Question

Glucose has _________ optical isomers.
(A) 4
(B) 8
(C) 16
(D) 10

Answer 1

Hint: To answer this question, you must know the structure of glucose. Glucose is an aldohexose sugar-containing two chiral centres and other centres are chiral. A general formula for obtaining the maximum number of optical isomers is $2^n$, where n is the number of chiral centres.

Complete step by step solution:
Molecular formula of glucose is $C_6{H}_{12}{O}_6$. Structure of glucose is:

As you can see in the above structure of glucose there are six carbon atoms, hence it is an aldohexose. We usually consider glucose as D-glucose (dextrose). The D-isomer of glucose is the one whose carbon 2 is in the R configuration or has OH in the right-hand side. The mirror image of dextrose is L-glucose, having S configuration of carbon 2. Glucose contains one aldehydic carbon and five alcoholic carbons. It contains four chiral centres. A chiral centre is the one that has four different groups bonded to it in such a manner that it has a non-superimposable mirror image. Thus, in glucose, carbon 2, 3, 4 and 5 are chiral centres and carbon 1 and 6 are achiral. Now, the formula to obtain the maximum number of possible optical isomers is $2^n$, where n is the total number of chiral centres. Since, glucose has four chiral centres, n=4 and we get,
$2^4=16$ optical isomers.

Hence, option (C) is correct.

Note: The 16 optical isomers of glucose are allose, altrose, galactose, glucose, gulose, idose, mannose, and tallose. The various conformations of glucopyranose rings are not considered since they are anomers rather than optical isomers.