Question

A comppound of molecular wight $180$ is acetylated to give a compound of molecular weight $390$. The number of Alcoholic groups in the compound is:

• A. $6$
• B. $5$
• C. $4$
• D. $3$

Answer 1

The correct option is B $5$

Acetylation of an alcohol takes place in the following manner:
$R-OH\stackrel{Acetylation}{\to }R-OAc$
Thus, it is clear that each $-OH$ group is replaced by a $-OAc$ group. Now, the weight of an acetyl group $\left(Ac\right)$ is 43 and the weight of the lost Hydrogen atom is $1$. Upon completion of reaction, for each $-OH$ group present in a molecule, there is an increase of $43-1=42$ units of weight.
Now, net increase in weight of the given molecule $(390-180)=210$
Thus, total number of $-OH$ groups in the original molecule is $\frac{210}{42}=5$
Hence, Option B is the correct answer.