. Carbohydrates: Carbohydrates are the most abundant organic compounds in every living organism. They are also known as saccharides (derived from Greek word ‘ sakcharon ’ which means sugar) as many of them are sweet. They are considered as hydrates of carbon, containing hydrogen and oxygen in the same ratio as in water.
Chemically, they are defined as polyhydroxy aldehydes or ketones with a general formula C n (H O) n . Some common examples are glucose (monosaccharide), sucrose (disaccharide) and starch (polysaccharide) Figure . . Structure of carbohydrates Carbohydrates are synthesised by green leaves during photo synthesis, a complex process in which sun light provides the energy to convert carbon dioxide and water into glucose and oxygen.
Glucose is then converted into other carbohydrates and is consumed by animals. Sun light 6CO2 + H2O C6H12O6 + 6O2 . . Configuration of carbohydrates: Almost all carbohydrates are optically active as they have one or more chiral carbons.
The number of optical isomers depends on the number of chiral carbons ( n isomers, where n is the total number of chiral carbons). We have already learnt in XI standard to represent an Starch D - Glucose CH OH HO CH OH D - Fructose AMYLOSE AMYLOPECTIN Sucrose XII XII - - - - organic compound using Fischer projection formula. Fischer has devised a projection formula to relate the structure of a carbohydrate to one of the two enantiomeric forms of glyceraldehyde (Figure . ).
Based on these structures, carbohydrates are named as D or L.The carbohydrates are usually named with two prefixes namely D or L and followed by sign either (+) or (-). Carbohydrates are assigned the notation (D/L) by comparing the configuration of the carbon that is attached to -CH OH group with that of glyceraldehyde.For example D-glucose is so named because the H and OH on C5 carbon are in the same configuration as the H and OH on C2 carbon in D-Glyceraldehyde. There + and – sign indicates the dextro rotatory and levo rotatory respectively. Dextro rotatory compounds rotate the