`D- (+) -` Glucose shows mutarotation because

Options

A. it undergoes interconversion with `D- (-) -` frustose
B. it is dextrorotatory
C. it undergoes interconversion between its `alpha - D- (+) -` glucospyranose and `beta-D-(+)` glucospyranose structures via the open chain structure
D. it undergoes interconversion between its pyranose structure and furanose structure

Answer

C

Explanation

Part of the evidence for the cyclic hemiacetal structure for `D - (+) -` glucose comes from experiments in which both `alpha` and `beta` forms have been isolated.
Ordinary `D - (+) -` glucose has a melting point of `146^(@)C`. However, when `D - (+) -` glucose is crystallised by evaporating an aqueous solution kept above `98^(@)C`, a second from of `D - (+) -` glucose with a melting point `150^(@)C` can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases and the rotation of the other increases, until both solutions show the same value. This change in rotation toward an equilibrium value is called mutarotation.
The explanation for this mutarotation lies in the existance of an equilibrium between the open-chain form of `D - (+) -` glucose and the `alpha` and `beta` forms of the cyclic hemiacetals. X-ray analysis as confirmed that ordinary `D - (+) - ` glucose has the `alpha` configuration at the anomeric carbon atom (also called glycosidci carbon) and but the higher melting form has the `beta` configuration.
The concentration of open-chain `D - (+) -` glucose in solution at equilibrium is very small. Thus, solutions of `D - (+) - ` glucose give on obserable `UV` of `IR` absorption band for carbonyl group, and solution of `D - (+) - ` glucose gives a negative test with Schiff's reagent, a special reagent that requires a relatively high concentration of a free aldehyde group (rather than a hemiacetal) in order to give a positive test.