We know that glucose gives Tollen's test because it's a reducing sugar, and having a hemiacetal group, on hydrolysis can be converted into an aldehyde group, giving the positive test.

But, when we come to tests like the Schiff's test or with sodium bisulphite, it does not show any reaction which are shown by aldehydes. Why? Is the answer that hemiacetal group cannot be converted into aldehyde or anything else? Probably there shouldn't be any water used in these reactions, so no hydrolysis, no conversion from hemiacetal to aldehyde. Is there any water present in Tollen's reagent?

  • $\begingroup$ Stronger oxidising agents will open the ring and oxidise the glucose but the mild oxidising agents won't do it. As it has hemiacetal group Schiff's base and sodium bisulphite doesn't react with glucose as they are mild oxidizing agents. I guess stearic repulsions may also be the reason. $\endgroup$ – Uday Feb 15 '18 at 10:34

This are two different types of reactions.

Tollens is a redox reaction in which silver is reduced and glucose is oxidized. This reaction depends on the redox potential of glucose only.

Schiff and bisulphite tests are addition reactions. The reagents condense with the aldehyde group. This occurs also in the case of glucose but it is not sufficient to give a positive result. In the case of bisulphite, the adduct must precipitate, but it is too soluble. In the case of the Schiff test, the adduct must dehydrate, but it forms the glucosamine derivative instead.

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