An optically active compound having molecular formula `C_(6)H_(12)O_(6)` is found in two isomeric forms [A] and [B] in nature. When [A] and [B] are dissolved in water, they show the following equilibrium.
`{:(,[A],hArr,"equilibrium mixture",hArr,B),([alpha]_(D)=,111^(@),," "52.2^(@),,19.2^(@)):}`
(i) What are such isomers called ?
(ii) Can they be called enantiomers ? Justify your answer.
(iii) Draw the cyclic structure of anomer [A].

Explanation

The optically active compound having M.F. `C_6H_12O_6` is glucose. It exists in two stereoisomeric forms (A) and (B). For structure.
(i) These two isomers which differ in configuration only at the glycosidic carbon are called anomers. The anomer (A) with `[alpha]_(D)=111^@` is called `alpha`-D-glucophyranose while the anomer (B) with `[alpha]_(D)=+19.2^@` is called `beta`-D-glucopyronase.
Since `alpha`-form is less stable than the corresponding `beta`-form, therefore , equilibrium mixture consists of approx, `36%` of `alpha`-form and `64%` of the `beta`-form.
(ii) Since these anomers are not mirror images of each other, they cannot be called as enantiomers.
(iii) The anomer (A) with `[alpha]_(D)=111^@` is called `alpha`-D-glucopyranose. Its Fisher projection and Haworth structure are given.