Question

A compound with molecular formula, C₄H₁₀O₃ 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 (CH₃CO – molar mass = 43 amu)

—OH + (CH₃CO)₂O → —O—COCH₃ + CH₃COOH

In other words acetylation of each OH group increases the molecular mass by 43 – 1 = 42 amu. Now that the molecular mass of the compound C₄H₁₀O₃ = 106 amu, while that of the acetylated product is 190 amu.

Therefore the number of ‘OH’ groups present in the compound is \(\frac{190 - 106}{42}\) = 2.