INTRODUCTION TO CARBOHYDRATE CHEMISTRY Classifications

The groups of naturally occurring compounds known as carbohydrates received their general name because of they often have the formula C x (H 2 O) y that is “ HYDRATES OF CARBON” for example formula of glucose is C 6 H 12 O 6 which is C 6 (H 2 O) 6. However it is important to note that all the compounds conforming to the formula C x (H 2 O) y are not necessarily carbohydrates, e.g- formaldehyde C(H 2 O) which is not a carbohydrate.

After thorough study carbohydrates are usually defined as polyhydroxy aldehyde or polyhydroxy ketones or substances. Simple carbohydrates r known as sugars or saccharides ( Latin: - saccharum, sugar) and the ending of the names of most sugars is – Ose. Thus such names as Sucrose for ordinary table sugar, Glucose for the principle sugar in blood Etc.

The simplest carbohydrates, those that can not be hydrolysed into smaller simple carbohydrates, are called Monosaccharides. On a molecular basis, carbohydrates that undergo hydrolysis to produce two molecules of same or different monosaccharides are called Disaccharides. For example:- 1 mole of maltose + H 3 O + 2moles of glucose (C 6 H 12 O 6 ) 1 mole of sucrose + H 3 O + one mole of glucose + one mole of fructose Carbohydrates that hydrolyse to yield 2 to 10 molecules of a monosaccharides are sometimes called Oligosaccharides. The carbohydrates hydrolyse to yield a large number ( >10) of molecules of a monosaccharides are known as Polysaccharides.

We encounter carbohydrates at almost every turn of our daily lives. The paper on which the book is printed is largely cellulose, so too is the cotton of our clothes and wood of our houses. The flour from which we make bread is mainly starch and starch is also a major constituents of many other foodstuffs, such as potatoes, rice etc. all are carbohydrates.

Nomenclature and Classification of Monosaccharides:- Monosaccharides are named and classified according to- a) The number of carbon atom present in the molecule and b) Whether they contains an aldehyde or keto group. Thus a monosaccharides containing three carbon atoms is called Triose, one containing four carbon atom is called Tetrose and so on. Again a monosaccharides containing an aldehyde group is called an Aldose, and one containing a keto group is called a Ketose. These two ways of naming and classifications are frequently combined. A four carbon aldose for example. Monosaccharides are named and classified according to- a) The number of carbon atom present in the molecule and b) Whether they contains an aldehyde or keto group. Thus a monosaccharides containing three carbon atoms is called Triose, one containing four carbon atom is called Tetrose and so on. Again a monosaccharides containing an aldehyde group is called an Aldose, and one containing a keto group is called a Ketose. These two ways of naming and classifications are frequently combined. A four carbon aldose for example. Is called an aldo tetrose

a six carbon ketose

D and L Designations of Configurations:- The simplest monosaccharides are aldotriose and ketotriose which are glyceraldehydes and dihydroxy acetone respectively… Glyceraldehyde Dihydroxy acetone Obviously of these two only glyceraldehydes contains a stereocentre. Glyceraldehyde exsists in two enantiomeric forms and the configurations are:-

(+) Glyceraldehyde (-) Glyceraldehyde

According to M.A. Rosanoff of New York University in 1906 first suggested that (+) glyceraldehydes is designated as D(+) Glyceraldehyde and (-) glyceraldehydes is designated as L(-) Glyceraldehyde. These two compounds, moreover, serve as configurational standards for all monosaccharides:- A monosaccharide whose highest-numbered stereocentre has the same configuration as D (+) Glyceraldehyde is designated as a D-Sugar one whose highest-numbered stereocentre has the same configuration as L (-) Glyceraldehyde is designated as L- Sugar. Thus,

D- Aldopentose L- Aldopentose (Hydroxyl group of highest numbereb stereo (Hydroxyl group of highest numbered stereo Centre in RHS) centre in LHS)

D and L designations are like (R) and (S) designations in that they are not necessarily related to the optical rotations of the sugars to which they are applied. Thus, one may encounter other sugars that are D(+) or D(-) and that are L(+) or L(-). A few of the aldoses are particularly important and their configurations and structures should be learned. D- Glucose, D- Mannose, D- Galactose and the structures are:-

D (+) Glucose D(+) Moannose D(+) Galactose

These are the most important aldohexoses because of their wide natural occurance. Now let us have a close look on the above structures we can notice that configurations of D (+) Mannose and D (+) Galactose differ in a simple way from the configuration of D (+) Glucose. i.e. D (+) Glucose and D (+) Mannose differ in configuration only at carbon-2 structures are given below:-

And D (+) Glucose and D (+) Galactose differ in configuration only at carbon-4 structures are given below:- Compounds that differ in configuration at only several stereocentres are called EPIMERS. Hence, D Glucose and D- Mannose are epimeric in carbon -2; D- Glucose and D- Galactose are epimeric carbon -4.