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 :

(ii) Draw the six-membered ring structure of \(\alpha\)-D(+)-glucose.

Hint:

Draw the Haworth pyranose ring with CH2OH up on C-5 and the anomeric C-1 OH down for the alpha form.

Answer 1

Step 1: Understanding the Concept:
In solution the -OH group on C-5 of open chain glucose adds to the -CHO group (C-1). This forms a six-membered cyclic hemiacetal called the pyranose ring.
C-1 becomes a new chiral centre, the anomeric carbon. In the alpha form its -OH group is on the opposite side to the \(\mathrm{CH_2OH}\) group, drawn down in the Haworth structure.

Step 2: Build the ring:
The ring has five carbon atoms (C-1 to C-5) and one oxygen atom. The oxygen joins C-1 and C-5.
The ring is drawn flat with the oxygen at the back right corner and C-1 on the right.

Step 3: Place the groups (Haworth projection of \(\alpha\)-D-glucopyranose):
C-1: -OH pointing down (alpha), H up.
C-2: -OH down, H up.
C-3: -OH up, H down.
C-4: -OH down, H up.
C-5: \(\mathrm{CH_2OH}\) pointing up (this fixes the D series), H down.

Final Answer:
\(\alpha\)-D-glucopyranose: a six-membered ring of C-1 to C-5 and one ring oxygen, with \(\mathrm{CH_2OH}\) up on C-5, and -OH groups down on C-1, C-2 and C-4 and up on C-3.