Question

The anomeric carbon in D-(+)-glucose is:
A. C-1 carbon
B. C-2 carbon
C. C-5 carbon
D. C-6 carbon

Answer 1

Hint: Think about the definition of the anomeric carbon and how the placement of the hydroxyl group around it enables it to form the $\alpha$- and $\beta$- anomers of and given sugar.

Complete step by step solution:
- We know that anomeric carbon is carbon whose stereochemistry determines the type of anomer that the compound is. It is used to refer to only the cyclized forms of sugars.
- In the open chain form of the sugar, the anomeric carbon is known to be the carbonyl carbon that is involved in the aldehydic or the ketonic group. After cyclization, the anomeric carbon is found near the oxygen atom in the pyranose or furanose ring.
- Here, we are concerned about the pyranose form of glucose. First let us look at the Fischer projection formula and how it undergoes cyclization.

Here, we can see the carbon atom that is next to the oxygen atom in the pyranose ring, and we can determine that it is the same carbon that is present in the aldehydic group in the Fischer projection formula.
- The ring can close in two different ways: with the new OH group down or up in the conventional Haworth projection. This carbon atom which can assume either configuration is referred to as anomeric carbon and the two different cyclised forms as anomers of each other called $\alpha$- D – glucopyranose and $\beta$- D – glucopyranose. When the hydroxyl group on the C-1 atom is pointing downwards, it is called the $\alpha$- anomer and when it is pointing upwards, it is called the $\beta$- anomer. The structures are:

Therefore, from above we can easily conclude that option A is the correct option to the given question.

Note: An anomer is a special case of an epimer. An epimer is an isomer where the stereochemistry of a sugar is changed at one and only one chiral center. In anomers, that chiral centre happens to be the C-1 atom. D-glucose and D-galactose are epimers of each other.