Q.Why carbohydrates are generally optically active.

Almost every carbohydrate is optically active because it has one or more chiral carbons, and the number of possible stereoisomers follows the 2ⁿ rule (n = number of chiral carbons). Emil Fischer's projection formula lets every carbohydrate be related back to one of the two enantiomeric forms of the simplest sugar, glyceraldehyde, and on this basis every carbohydrate is labelled D or L.

The assignment rule compares the configuration at the highest-numbered chiral carbon (nearest the terminal -CH₂OH) with the reference carbon of glyceraldehyde: D-glucose is 'D' because the H/OH arrangement at its C5 matches the H/OH arrangement at C2 of D-glyceraldehyde. This D/L configurational label is completely independent of the (+)/(-) sign that records the actual, measured **direction of optical rot …