Question

Which of the following does not give a silver mirror test with Tollen's reagent?
A) Sucrose
B) Glucose
C) Fructose
D) Lactose

Answer 1

The Tollen’s test is a very useful way to distinguish between aldehydes and ketones. This qualitative laboratory test is also called the silver mirror test. The aldehydes give positive Tollens test or silver mirror test.

Complete answer:
The Tollens reagent refers to the chemical reagent used in the detection of a functional group of Aldehyde, a functional group of aromatic Aldehyde, or a functional group or alpha hydroxy ketone in a certain test substance.
Tollens reagent takes its name from Bernhard Tollens, a German chemist who discovered this reagent and uses it. The Tollens reagent is a solution of silver nitrate $\left( {AgN{O_3}} \right)$ and ammonia $\left( {N{H_3}} \right)$ .
The silver mirror test with Tollen’s reagent is given by molecules containing aldehyde groups. But sucrose has no aldehyde groups, and the rest of the molecule has aldehyde groups. Therefore, glucose, fructose, and lactose, except sucrose, give a positive silver mirror test.
Hence, the correct option is A) sucrose.

Note:
Aldehyde contains – CHO functional group. A hydrogen atom is connected to the carbonyl group. Aldehydes can be oxidized to carboxylate ions. Therefore, Aldehydes give you a silver mirror test. The ketones contain the functional group double bonded CO. No hydrogen atom joins the carbonyl group. As we know, ketones cannot be rusted with carboxylate ions. Therefore, the ketones do not give a silver mirror test.