Q.Define optical activity. How many optical isomers are possible for glucose?

Glucose's open-chain structure, CHO-(CHOH)4-CH2OH (an aldohexose), was established entirely from chemical evidence: its molecular formula C6H12O6; a straight, unbranched six-carbon chain (shown by HI reduction to n-hexane); exactly one carbonyl group (shown by forming one oxime with NH2OH and one cyanohydrin with HCN); that carbonyl being specifically an aldehyde, not a ketone (shown by mild bromine-water oxidation to the monocarboxylic gluconic acid); five hydroxyl groups on five different carbons (shown by acetylation to glucose pentaacetate); and one primary alcoholic (-CH2OH) group (shown by nitric-acid oxidation of both glucose and gluconic acid giving the same dicarboxylic saccharic acid). Glucose has four chiral carbons (C-2 to C-5), is optically active ([alpha]D20 = +52.7 degrees, hence 'dextrose'), and is assigned the D-configuration (by Emil Fischer, 1902) because its lowest chiral carbon, C-5, matches D-glyceraldehyde's configuration. In …