Question

A hydroxy compound $(X)$ with molar mass $122\,\text{g mol}^{-1}$ is acetylated with acetic anhydride, using a large excess of the reagent ensuring complete acetylation of all hydroxyl groups. The product obtained has a molar mass of $290\,\text{g mol}^{-1}$. The number of hydroxyl groups present in compound $(X)$ is ___.

• A. $3$
• B. $5$
• C. $4$
• D. $2$
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Hint:

Acetylation is commonly used to determine the number of hydroxyl groups in organic compounds.

Answer 1

The Correct Option is C

Step 1: Understanding acetylation.
Each hydroxyl group $(-OH)$ when acetylated adds an acetyl group $(-COCH_3)$, increasing molar mass by:
\[ \text{Increase per } -OH = 42\,\text{g mol}^{-1} \] Step 2: Calculating total increase in molar mass.
\[ \Delta M = 290 - 122 = 168\,\text{g mol}^{-1} \] Step 3: Finding number of hydroxyl groups.
\[ n = \frac{168}{42} = 4 \] Step 4: Final conclusion.
The compound $(X)$ contains $4$ hydroxyl groups, corresponding to option (3).