Representation of Fructose structure

Fructose (C6H12O6) is the second monosaccharide studied in detail in this chapter -- a laevorotatory ketohexose, with specific rotation [alpha]D20 = -92.4 degrees; this strong laevorotation is why fructose is also called laevulose. Even though its carbonyl group is a ketone rather than an aldehyde, fructose IS a reducing sugar, because it is specifically an alpha-hydroxy ketone (a ketone with an adjacent -OH), and this arrangement of groups is oxidisable under the same mild conditions used to test for reducing sugars, just as an aldehyde is. Like glucose, fructose exists mainly in cyclic ring forms rather than as the open chain: in its FREE state (e.g. as free fructose in solution) it exists predominantly as fructopyranose, a six-membered ring, with a smaller proportion present as fructofuranose, a five-membered ring; but in its COMBINED state -- for example, as the fructose unit within sucrose (section 14.2.9a) -- it is found exclusively in the five-membered fructofuranose ring form. The name 'furanose' follows the same analogy pattern as glucose's 'pyranose': it is named after the simple five-membered oxygen heterocycle furan. Because fructose's ring-closing carbonyl is a ketone rather than an aldehyde, the cyclic structure formed is technically a hemiketal (rather than glucose's hemiacetal) -- the same underlying idea of an intramolecular addition to the carbonyl, just starting from …