Question

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

Answer 1

Following reactions and facts could not be explained by open-chain structures of glucose.

• Despite having the aldehyde group, glucose does not give 2, 4 − DNP test, Schiff's test and it does not form the hydrogen sulphide addition product with \[\ce{NaHSO3}\]​.

• The pentaacetate of glucose does not react with hydroxylamine indicating the absence of a free −CHO group.

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 −OH at C − 5 is involved in a ring formation.