Taking an example,write the reaction of glycoside formation in monosaccharides.What is the significance of this reaction in carbohydrate chemistry?

1 Answer
Mar 7, 2018

Glycoside formation is the reaction of a hemiacetal (or hemiketal) with an alcohol to produce an acetal (or ketal) linkage. In carbohydrate chemistry, this bond is called a glycosidic bond, and its significance is that this bond produces polysaccharides.

Explanation:

Sugars contain both aldehydes and ketones. I'll only discuss aldehydes, but the same is true for ketones.

An aldehyde reacts with an alcohol under acidic conditions to produce a hemiacetal. This is true for a straight chain Aldose sugar like Glucose. However, in glucose, the alcohol group that forms the hemiactal actually comes from the same glucose molecule (5th carbon hydroxyl group). When the glucose molecule undergoes cyclization, a hemiacetal is forming at carbon 1, and this carbon is called the "anomeric carbon".

A hemiacetal can react with a second alcohol group to form an acetal. Cyclic glucose has a hemiacetal at carbon#1, and a second molecule of glucose has a number of alcohol groups (called hydroxy groups). Typically a hydroxy group on carbon 4 of a second glucose molecule will react with the hemiacetal group of the first sugar, and the result is a linkage and the formation of an acetal. The linkage is the formation of a glycoside, and the bond is called a glycosidic bond. This is how sugars polymerize (make polysaccharides, instead of just monosaccharides).