Question

in preparation of aspirin, please rank the following catalysts in increasing order of reactivity with salicyclic acid and explain why: a) sodium acetate b) pyridine c) boron trifluoride d) sulfuric acid

Answer 1

The order of reactivity of the catalysts is as following:

            H₂SO₄ > C₅H₅N > BF₃ > CH₃COONa

Sulfuric Acid protonates the carbonyl group of the acetic anhydride used for acylation of salicylic acid which facilatates the nucleophilic attack at the carbonyl carbon by the phenolic -OH group of salicylic acid.

When pyridine is used the phenolic -OH group is deprotonated to increase the nucleophilicity. But there is a carboxylic acid group which is also an acidic group and more acidic as compared to the phenoilc -OH. Pyridine is acetylated in presence of acetic anhydride which lowers the catalytic activity.

Boror Trifluoride also acts as a catalyst but as the compound is itself very reactive a higher catalytic load is always required.

Sodium Acetate acts as a buffer and shows a minute catalytic activity.