# 20.1: Prelude to Carbohydrates


Carbohydrates are a major class of naturally occurring organic compounds, which come by their name because they usually have, or approximate, the general formula $$\ce{C}_n \ce{(H_2O)}_m$$, with $$n$$ equal to or greater than three. Among the well-known carbohydrates are various sugars, starches, and cellulose, all of which are important for the maintenance of life in both plants and animals.

Although the structures of many carbohydrates appear to be quite complex, the chemistry of these substances usually involves only two functional groups - ketone or aldehyde carbonyls and alcohol hydroxyl groups. The carbonyl groups normally do not occur as such, but are combined with hydroxyl groups to form hemiacetal or acetal linkages of the kind discussed in Section 15-4E.

An understanding of stereochemistry is particularly important to understanding the properties of carbohydrates. Configurational and conformational isomerism play an important role. For this reason, you may wish to review Chapter 5 and Sections 12-3 and 19-5.