Many organic reactions are slow at room temperature, so the mixture must be heated for a long time. But organic reactants and products are often volatile and would boil away from an open flask. Heating under reflux solves this: the mixture is heated with a condenser fitted vertically above the flask, so vapour condenses and drips back into the flask. The reaction can continue for as long as needed without losing material.
Anti-bumping granules: added so the liquid boils smoothly rather than in sudden violent bursts.
Condenser: a Liebig condenser fitted vertically. Cooling water enters at the bottom and leaves at the top, so the jacket stays full of cold water.
Open top: the top of the condenser is never sealed, or pressure would build up and the apparatus could burst.
Heat source: an electric heating mantle, a water bath or an oil bath, because most organic liquids are flammable.Worked example: oxidising ethanol. Heating ethanol under reflux with excess acidified potassium dichromate(VI) keeps any ethanal that forms in the flask, so it is oxidised further to ethanoic acid. If ethanal is wanted instead, the apparatus is set up for distillation and the ethanal is distilled off as soon as it forms, before it can be oxidised further.After refluxing, the apparatus can be rearranged for distillation to separate the product from the reaction mixture.
Anti-bumping granules: added so the liquid boils smoothly rather than in sudden violent bursts.
Condenser: a Liebig condenser fitted vertically. Cooling water enters at the bottom and leaves at the top, so the jacket stays full of cold water.
Open top: the top of the condenser is never sealed, or pressure would build up and the apparatus could burst.
Heat source: an electric heating mantle, a water bath or an oil bath, because most organic liquids are flammable.Worked example: oxidising ethanol. Heating ethanol under reflux with excess acidified potassium dichromate(VI) keeps any ethanal that forms in the flask, so it is oxidised further to ethanoic acid. If ethanal is wanted instead, the apparatus is set up for distillation and the ethanal is distilled off as soon as it forms, before it can be oxidised further.After refluxing, the apparatus can be rearranged for distillation to separate the product from the reaction mixture.