Carbohydrates — Part 2: Fructose, disaccharides and polysaccharides

Fructose, C₆H₁₂O₆, is a ketohexose: HOCH₂–CO–(CHOH)₃–CH₂OH, with the ketone at C2. Like glucose it gives n-hexane with HI and a pentaacetate. Its cyanohydrin, hydrolysed and reduced with HI, gives 2-methylhexanoic acid, which places the C=O at C2 (glucose gives straight-chain heptanoic acid). In solution the C5 –OH adds to C2, giving a five-membered furanose ring (a hemiketal) with α and β anomers. Natural fructose is D-(−), laevorotatory, [α]D = −92.4°.

Bromine water is a mild oxidant that attacks aldehydes but not ketones (and it is not alkaline, so fructose cannot rearrange). Glucose decolourises the orange-brown bromine water, forming gluconic acid; fructose does not. Tollens’ and Fehling’s cannot distinguish them: in alkaline solution fructose isomerises (via an enediol) and so is also a reducing sugar. Both give the same osazone, because only C1 and C2 react and the two sugars have the same configuration at C3–C5.