Question

In which of the following carbohydrate, molecular mass increases by 84u after complete acetylation?

• A. Aldotriose
• B. Aldotetrose
• C. Ketotetrose
• D. Ketopentose

Hint:

Chemistry Tip: Number of acetyl groups introduced = number of hydroxyl groups present.

Answer 1

The Correct Option is A

Step 1: Understand acetylation. During acetylation, each hydroxyl group $(-OH)$ is converted into acetate group $(-OCOCH_3)$. Net increase in mass for one hydroxyl group is: $$ CH_3CO - H = 43 - 1 = 42u $$ So, each $-OH$ group increases molecular mass by $42u$.

Step 2: Total increase given. Total increase in mass after complete acetylation: $$ 84u $$

Step 3: Find number of hydroxyl groups. $$ \text{Number of } -OH \text{ groups}=\frac{84}{42}=2 $$

Step 4: Identify carbohydrate with two hydroxyl groups. Aldotriose structure: $$ CHO-CHOH-CH_2OH $$ It contains exactly two hydroxyl groups.

Step 5: Conclusion. Hence the carbohydrate must be aldotriose. $$ \therefore \text{Correct option is (A) Aldotriose.} $$