Glycosides

A hemiacetal is the product of a reaction between an aldehyde and an alcohol. A hemiketal is the product of the reaction between a ketone and an alcohol. A ketal is the product of a reaction between a ketone and a large amount of alcohol.

Carbohydrates are determined by the number of sugar monomers associated with them. While we talk about just a few, there are more than two hundred known monosaccharides. Disaccharides, oligosaccharides, and polysaccharides are made by the hydrolysis of different monosaccharide monomers together. The disaccharide sucrose, for example, is made from a glycosidic bond between glucose and fructose.

GLYCOSIDES

Glycosides are important in sugar chemistry. It takes a hemiacetal sugar and alcohol under acidic conditions to make an acetal or a glycoside. It is in simplified form in figure 112 but is very complex when you remember that a sugar molecule consists of many alcohol groups that can act as the “alcohol” and alcohols basically have a hemiacetal group at the anomeric carbon:

Figure 112.

In such cases, the ROH is another OH bond attached to another sugar, leading to a disaccharide that can go on to add more and more sugars to make oligosaccharides and polysaccharides. In the case of sucrose, both anomeric carbons are involved in an acetal or glycosidic bond, so that the molecule is not self-replicating as the anomeric carbon is “used up” on both the glucose and fructose molecules, making no other anomeric carbon available for bonding. Figure 113 shows the sucrose molecule: