From the given following (A to D) cyclic structures, those which will not react with Tollens' reagent are: [figure]
- A and D
- A and B
- B and D
- B and C
Answer
(4) B and C
Solution
A sugar reduces Tollens' reagent only if its anomeric carbon (the carbon joined to the ring O) carries a free –OH (hemiacetal), because then the ring opens in solution to give the open-chain –CHO form.
A: β-D-glucopyranose; anomeric C has –OH (hemiacetal) reacts
B: methyl glucopyranoside; anomeric C has -OCH3 (acetal, glycoside) ring cannot open, does not react
C: methyl furanoside; anomeric C has -OCH3 (acetal, glycoside) does not react
D: α-D-galactopyranose; anomeric C has –OH (hemiacetal) reacts
B and C do not react with Tollens' reagent.
Answer: (4)