Carbohydrates & Their Classification | Organic Chemistry for NEET PDF Download

CARBOHYDRATES:

1. Introduction:

Carbohydrates received their name becasue of their general formula C_x(H₂O)_y, according to which they appear to be hydrates of carbon.

xCO₂ +  yH₂O  C_x(H₂O)_y +  xO₂

Photosynthesis:

6CO₂ + 6H₂O+ 18 ATP  C₆H₁₂O₆ + 6O₂

Cellular Respiration:

C₆H₁₂O₆  +6O₂  6CO₂  +6H₂O +38 ATP (36 ATP net gain)

2. Classification and structure of Carbohydrates:

Carbohydrates are polyhydroxy aldehydes and ketones and substances which hydrolyse to polyhydroxy aldehydes and ketones.

The simplest carbohydrates are called sugars or saccharides, (Latin: Saccharum, sugar). Carbohydrates can be classified as monosaccharides, oligosaccharides and polysaccharides.

3. General Characteristic of Monosaccharides:

The important characteristics of monosaccharides as follows:

(i) All monosaccahrides are water soluble due to the presence of hydrogen bonding between the different OH groups and surrounding water molecules.

(ii) Monosaccharides have sweet taste and dependent upon heating they get charred and give the smell of burning sugar.

(iii) Monosaccharides are optically active in nature due to the presence of chiral carbon atoms.

(iv) The chemical characteristics of monosaccharides are due to OH groups and carbonyl group which may be either aldehydic or ketonic group.

Glyceraldehyde contains one asymmetric carbon atom (marked by an astrisk) and can thus exist in two optically active forms, called the D-form and the L-form. Clearly, the two forms are mirror images that cannot be superimposed, that is they are enantiomers.

All four isomers have been prepared synthetically. The D-and L-erythrose are mirror images, that is, they are enantiomers. They have exactly the same degree of rotation but in opposite directions. Equal amounts of the two would constitute a racemic mixture, that is a mixture that would allow a plane-polarised light to pass through the solution unchanged.

Supplying hydrogen atoms to the five carbon atoms to satisfy their tetravalency, following structure (open chain) may be assigned to glucose: (* indicates assymetric carbon atom).