Classification of Carbohydrates: Aldoses and Ketoses

A monosaccharide is classified as an aldose or a ketose purely on the basis of which kind of carbonyl group it carries. An aldose has an aldehyde group, $-\text{CHO}$, which by the standard numbering convention is always placed at carbon C1, the very end of the chain. A ketose instead has a ketone carbonyl group, $\text{C}{=}\text{O}$, which for the naturally occurring sugars this chapter deals with sits at C2, one carbon in from the end.

Monosaccharides are further named according to how many carbon atoms their chain contains: a three-carbon sugar is a triose, a four-carbon sugar a tetrose, a five-carbon sugar a pentose, and a six-carbon sugar a hexose. Combining the two classification schemes gives the precise name of a sugar. Glucose has six carbons and an aldehyde group at C1, so it is classified as an aldohexose. Fructose also has six carbons, but its carbonyl group is a ketone at C2, so it is classified as a ketohexose. Both belong to the hexose family and share the same molecular formula, $\text{C}_6\text{H}_{12}\text{O}_6$, which makes them constitutional isomers of one another -- same atoms, different connectivity of the carbonyl group. …