Hemiacetal formation of carbohydrates

Generally, hemiacetal formation occurs when the nucleophilic hydroxyl group of an alcohol attacks the carbonyl carbon of an aldehyde functional group. In the case of glucose specifically, this reaction occurs intramolecularly between the hydroxyl group of Carbon-$5$‍ and the aldehyde carbon resulting in the formation of a more stable, six-membered ring. In the process, a new chiral center is formed at the former carbonyl carbon, which is now called the anomeric carbon. This nucleophilic attack can occur above or below the pi bond resulting in two diastereomer products called anomers. In the β-anomer of glucose, the anomeric (Carbon-$1$‍) hydroxyl group is cis with respect to Carbon-$6$‍. Alternatively, the α-anomer contains an anomeric hydroxyl group that is trans with respect to Carbon-$6$‍. These are represented in Figure 1. In water, the ring-closing hemiactel reaction of glucose occurs in the forward and reverse direction, resulting in the spontaneous opening and closing of the ring and an equilibrium concentration of the two anomers. Due to steric hindrance, the β-anomer of glucose accounts for ~ $64\mathrm{\%}$‍ of concentration, and the α-anomer accounts for ~$34\mathrm{\%}$‍. The straight chain form accounts for <$1\mathrm{\%}$‍ of concentration.

Carbohydrates that form six membered rings (such as glucose) are referred to as pyranoses, while those forming five membered rings are referred to as furanoses.