Question

A compound with molecular formula, $$C_4 H_{10} O_3$$ on acetylation with acetic anhydride gives a compound with molecular weight $$190$$. Find out the number of hydroxyl groups present in the compound.

Answer 1

During acetylation, one H-atom (at. mass = 1 amu) of the $$OH$$ group is replaced by an acetyl group. i.e., $$CH_3 CO$$ (mol. mass $$= 43 $$ amu).
$$-OH + (CH_3 CO)_2O \rightarrow -O-COCH_3 + CH_3COOH$$
In other words, acetylation of each $$OH$$ group increases the mass by $$43 - 1 = 42$$ amu. Now the mol. mass of $$C_4H_{10} O_3 = 106$$ amu while that of the acetylated product is $$190$$ amu, therefore, the number of $$OH$$ group in the compound $$= \dfrac{190 - 106}{42} = 2$$.