reflux

Reflux to apply energy to chemical reactions

Laboratory reflux apparatus for adding energy to a chemical reaction

A liquid reaction mixture is placed in a vessel open only at the top. This vessel is connected to a Liebig condenser, such that any vapours given off are cooled back to liquid, and fall back into the reaction vessel. The vessel is then heated vigorously for the course of the reaction. The purpose is to thermally accelerate the reaction by conducting it at an elevated temperature (i.e. the solvent's boiling point).

The advantage of this technique is that it can be left for a long period of time without the need to add more solvent or fear of the reaction vessel boiling dry as any vapour is immediately condensed in the condenser. In addition, as a given solvent will always boil at a certain temperature, one can be sure that the reaction will proceed at a constant temperature. By careful choice of solvent, one can control the temperature within a very narrow range. The constant boiling action also serves to continuously mix the solution, although a magnetic stirring rod mechanism is often used to achieve a uniform solution. This technique is useful for performing chemical reactions under controlled conditions that require substantial time for completion.

The diagram shows a typical reflux apparatus for applying energy to chemical reactions. It includes an optional beaker of water between the reactants and the heat. This is often used as a safety precaution when using flammable reactants and a Bunsen burner in order to keep the flame away from the reactants. In modern laboratories, open flames are avoided due to the many flammable solvents often in use, and electrical heating, (i.e., with a hot plate or mantle) is preferred. Furthermore, a high boiling, thermally stable silicone oil is generally used to immerse the reaction vessel, rather than water which evaporates too readily to be useful for lengthy reactions. Using an oil bath, temperatures of up to several hundred degrees can easily be achieved, which is higher than the boiling point of most commonly used solvents. If even higher temperatures are required, the oil bath can be replaced with a sand bath.

Reflux in beverage distillation

By controlling the temperature of the condenser, a reflux still may be used to ensure that higher boiling point components (which are also of higher molecular weight) are returned to the flask while lighter elements are passed out to a secondary condenser. This is useful in producing high quality alcoholic beverages, while ensuring that less desirable components (such as fusel alcohols) are returned to the primary flask. This is particularly effective in the production of alcoholic beverages in which it is appropriate to retain the flavors and aromas of the source fruit - such as applejack, although of course applejack is traditionally freeze-distilled from cider and thus contains relatively high levels of fusels and other impurities. For high quality neutral spirits (such as vodka), or post distillation flavored spirits, a process of multiple distillations or charcoal filtering may be applied to obtain a product lacking in any suggestion of its original source material for fermentation.

See also

• Batch distillation
• Fractional distillation
• Fractionating column
• McCabe-Thiele method

References

1. ^ Kister, Henry Z. (1992). Distillation Design (1st Edition ed.). McGraw-Hill. ISBN 0070349096. 
2. ^ Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook (6th Edition ed.). McGraw-Hill. ISBN 0070494797. 

External links

• Distillation column components

Gallery

Toluene is refluxed with sodium-benzophenone desiccant before it is distilled to give pure oxygen- and water-free toluene.

Industrial fractionating columns all of which use reflux

Organic synthesis apparatus using reflux

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