Re: Is mannitol a non-reducing sugar?

Original question:

I know that most if not all monosaccharides are reducing agents due to their aldehydes.  I also know that mannitol is not, strictly speaking, a sugar since it is a polyol.  But, given only the following two classification options, would you consider mannitol a reducing or a non-reducing sugar?

Hmmm...... that's not a simple question!

As you state, mannitol is actually not a sugar, so the simplest answer to your question is "no", since mannitol is not a sugar, it cannot be a reducing sugar.

But given only the two choices of "reducing sugar" or "non-reducing sugar", I have to say it depends on the exact definition.

One source I found defines a reducing sugar as having a ketone or aldehyde group, which allows the sugar to act as a reducing agent. Mannitol has neither an aldehyde or a ketone group.

Another source mentions that reducing sugars give positive tests with Tollens' or Benedict's solutions, while non-reducing sugars are carbohydrates that contain only acetal groups and do not give positive tests with either reagent.

Because of these two definitions, and the fact that the chemical reactions of mannitol are very different than the kind of reaction and structure that defines reducing sugars, and given only the two choices: I'd say mannitol is more like a non-reducing sugar.

HOWEVER, Chromium(VI) and chromium(V) are both able to oxidize D-mannitol through a redox reaction. Mannitol is the reducing agent, so it is not so easy if we are limited to only two choices!

I hope this helps!

-Jason

The PubChem project from the NIH (entry for mannitol) http://pubchem.ncbi.nlm.nih.gov/

A full page on carbohydrates from the Virtual Text of Organic Chemistry ©1999 William Reusch, All rights reserved http://www.cem.msu.edu/~reusch/VirtualText/carbhyd.htm

Abstract of the paper "Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) by D-Glucitol and D-Mannitol" (Roldán et al (2000) Helv. Chim. Acta 83 pp3211-3228) can be found with a search for the word mannitol at http://www.wiley-vch.de/contents/jc_2217/200012.html

And lastly, three textbooks:

Chemistry, 6th edition (1998) by Raymond Chang

Organic Chemistry, 6th edition (1996) by T. W. Graham Solomons

Fundamentals of Biochemistry (1999) by Voet, Voet and Pratt