Illustrated Glossary of Organic Chemistry

Le Chatelier's Principle (Chatelier's Principle): In simple terms, when one side of an equilibrium is altered by changing concentration, pressure, etc. the equilibrium shifts to counteract this change, and restore the equilibrium.  For example, when the concentration of a reactant is increased, the equilibrium shifts to use up the increased amount of reactant, thereby producing more product. Decreasing the concentration of a product by removing it from the equilibrium (for example by precipitation or evaporation) causes more reactants to be consumed to restore the product concentration. When the product is continuously removed the equilibrium will continuously shift, producing more product, until all of the reactants are consumed.

Heating a 1:1 mixture of benzoic acid and methanol with a catalytic amount of sulfuric acid produces an ester and water. At equilibrium, the concentrations of the reactants and products are about equal, and K_eq ~ 1. This is not a very efficient way to transform a carboxylic acid into an ester.


The Fischer esterification reaction takes advantage of Le Chatelier's principle to increase the amount of carboxylic acid that is esterified. The equilibrium is shifted towards products by using a large excess of the alcohol (it is used as the reaction solvent), and (in some cases) also removing water as it it formed.