Question

Which of the following pair(s) give positive Tollen's test:

• A. Glucose, Sucrose
• B. Glucose, Hexanal
• C. Hexanal, acetophenone
• D. Fructose, hydroxyacetone

Answer 1

Answer: (B), (D)

Fructose, hydroxyacetone

Aldehydes and $\alpha$-hydroxy ketones give positive Tollen's test. Glucose has aldehyde group at $C1$ and fructose is a $\alpha$-hydroxy ketone, so both of these give positive Tollen's test. Sucrose does not have any free aldehyde or $\alpha$-hydroxy ketone group because they are involved in a glycosidic linkage.
Hexanal has an adehyde group but acetophenone does not have any of these two groups. Hence, though hexanal give positive test with Tollen's reagent, acetophenone is negative to it.
Hydroxyacetone is $\alpha$-hydroxy ketones, hence it shows positive Tollen's test.
Hence, (b) and (d) are correct.