INTRODUCTION

Old Definition
The group of compounds known as carbohydrates received their general name because of early observations that they often have the formula C_x(H₂O)_y - that is, they appear to be hydrates  of carbon.

Limitations of the old definition: 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, C_x(H₂O)_y.

New definition

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.