Question

Which of the following represents the structure of $\alpha -D-$mannopyranose ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Mannose is a C-2 epimer of glucose so the open chair struture of mannose is

Mannose form a six membered pyranose ring in which $-OH$ at C5 is involved in ring formation which leads to hemiacteal linkage.
In the pyranose ring, $C{H}_{2}OH$ group is added at the terminal, placed above the plane of the hexagon ring always. Groups which are present on the left-hand side in Fischer projection are placed above the plane of the ring and placed all the groups of right hand below the plane of the ring. Due to this arrangement, a new asymmetric carbon atom is created at the carbonyl carbon and it is referred to as anomeric carbon atom. Anomers are two different configurations that are possible for the anomeric carbon atom. The position of OH group differs at C1 in both the structures, where alpha stands for 'down' OH group and beta for 'up' OH group.
Hence, the structure of $\alpha -D-$mannopyranose is option (c).