The reference compound shown gives a positive Tollens test. Which of Structures A to D also gives a positive Tollens test?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

A positive Tollens test needs a free aldehyde or a hemiacetal that can open to one. Beta-D-galactopyranose has a free anomeric OH and equilibrates with its aldehyde form. The alditol has no C=O, the methyl glycoside is an acetal and gluconic acid has had its CHO oxidised to COOH.

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