Assertion (A): Naturally occurring glucose is assigned the D-configuration.
Reason (R): Naturally occurring glucose is a dextrorotatory compound.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
In the name D-(+)-glucose, the two symbols say different things: ‘D’ gives the configuration, and ‘(+)’ gives the direction of rotation of plane polarised light.
Assertion (A): Naturally occurring glucose is assigned the D-configuration. — True. Glucose is correctly named D-(+)-glucose.
Reason (R): Naturally occurring glucose is a dextrorotatory compound. — True. The ‘(+)’ in its name shows that it is dextrorotatory.
Why R does not explain A?
‘D’ and ‘L’ have no relation with optical activity, so being dextrorotatory cannot be the reason for the D-configuration.
Glucose is D because the –OH on its lowest asymmetric carbon lies on the right side, as in (+)-glyceraldehyde, which has the D-configuration.
NCERT (2026-27 edition):
10.1.2.1 Glucose – “Glucose is correctly named as D(+)-glucose. ‘D’ before the name of glucose represents the configuration whereas ‘(+)’ represents dextrorotatory nature of the molecule. … ‘D’ and ‘L’ have no relation with the optical activity of the compound.”
“(+) Isomer of glyceraldehyde has ‘D’ configuration.” … “For assigning the configuration of monosaccharides, it is the lowest asymmetric carbon atom … which is compared. As in (+) glucose, —OH on the lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so (+) glucose is assigned D-configuration.”