A hydroxy compound (X) with molar mass 122 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 g mol-1. The number of hydroxyl groups present in compound (X) is:
- 2
- 4
- 3
- 5
Answer
(2) 4
Solution
Acetylation replaces H of each –OH by an acetyl group:
R-OH + (CH3CO)2O R-OCOCH3 + CH3COOH
Each –OH → –OCOCH3 increases the molar mass by 43 - 1 = 42 g mol-1.
Increase in molar mass = 290 − 122 = 168
Number of –OH groups = 16842 = 4
(e.g. erythritol, C4H6(OH)4, M = 122)
Answer: (2)