Source: Laboratory of Dr. Philip Miller — Imperial College London

Many chemical experiments require elevated temperatures before any reaction is observed, however heating solutions of reactants can lead to loss of reactants and/or solvent via evaporation if their boiling points are sufficiently low. In order to ensure no loss of reactants or solvent, a reflux system is used in order to condense any vapors produced on heating and return these condensates to the reaction vessel. 

1. Preparation of Glassware

1. Ensure the glassware is free of chemical contaminants and if necessary of moisture by allowing to heat in an oven at 100 °C for approximately 30 min.
2. Once cooled, ensure all ground glass joints are contaminant free using either a dry or acetone soaked cleaning tissue specially made for laboratory applications.
3. After dissolving regents in a suitable solvent in an appropriate reaction vessel (typically a round-bottomed flask) and adding a magnetic stirrer bar

The outcome can be observed after spectroscopic characterization of the resultant solution, as the two reagents should now have reacted to form a new product. Typically, various purification strategies will be required to separate the desired product from undesired side reactions.

In this example, a transesterification reaction between dimethyl terephthalate (DMT) and ethylene glycol has occurred to afford bis(2-hydroxyethyl) terephthalate and methanol (Scheme 1). The refluxin

Performing reactions under reflux is an important technique to understand. Apart from providing a system whereby solvent and volatile reagents are recycled, it also allows fine control of reaction temperature, as this will be held constant at the boiling point of the chosen solvent. By careful choice of solvent, one can control the temperature within a very narrow range.

More advanced techniques can utilize refluxing solvents to perform sophisticated purification techniques such as Soxhlet ext