Read the passage carefully and answer the questions.
Carbohydrates such as cane sugar, glucose, starch etc. belong to a class of naturally occurring organic compounds. Structurally these are polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. Carbohydrates which are sweet in taste are called sugars. Theses sugars can be reducing or non-reducing. Monosaccharides cannot be hydrolyzed to simpler units whereas Oligosaccharides yield two to ten monosaccharide units on hydrolysis. Polysaccharides yield large number of monosaccharide units on hydrolysis. This class of biomolecules is essential for living systems as they serve as storage molecules in plants and animals. In addition, they provide raw materials for important industries like furniture, textiles, paper, lacquers and breweries. Two aldopentose form important constituents of nucleic acids. Carbohydrates are also called saccharides (Greek: sakcharon which means sugar). Sugar which is used in household and in milk also comes under this class of biomolecules.
Which of the following is incorrect for glucose?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → It is named as D (-) glucose
In a name such as D-(+)-glucose, the letter D compares the –OH group on the lowest asymmetric carbon in the Fischer projection with that of glyceraldehyde, and the sign gives the measured direction of rotation.
Natural glucose has the D configuration and is dextrorotatory, so its correct name is D-(+)-glucose. The name “D (−) glucose” gives the wrong sign of rotation; it is fructose that is laevorotatory, D-(−)-fructose.
Option (1) It is named as D (-) glucose — Not true for glucose (it is D-(+)-glucose). Natural glucose is dextrorotatory; the (−) sign belongs to fructose.
Option (2) A reducing sugar — True for glucose. Glucose has a free aldehyde group in its open-chain form, so it reduces Tollens’ reagent and Fehling’s solution.
Option (3) It is an aldose — True for glucose. Glucose, \( \ce{CHO-(CHOH)4-CH2OH} \), is a six-carbon monosaccharide with an aldehyde group, that is, an aldohexose.
Option (4) Six membered cyclic structure is called pyranose — True for glucose. The –OH group on C-5 adds to the aldehyde group and closes a ring of five carbon atoms and one oxygen atom; by analogy with pyran, this six-membered ring form is called the pyranose structure.
NCERT (2026-27 edition):
Glucose – …
Cyclic Structure of Glucose – “… The six membered cyclic structure of glucose is called pyranose structure (α– or β–), in analogy with pyran. Pyran is a cyclic organic compound with one oxygen atom and five carbon atoms in the ring.”