Question

In the reaction between acetic acid and salicylic acid for the formation of the ester, why is acetic anhydride used instead of acetic acid to attach this group?

Answer 1

Because the reaction with acetic acid is unfavorable: the reaction has a number of flaws, but most notable is the fact that alcohol+ carboxylic acid = ester + water, and the water formed, along with the water present in the environment, hampers the reaction. Because both sides of the reaction are very close in energy, a small amount of aspirin will form, but the yield will be very poor- both sides form an equilibrium in the presence of a catalyst. Best case is 50% or so yield.

The anhydride, though, is "dehydrated" in a sense: anhydride + water = 2 carboxylic acids. There's a lot of energy pumped into the anhydride to make it. So anhydride plus alcohol = ester plus carboxylic acid, leaving another carboxylic acid to make another ester before it forms water. The high energy of the anhydride sits it on a potential energy "hill", making the reaction all downhill and favorable, rather than mediocre and equilibrium driven. The yield is much higher, and can be over 90% relative to starting alcohol.

I choose to look at it this way: you can either remove water before or after, and removing it before turns out to work better.