol” form is the more preferred form in the equilibrium due to the stability of the compound provided by either hydrogen bonding, or resonance, or any other factor, as may be the case.