Acetylation or Esterification to get Aspirin?

There are many methods for obtaining esters. In the pharmaceutical industry, the most important factors determining the choice of the process are the costs and the ease of obtaining a clean product or its purification. Normal esterification (acid and alcohol) is an equilibrium reaction. The use of acetic anhydride or acetyl chloride bypasses the equilibrium process.

Acetate esters in particular are often best made via acetic anhydride.  While reaction with acetyl chloride is also favorable, it requires an equivalent of base (usually pyridine or trimethylamine) to remove the HCl byproduct.  With acetic anhydride, once can use sodium acetate as the base.  This is milder, less toxic, and can be used catalytically (0.1 equivalent or so).  Other anhydrides are not commercially available and inexpensive (as acetic anhydride is), so this method is not as attractive for making butanoates and so forth. 

thx AWK!