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.
A reaction equilibrium can be influenced with an excess of reagent or removal of a formed product in order to drive the equilibrium to the product side. Equilibria can be also influenced by temperature or pressure. This underlying principle is called Le Chatelier's principle and states that a change in temperature, pressure, concentration, or volume will cause a readjustment of the reaction to establish a new equilibrium. By adding an excess of reagent, the concentration changes and a new equilibrium establishes, favoring the product side. For instance, driving the equilibrium of a hydrolysis can be easily achieved by adding an excess of water.
Influencing the equilibrium of a reaction where water is formed as a byproduct, like an esterification, is not straightforward and requires special glassware. This special piece of glassware is called a Dean-Stark trap and helps to remove the formed water from the reaction medium (Figure 1). Solvents that form an azeotrope with water, like toluene, are commonly employed. An azeotrope is a point in a distillation where the composition of the liquid phase is equal to the composition of the gas phase. A further separation through a simple distillation past the azeotropic point is not possible. This is an advantage when using the Dean-Stark trap to influence equilibria, because it will ensure the continuous removal of water. Upon heating the reaction mixture, the formed toluene/water azeotrope will distill over, condense in the condenser, and flow into the Dean-Stark trap. Toluene and water will form two separate layers with toluene as the top layer and water as the bottom layer. While toluene can flow back into the reaction flask, water gets trapped as the bottom layer and ultimately removed from the reaction equilibrium thus driving the reaction towards the product side.
Figure 1. Dean-Stark apparatus
- Take a 250 mL round bottom flask equipped with a magnetic stir bar.
- Place an oil bath under the round bottom flask on a magnetic stirrer.
- Fill the round-bottom flask with 7.5 g (0.05 mol) m-Nitrobenzaldehyde and add 75 mL of toluene.
- Add 3.1 mL (3.45 g, 0.055 mol) ethylene glycol.
- Attach the Dean-Stark trap to the round bottom flask.
- Attach a reflux condenser on top of the Dean-Stark trap.
2. Running the Reaction
- Set the oil bath temperature to 170 °C and heat the reaction mixture to reflux.
- Monitor the reaction by measuring the water amount in the Dean-Stark trap.
- The reaction is done when no further water becomes trapped in the side arm of the Dean-Stark trap.
- After approximately 2 h, the total amount of collected water is approximately 0.8 mL.
- Release the water and remove the combined organic solvent from the reaction mixture under reduced pressure in a rotary evaporator.
- Dissolve the yellow residue in 8 mL ethanol under reflux.
- Cool down the solution.
- The desired acetal will crystallize.
- Filter the solid and dry it under reduced pressure.
The Dean-Stark trap is used to shift the equilibrium of organic reactions to the product side.
According to Le Châtalier's principle, an equilibrium can be driven toward the products using an excess of one of the reactants, by continuously removing one of the products, or by changing the temperature or the pressure at which the reaction is carried out. Perhaps the most commonly encountered equilibrium reactions are those involving water as a product.
As stated previously, the removal of this water can drive the reaction to completion. A Dean-Stark trap is a specialized piece of glassware used for continuously removing water formed in a chemical reaction.
This video will illustrate the principles of the Dean-Stark trap, a laboratory procedure in which the apparatus is used, and several applications.
Reactions such as the conversion of boronic acid to an ester result in the formation of water, which can hydrolyze the ester back to the acid, decreasing the overall yield.
As the reaction progresses, the water produced in the reaction may be continuously removed from the flask with the use of a Dean-Stark trap. In order to do that, first add the reaction components to a flask along with a hydrocarbon such as toluene and heat the mixture. As the reaction progresses, the water is released. Now toluene and water, which boil at 110 and 100 degrees, respectively, form an azeotrope, which boils at 84 degrees. Upon cooling in the condenser, the solvent vapors condense back to liquid, which drips into the collection vessel of the trap, and any overflow is returned to the reaction vessel.
The condensed liquid mixture eventually separates into two immiscible layers, with the denser component on the bottom. This is usually the water layer, which is then drained off. The same process is continued until no more water is produced, which indicates the completion of the reaction.
Now that we have discussed the principles of the Dean-Stark trap, let's look at a laboratory procedure in which the apparatus is used.
In this procedure, we will react an aromatic aldehyde with ethylene glycol to yield an acetal protecting group, which shields the reactive aldehyde from further chemical reactions in a multistep synthesis. To begin, add to a 250-mL round-bottomed flask a stirbar, 7.5 g of 3-nitrobenzaldehyde, 75 mL of toluene, and ethylene glycol. Then attach the Dean-Stark trap to the flask and a reflux condenser onto the top of the trap.
Lower the flask and its contents in an oil bath, turn on the water in the condenser, and stir at 170 degrees. Allow the azeotropic mixture to condense and collect in the trap, and continue until the water formation ceases. After the two layers separate, measure the amount of water produced, and compare it to the theoretical yield. To verify the completion of the reaction, run the starting material and products on a TLC plate.
Once the reaction is complete, remove the flask from the heat source and allow it to reach room temperature. Discard the contents of the Dean-Stark trap, as they should not contain any product, and concentrate the contents of the flask under reduced pressure with a rotary evaporator.
To remove impurities, dissolve the yellow residue in 8 mL of hot ethanol and allow it to cool to room temperature, allowing the product to crystallize. Then, filter the solid, rinsing with cold ethanol, and dry it under vacuum.
Now that we have seen a laboratory procedure let's look at some applications for which a Dean-Stark trap is used.
Enamines are substituted vinylamine compounds useful to form carbon-carbon bonds alpha to carbonyl groups. Enamines are prepared by heating a secondary amine, such as pyrrolidine, and an aldehyde or ketone, and removing the water byproduct with a Dean-Stark trap.
In addition to water, a Dean-Stark trap can be used to collect other compounds. Here, it was used to collect the product of an esterification reaction between benzoic acid and 1-butanol, which is also the reaction solvent. The 1-butanol is immiscible with and less dense than the product, and flows back into the reactor. The esterification product, which is hydrophobic, is also easily separated from the water byproduct.
An additional use for Dean-Stark traps is the determination of water content in foodstuffs. This is accomplished by placing a known amount of the food and boiling it in a hydrocarbon solvent. The volume of water collected from the distillate is measured, and divided by the weight of the food item to calculate the moisture percentage.
You've just watched JoVE's introduction to Driving Equilibria with Dean-Stark Traps. You should now understand the principles of Dean-Stark traps, how to perform a laboratory procedure, and some of its applications. Thanks for watching!
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.
Applications and Summary
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 organic solvent, which will form an azeotrope with water but does not mix with water in the liquid phase. The addition of an entrainer ensures the continuous removal of water, which becomes trapped in the side arm of the Dean-Stark trap. Unlike the Dean-Stark trap, a normal distillation apparatus requires the continuous addition of an entrainer since the distilled entrainer cannot flow back to the solvent mixture.
The Dean-Stark trap can also be used to drive the equilibria of reactions, where water forms as a byproduct, like in an ester or acetal formation. Through an azeotropic distillation where the solvent is also the entrainer, water is removed from the reaction and therefore from the equilibrium.
Finally, an azeotropic distillation with a Dean-Stark trap can also be used to determine the water content of solvents or solvent mixtures. Not only can water be removed with a Dean-Stark trap, but also volatile alcohols by placing 5-Å molecular sieves in the trap.