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Organic Chemistry II

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Dean-Stark Trap



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!

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