Assembly of a Reflux System for Heated Chemical Reactions

A reflux system is normally operated using a vertical connection of a water-cooled-glass column (reflux condenser) to the outlet of the reaction vessel. This piece of glassware consists of a jacketed column with water inlet and outlet ports that allow cold tap water to flow through the outer jacket, while reaction vapors are forced through the inner column. The flowing cold water ensures these vapors are condensed on the walls of the inner column and gravity will return these condensates to the reaction vessel. Once a steady reflux has been reached a constant drip of fluid back to the reaction solution should be established. In this fashion, reactions can be left indefinitely without ever needing more solvent to be added.  This video will explain the process of connecting the glassware and establishing a steady reflux.

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 refluxing solvent will be the methanol that is being produced (b.p. 65 °C). After heating the starting material (Figure 1) under reflux for 45 min, nuclear magnetic resonance (NMR) spectroscopy can be used to ensure product formation, as shown in Figure 2.

Scheme 1. Transesterification reaction between dimethyl terephthalate and ethylene glycol.

Figure 1. ¹H NMR spectrum of starting material: dimethyl terephthalate (DMT).

Figure 2. ¹H NMR spectrum of product: bis(2-hydroxyethyl) terephthalate.

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 extractions or fractional distillation. The later of which is used industrially on a huge scale, for instance in petroleum refineries in order to separate crude oil into various petrol fractions of different boiling points.