Why reflux is used
Some organic reactions require prolonged heating. Reflux allows this without continuously losing volatile material from the reaction flask.
Vapour produced in the heated flask rises through a vertical condenser. Cooling converts the vapour back into liquid, which then runs down into the same flask.
- The reaction mixture is heated in a round-bottomed flask.
- Vapour travels upwards through the central tube of the condenser.
- Cooling water flows through the surrounding condenser jacket.
- The condensed liquid returns to the reaction mixture rather than being collected separately.
- The central tube remains open at the top so pressure cannot build up inside the heated apparatus.
Water is supplied through the lower side-arm and leaves through the upper side-arm.
Safe, smooth heating
Anti-bumping granules help the liquid boil more steadily by favouring the production of small bubbles instead of sudden large ones.
Electric heating is appropriate for many organic mixtures because a naked flame could ignite flammable organic substances.
Complete oxidation of a primary alcohol
Heating a primary alcohol under reflux with excess acidified potassium dichromate(VI) allows oxidation to continue to a carboxylic acid.
For propan-1-ol:
The dichromate(VI) mixture changes from orange to green as forms. After the oxidation is complete, distillation can be used to remove the organic product from the reaction mixture.
Exam Tip: Draw reflux with a vertical condenser and an open central tube. Show water entering low down and leaving higher up. The outer water jacket is closed at its top and bottom, while the two water connections remain open. Avoid drawing an extra closing line across the joint between the flask and condenser.