Summary

A Dean Stark is a form of glassware used in certain chemical experiments to extract water from a reaction mixture. This apparatus was invented by Ernest W. Dean and David D. Stark in 1920. They described it as "an original method for the determination of the quantity of water and other organic emulsions contained in petroleum".

Learning Goals

• To illustrate what the Dean-Stark apparatus consists of, and how it is used.
• To illustrate how a Dean Stark apparatus can shift a chemical equilibrium, and so increase the yield of the reaction.
• To illustrate the separation of water and ester in order to shift an equilibrium point.

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An ester is an organic compound that is much used in industry (textile fibers, waxes, solvents) notably for their olfactive properties (aromas for the food inductry, perfumes...). The chemical reaction that enables us to obtain them is called esterification.  An esterification reaction can be represented as follows:

Carboxylic Acid  + alcohol  = ester  + water

This reaction is slow and limited. Thus, with the reactants present in stoichiometric proportions, the rate of reaction is 66% when alcohol is used.

This reaction is slow and limited because it is reversible. In effect, a part of the ester and water produced by the reaction undergoes the reverse reaction (hydrolysis), which destroys some of the ester, converting it back to acid and alcohol.

One can improve the yield of the esterification and its rate of the reaction that produces it, by inhibiting hydrolysis. One can do this by using a catalyst (another acid), which will remove some of the water produced in the reaction, and so limit the rate of hydrolysis. One can also use heat to increase the rate of the reaction.

Esters and water are not miscible in solution and it is this property which the Dean Stark apparatus exploits. By eliminating a product of the reaction mixture (here, the water), one shifts the equilibrium of the reaction in favor of further esterification, and so increase the yield of ester.