Question

Write the reactions of $D$−glucose which can’t be explained by its open-chain structure. How can the cyclic structure of glucose explain these reactions?

Answer 1

Reactions of $D$−glucose which can’t be explained by its open-chain structure.

1. Despite having the aldehyde group, glucose does not give Schiff’s test and it does not form the hydrogensulphite addition product with $NaHS{O}_3$.

2. The pentaacetate of glucose does not react with hydroxylamine, indicating the absence of free $—CHO$ group.

Cyclic structure of glucose that can explain the above reactions

It was proposed that one of the $—OH$ groups may add to the $—CHO$ group and form a cyclic hemiacetal structure. It was found that glucose forms a six-membered ring in which $—OHatC-5$ is involved in ring formation. This explains the absence of $—CHO$ group and the existence of glucose in two forms. These two cyclic forms exist in equilibrium with open-chain structure




Due to formation of cyclic structures of glucose, $-CHO$ group of glucose is not free to show the reactions.