Assertion (A): Rhamnose is classified as a carbohydrate even though its molecular formula, C6H12O5, does not fit the general formula Cx(H2O)y.
Reason (R): Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Carbohydrates got their name from the old idea of “hydrates of carbon”, Cx(H2O)y. The formula does not hold for every carbohydrate, so they are now defined by their functional groups.
Assertion (A): Rhamnose is classified as a carbohydrate even though its molecular formula, C6H12O5, does not fit the general formula Cx(H2O)y. — True. With six carbon atoms and twelve hydrogen atoms, the formula would need six oxygen atoms to be C6(H2O)6; rhamnose has only five, yet it is a carbohydrate.
Reason (R): Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. — True. This is the chemical definition, based on the functional groups shown by their reactions.
Why R explains A?
Since carbohydrates are defined by their functional groups and not by the formula Cx(H2O)y, a compound that does not fit the formula can still be a carbohydrate.
That is why rhamnose is classified as one, so R gives the reason for A.
NCERT (2026-27 edition):
10.1 Carbohydrates – “Most of them have a general formula, Cx(H2O)y, and were considered as hydrates of carbon from where the name carbohydrate was derived. … Similarly, rhamnose, C6H12O5 is a carbohydrate but does not fit in this definition. A large number of their reactions have shown that they contain specific functional groups. Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.”