Fructose

Fructose is another commonly encountered monosaccharide, sharing the same molecular formula as glucose (C₆H₁₂O₆) but differing in its properties: it is levorotatory and is a ketohexose rather than an aldohexose. It occurs abundantly in fruits, which is why it is also called fruit sugar, and it is the sweetest of all known sugars, being readily soluble in water.

Preparation. Fructose is obtained in two standard ways. From sucrose, heating with dilute H₂SO₄ or treating with the enzyme invertase hydrolyses sucrose into equal parts glucose and fructose (C₁₂H₂₂O₁₁ + H₂O --H⁺ or invertase--> C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)); the fructose is then separated out by crystallisation, and the equal-parts glucose-fructose mixture left behind is called invert sugar. Commercially, fructose is more often prepared by the acidic hydrolysis of inulin, a polysaccharide built entirely of fructose units ((C₆H₁₀O₅)ₙ + nH₂O --H⁺--> nC₆H₁₂O₆).

Structure elucidation (open-chain form). A freshly prepared solution of fructose has a specific rotation of -133°, which changes on standing to an equilibrium value of -92° through mutarotation. Its structure, like glucose's, was deduced from a parallel set of chemical facts. (1) Elemental analysis and molecular-weight determination show it has the same molecular formula as glucose, C₆H₁₂O₆. (2) Reduction with HI and red phosphorus gives a mixture of n-hexane (major product) and 2-iodohexane (minor product), showing that its six carbon atoms also form a straight, unbranched chain. (3) It reacts with hydroxylamine and with HCN, showing the presence of a carbonyl group. (4) It reacts with acetic anhydride in the presence of pyridine to form a penta-acetate, showing that five -OH groups are present. (5) Unlike glucose, fructose is NOT oxidised by bromine water, which rules out the presence of an aldehyde (-CHO) group at all -- the carbonyl group must instead be a ketone. (6) Partial reduction with sodium amalgam and water produces a mixture of sorbitol and mannitol, which are epimers at their second carbon; because this reduction creates a brand-new asymmetric carbon at C2, it confirms that C2 was originally the site of a keto group. (7) Oxidation with nitric acid cleaves the chain to give glycollic acid and tartaric acid, both containing fewer carbon atoms than fructose itself, which further confirms a keto group at C2 and shows that primary alcohol groups are present at both C1 and C6. Taken together, this evidence gives the accepted open-chain structure of D-(+)-fructose: a -CH₂OH at C1, a C=O (keto) at C2, three consecutive -CHOH- asymmetric carbons at C3-C5, and a terminal -CH₂OH at C6. …