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Ring-straight chain isomerism

Monosaccharides can exist as a linear chain or as a ring-shaped molecule; in aqueous solutions they are usually found in the ring form. When Glucose is present in a ring form it can have two different arrangements of the hydroxyl group (OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below the plane of carbon 1 in the sugar, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (β) position.

Glucose has the chemical formula, C 6, H 12, O 6. In the straight chain form, the carbons can be numbered from 1 to 6 starting at the carbonyl group. One O H group is attached to each of the carbons from carbons 2 to 6. Carbon 1 is an aldehyde, so the carbon is double-bonded oxygen and single bonded hydrogen. To form the ring, the O H group at carbon 5 reacts with the carbonyl group. The 6-membered ring form of glucose is made of carbons 1 to 5, with oxygen between carbon 1 and carbon 5. The oxygen in the ring came from the O H group at carbon 5. The new O H group at carbon 1 is made from the oxygen that was in the carbonyl, and the hydrogen from the O H group at carbon 5. The alpha form of glucose has the O H at carbon 1 pointing down in the same direction as the O H groups on carbons 2 and 4. The beta form of glucose has the O H group at carbon 1 pointing up in the same direction as the O H group at carbon 3 and the C H 2 O H at carbon 5.

Figure 1: Conversion between linear and ring forms of glucose.

Five-membered rings can also be formed.

Referred from:

Article
Glucose