The group of compounds known as carbohydrates received their general

name because of early observations that they often have the formula

Cx(H2O)y - that is, they appear to be hydrates of carbon.

The above definition could not survive long due to the following reasons:

(i) A number of compounds such as rhamnose, (C6H12O5) and

2-deoxyribose (C5H10O4) are known which are carbohydrates by their

chemical behaviour but cannot be represented as hydrates of carbon.

(ii)There are other substances like formaldehyde (HCHO, CH2O) and

acetic acid [CH3COOH, C2 (H2O)2] which do not behave like

carbohydrates but can be represented by the general formula, Cx(H2O)y.

Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy

ketones or substances which give these on hydrolysis and contain at least

one chiral carbon atom. It may be noted here that aldehydic and ketonic

groups in carbohydrates are not present as such but usually exist in

combination with one of the hydroxyl group of the molecule in the form of

hemiacetals and hemiketals respectively.

Classification

The carbohydrates are divided into three major classes depending upon

whether or not they undergo hydrolysis, and if they do, on the number of

products formed.

(i)Monosaccharides: The monosaccharides are polyhydroxy aldehydes or

polyhydroxy ketones which cannot be decomposed by hydrolysis to give

simpler carbohydrates. Examples are glucose and fructose, both of which

have molecular formula, C6H12O6.

(b) Trisaccharides, which yield three monosaccharide molecules on

hydrolysis. Example is raffinose, which has molecular formula,

C18H32O16.

(c)Tetrasaccharides, etc.

(iii)Polysaccharides: The polysaccahrides are carbohydrates of high

molecular weight which yield many monosaccharide molecules on

hydrolysis. Examples are starch and cellulose, both of which have

molecular formula, (C6H10O5)n.