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, \((\mathrm{CH_3CO})_2\mathrm{O}\) and nitric acid.

Despite having the \(-\mathrm{CHO}\) 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 \(-\mathrm{OH}\) groups add to the \(-\mathrm{CHO}\) group and form a cyclic hemiacetal structure.

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

Hint:

Draw a pyranose ring (five C and one O) with C-1, C-2 and C-4 -OH down, C-3 -OH up and -CH2OH up on C-5.

Answer 1

Step 1: Understanding the Concept:
In the six-membered ring form, the -OH on C-5 adds to the -CHO group at C-1. This forms a cyclic hemiacetal.
The ring has five carbon atoms (C-1 to C-5) and one oxygen atom. It is called the pyranose ring.

Step 2: Key Rule for the Haworth drawing.
1. Groups on the right side in the Fischer projection point down in the ring.
2. Groups on the left side in the Fischer projection point up in the ring.
3. For a D-sugar, the -CH2OH group on C-5 points up.
4. In the alpha form, the anomeric -OH on C-1 is on the side opposite to the -CH2OH group, so it points down.

Step 3: Draw the ring.
Draw a flat hexagon with the ring oxygen at the back right corner. Number the carbons C-1 (right of the oxygen) to C-5 (left of the oxygen).
C-1 has -OH down and H up (alpha).
C-2 has -OH down.
C-3 has -OH up.
C-4 has -OH down.
C-5 carries -CH2OH up, and it is bonded to the ring oxygen.

Final Answer:
\(\alpha\)-D-glucopyranose is a six-membered ring (five C and one O) with -OH at C-1, C-2 and C-4 pointing down, -OH at C-3 pointing up, and -CH2OH at C-5 pointing up.