Question

Which of the following pairs of carbohydrates are an exception to the general formula of carbohydrates?

• A. Fructose and glucose
• B. Dihydroxyacetone and glyceraldehyde
• C. Rhamnose and deoxyribose
• D. Erythrose and erythrulose

Answer 1

The correct option is C Rhamnose and deoxyribose

The general formula for carbohydrates is $C_n{H}_{2n}{O}_n$If n=3 (trioses), the formula becomes
$C_3{H}_6{O}_3$.
If n=4 (tetroses), the formula becomes $C_4{H}_8{O}_4$.
If n=5 (pentoses), the formula becomes $C_5{H}_{10}{O}_5$.

But rhamnose has a formula of $C_5{H}_{10}{O}_4$.
If n=6 (hexoses), the formula becomes $C_6{H}_{12}{O}_6$. But deoxyribose has a formula of $C_6{H}_{12}{O}_5$.
So rhamnose and deoxyribose are exceptions to the general formula of carbohydrates.