Reducing nature of glucose

The cyclic hemiacetal group present in glucopyranose is not a permanently 'closed' structure -- it sits in a fast equilibrium with a small, ever-present amount of the open-chain form carrying a genuine, free aldehyde group. Because that free aldehyde is always present in solution, however briefly and in however small a proportion at any instant, the hemiacetal functions chemically as a 'potential aldehyde' group, and glucose behaves overall as a genuine reducing sugar. This is confirmed by the two classic qualitative tests for a reducing aldehyde sugar: glucose gives a positive Tollens' test (the aldehyde reduces the diammine-silver(I) reagent to metallic silver, depositing a bright silver mirror on the test-tube wall) and a positive Fehling's test (the aldehyde reduces the deep-blue Cu2+ complex in Fehling's solution to a brick-red precipitate of copper(I) oxide, Cu2O). Both tests, discussed in the aldehydes/ketones chapter (s …