Glucose

Glucose is a simple sugar that serves as the major energy source for the human body. It is the most important and most abundant monosaccharide, occurring free in honey and in sweet fruits such as grapes and mangoes, and combined in sucrose, starch and cellulose. Human blood normally carries about 100 mg/dL of glucose, which is why it is also called blood sugar.

Preparation. Glucose can be obtained in two standard ways. First, boiling sucrose (cane sugar) with dilute H₂SO₄ in alcoholic solution hydrolyses it into equal parts glucose and fructose (C₁₂H₂₂O₁₁ + H₂O --H⁺--> C₆H₁₂O₆ + C₆H₁₂O₆). Second, and more importantly on a commercial scale, starch is hydrolysed with dilute HCl at high temperature (393 K) and pressure (2-3 atm), each glucose unit of the starch chain being released as free glucose ((C₆H₁₀O₅)ₙ + nH₂O --H⁺--> nC₆H₁₂O₆).

Structure elucidation (open-chain form). Glucose is an optically active aldohexose with four asymmetric carbons; its aqueous solution is dextrorotatory, which is why it is also called dextrose. Its open-chain structure was pieced together from eight independent lines of chemical evidence. (1) Elemental analysis and molecular-weight determination fix its molecular formula as C₆H₁₂O₆. (2) Vigorous reduction with concentrated HI and red phosphorus at 373 K gives mainly n-hexane, with 2-iodohexane as a minor product, showing that all six carbons are joined in an unbranched, linear chain. (3) Glucose reacts with hydroxylamine to give an oxime and with HCN to give a cyanohydrin, both reactions being characteristic of a carbonyl group. (4) Mild oxidation with bromine water converts glucose to gluconic acid, showing that the carbonyl group is specifically an aldehyde sitting at one end of the chain, while vigorous oxidation with concentrated HNO₃ gives glucaric (saccharic) acid -- a diacid -- showing that the other end of the chain carries a primary alcohol group. (5) Glucose reduces ammoniacal silver nitrate (Tollens' reagent, depositing a silver mirror) and alkaline copper sulphate (Fehling's solution, giving a red precipitate of cuprous oxide), both classic confirmations of an aldehyde group. (6) Reaction with acetic anhydride gives a penta-acetate derivative, showing that five separate -OH groups are present. (7) Glucose does not dehydrate easily, which rules out two -OH groups sharing a single carbon, so its five -OH groups must sit on five different carbons, leaving the sixth carbon as the aldehyde. (8) Emil Fischer used these facts to work out the exact spatial arrangement of the -OH groups, arriving at the structure now written as D-(+)-glucose, so named because it has the D configuration (§14.1.1) and is dextrorotatory. …