D-Glucose (shown as the open chain in Panel 2) reacts to form an internal hemiacetal. Carbon #1 (blue halo) reacts with the oxygen (yellow halo) of carbon #5. As a result of this reaction, carbon #1 becomes chiral, and two stereoisomers ( anomers) are formed. One anomer, α-D-glucopyranose, is shown in Panel 1 from the perspective of looking on the edge of the Haworth structure. This is the perspective that is often shown in text books, and the anomeric carbon, carbon #1 with the blue halo, is shown on the right. Notice that the pyranose ring is not planear and that in the α configuration the hydroxyl group of the anomeric carbon is projecting below the pyranose ring. β-D-glucopyranose is shown in Panel 3, rotate its structure so that you view it from the same perspective as you view the structures in Panels 1 and 2, and then observe the position of the hydroxyl group on carbon #1, blue halo. Any aqueous solution of D-glucose will contain an equilibrium mixture of all three of these molecular forms, and therefore the aldehyde group, which is oxidized by a variety of reagents, is always present in an aqueous solution of glucose. Glucose is, therefore, called a reducing sugar.