Purpose of reflux
Heating under reflux allows an organic reaction mixture to be heated for an extended period while reducing the loss of volatile substances. Vapour rises into a vertical condenser, cools and returns to the reaction flask as liquid.
The preparation of an ester provides a suitable example. Ethanol and ethanoic acid can be heated with concentrated sulfuric acid to prepare ethyl ethanoate.
Setting up the reaction
- Add of ethanol and of glacial ethanoic acid to a pear-shaped flask.
- Add 15 drops of concentrated sulfuric acid.
- Add a few anti-bumping granules.
- Fit the condenser vertically above the reaction flask.
- Partly immerse the flask in a water bath and heat the mixture under reflux for about 15 minutes.
- Stop heating and allow the mixture to cool.
Anti-bumping granules help the mixture boil more evenly by encouraging small bubbles rather than sudden large ones.
Condenser and heating details
- Cooling water enters through the lower condenser connection and leaves through the upper connection.
- The top of the condenser remains open so that pressure cannot build inside the heated apparatus.
- A water bath or electric heater avoids placing flammable organic liquids close to a naked flame.
Safety
- Concentrated sulfuric acid and ethanoic acid are corrosive, so suitable eye and hand protection is required.
- Ethanol is flammable and should be kept away from naked flames.
- Ethyl ethanoate is flammable, toxic and strongly odorous, so it should be handled in a fume cupboard.
Exam Tip: In a reflux diagram, draw the condenser vertically and leave its upper end open. Show cooling water entering through the lower connection and leaving through the upper connection.