Question

Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly classified into three groups - monosaccharides, oligosaccharides and polysaccharides. The carbohydrates may also be classified as either reducing or non-reducing sugars. An important monosaccharide glucose is an aldohexose and is correctly named as D(+)-glucose. It was assigned the open structure on the basis of its reactions with HI, \(\mathrm{NH_2OH}\), \(\mathrm{Br_2}\) water, \(\mathrm{(CH_3CO)_2O}\) and nitric acid.

Despite having the - CHO group, glucose does not give Schiff's test and hydrogen sulphite addition product with \(\mathrm{NaHSO_3}\). It was found that glucose forms a six-membered ring in which one of the - OH groups add to the - CHO group and form a cyclic hemiacetal structure.

Answer the following questions :

(i) In D(+)-glucose, what do the letter 'D' and sign '(+)' represent ?

Hint:

D shows the -OH on C-5 is on the right in the Fischer projection, and (+) shows glucose rotates polarised light to the right.

Answer 1

Step 1: Understanding the Concept:
The name D(+)-glucose carries two separate labels. One describes the spatial arrangement of the molecule and the other describes its effect on polarised light. They are not linked to each other.

Step 2: Meaning of the letter D:
D describes the configuration. In the Fischer projection of glucose, the -OH group on the last chiral carbon (C-5, farthest from the -CHO group) is on the right-hand side.
This is the same arrangement as in D-glyceraldehyde, the reference compound.

Step 3: Meaning of the sign (+):
The sign (+) describes the optical rotation. It means glucose is dextrorotatory.
A solution of glucose turns the plane of plane polarised light to the right (clockwise).
This is found by experiment and cannot be predicted from the letter D.

Final Answer:
D means the -OH on C-5 lies on the right in the Fischer projection (configuration like D-glyceraldehyde). (+) means glucose rotates plane polarised light to the right (dextrorotatory).