Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA

A Dean-Stark trap is a special piece of glassware, which allows the collection of water during a reaction through an azeotropic distillation. The desire to collect water from a reaction can have various reasons. It can drive the equilibria in reactions, where water is formed as a byproduct. According to Le Chatelier's principle, a change in temperature, pressure, concentration, or volume will cause a readjustment of a reversible reaction to establish a new equilibrium. An acetal formation is a reversible reaction, where water is formed as a byproduct. In such cases, achieving good yields is possible by driving the equilibrium towards the product side via the removal of water. The Dean-Stark trap also allows the determination of water content or can be used to remove water from a solvent mixture through an azeotropic distillation.

1. Preparation

1. Take a 250 mL round bottom flask equipped with a magnetic stir bar.
2. Place an oil bath under the round bottom flask on a magnetic stirrer.
3. Fill the round-bottom flask with 7.5 g (0.05 mol) m-Nitrobenzaldehyde and add 75 mL of toluene.
4. Add 3.1 mL (3.45 g, 0.055 mol) ethylene glycol.
5. Attach the Dean-Stark trap to the round bottom flask.
6. Attach a reflux condenser on top of the Dean-Stark trap.

2. Running the

Water will form and becomes trapped over the course of the reaction. The theoretical amount of formed water upon complete conversion can be calculated and compared with the measured amount of the trapped water to determine the reaction progress.

This experiment demonstrates vividly Le Chatelier's principle and how it can drive an equilibrium.

Dean-Stark traps are commonly used to remove water from a solvent mixture under various circumstances. For example, the removal of water through a simple distillation when water does not form an azeotrope with the other solvent, is possible with a Dean-Stark trap based on its design. In the case of an azeotropic distillation, the addition of an entrainer is necessary. An entrainer is an organ