Carbohydrates are optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. They have been broadly…

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 $\displaystyle \mathrm{HI}, \mathrm{NH}_{2} \mathrm{OH}, \mathrm{Br}_{2}$ water, $\displaystyle \left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}$ and nitric acid. Despite having the - CHO group, glucose does not give Schiff's test and hydrogen sulphite addition product with $\displaystyle \mathrm{NaHSO}_{3}$. 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 :

How do you explain the following ?

Presence of a carbonyl group in glucose

Presence of five - OH groups in glucose which are attached to different carbon atoms.

What type of carbohydrates are called reducing sugars ?

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

Draw the six-membered ring structure of $\displaystyle \alpha-\mathrm{D}(+)-$ glucose.