Question

Hydroxy compound ($\text{A}$) with molecular mass $= 122$ react with excess of acetic anhydride and gives compound ($\text{X}$) with molecular mass $= 290$, then find the no. of hydroxy groups in given compound ($\text{A}$).

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

Hydroxyl group estimation using acetylation relies on mass conservation. Calculate the total mass increase and divide by $42 \text{ g/mol}$ (the mass of the $\text{COCH}_3$ group replacing the $\text{H}$ atom).

Answer 1

The Correct Option is C

The reaction of an alcohol ($\text{R}-(\text{OH})_n$) with acetic anhydride acetylates $n$ hydroxyl groups.
$\text{R}-(\text{OH})_n \rightarrow \text{R}-(\text{OCOCH}_3)_n$.
Mass change per $\text{OH}$ group $=\text{Mass}(\text{COCH}_3) - \text{Mass}(\text{H}) = 43 - 1 = 42 \text{ g/mol}$.
Total mass change $\Delta M = M_X - M_A = 290 - 122 = 168 \text{ g/mol}$.
Number of $\text{OH}$ groups $n = \frac{\Delta M}{\text{Mass added per } \text{OH}}$.
$n = \frac{168}{42}$.
$n = 4$.