Ring structure of glucose

The open-chain Fischer structure of glucose, though it correctly explains many of glucose's properties, cannot account for every chemical property glucose is observed to show -- most notably, glucose does not react with all the reagents a genuinely free aldehyde would be expected to react with, and it also shows an unusual property called mutarotation (a slow change in optical rotation when freshly dissolved). The resolution is that in aqueous solution, glucose exists mainly as one of two CYCLIC (ring) structures, in dynamic equilibrium with a small amount of the open-chain form. The ring closes when the -OH group at C-5 reacts internally with the aldehydic carbon at C-1, forming a cyclic hemiacetal -- exactly the same hemiacetal-forming reaction an aldehyde undergoes with any external alcohol, except here the alcohol (the C-5 -OH) is part of the SAME molecule. This creates a six-membered ring: five carbons (C-1 through C-5) plus the one oxygen that bridged across from C-5, so the ring is called a pyranose ring, by analogy with the simple six-membered oxygen heterocycle pyran -- glucose in this cyclic form is accordingly called glucopyranose. Because C-1 was not previously a stereocentre in the open-chain form (it was a flat, sp2 aldehydic carbon) but becomes one upon ring closure (bonded to four different groups once the ring's own oxygen and the new -OH are both attached), ring closure creates a brand-new chiral centre at C-1, called the anomeric carbon. The two possible configurations at this new centre give two distinct ring structures called anomers: the alpha-anomer and the beta-anomer, which differ ONLY in the spatial arrangement at C-1 (every other stereocentre, C-2 through C-5, is identical between them). The clearer way to draw these ring structures is the Haworth formula, in which the pyranose ring is drawn as if viewed edge-on, perpendicular to the plane of the paper: the lower face of the ring is conventionally called the alpha-side and the upper face the beta-side. The alpha-anomer has its anomeric -OH (at C-1) drawn pointing down, on the alpha-side; …