Reactions of Monosaccharides at the OH Groups

Because monosaccharides contain OH groups, they undergo reactions typical of alcohols—that is, they are converted to ethers and esters. Because the cyclic hemiacetal form of a monosaccharide contains an OH group, this form of a monosaccharide must be drawn as the starting material for any reaction that occurs at an OH group. 

The forms of glucose in aqueous solution are quite complex and small proportions exist as furanose structures. These arise from the reaction between the OH group at C-4 and the aldehyde group in aldoses. The five-membered ring system is quite stable but pyranose forms are favoured over furanose forms by most monosaccharides notable exceptions are the ketohexose fructose and the aldopentose ribose. Too much strain would be involved in reducing bond angles to produce smaller rings, so trioses and tetroses do not form intermolecular cyclic systems. 

Discussion on any kind of polymers is logically preceded by at least a quick survey of properties of parent monomers. This is particularly relevant in the case of polysaccharides because monosaccharides are rather special monomers, with peculiar configurational-conformational properties and reactivity. Monosaccharides are multifunctional compounds, usually with three or four OH groups of approximately equal chemical reactivity reducing sugars have, of course, additional reaction capabilities. 

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