Glucose: how it is made and the open-chain evidence

Glucose occurs free in sweet fruits, honey and ripe grapes, and in combined form in larger carbohydrates.

From sucrose: boiling cane sugar with dilute HCl or H₂SO₄ in alcoholic solution gives equal amounts of glucose and fructose; one sucrose plus one water yields one of each hexose.

From starch (the commercial route): boiling with dilute H₂SO₄ at 393 K under 2–3 atm pressure. (C₆H₁₀O₅)n + nH₂O → nC₆H₁₂O₆.

Glucose, also called dextrose, is an aldohexose and the monomer of starch and cellulose. NCERT calls it probably the most abundant organic compound on earth.

Evidence 1 and 2: the molecular formula is C₆H₁₂O₆, and long heating with HI gives n-hexane, so the six carbons form an unbranched chain.

Evidence 3: glucose forms an oxime with hydroxylamine and adds HCN to give a cyanohydrin, so it has a carbonyl group, >C=O.

Evidence 4: a mild oxidant, bromine water, turns it into gluconic acid, a six-carbon carboxylic acid. Only an aldehyde is oxidised so easily, so the carbonyl is –CHO.

Evidence 5: acetic anhydride gives glucose pentaacetate, so there are five –OH groups. The pentaacetate is stable, so the five –OH groups sit on different carbons.

Evidence 6: nitric acid oxidises both glucose and gluconic acid to saccharic acid, a dicarboxylic acid. The second –COOH comes from a primary alcohol, –CH₂OH, at C6.

Result: OHC–(CHOH)₄–CH₂OH. Fischer later fixed where each –OH points (structure I); gluconic acid (II) and saccharic acid (III) keep that arrangement.