Two heating methods are used when preparing organic liquids, for example in the oxidation of alcohols.Heating under reflux
The reaction mixture is heated in a round-bottomed flask with a condenser fitted vertically above it. Vapour rises, condenses in the condenser and drips back into the flask. So volatile reactants and products cannot escape, and the mixture can be heated for a long time until the reaction is complete. Flammable vapours are also kept in.
Cold water enters the condenser at the bottom and leaves at the top. The top of the condenser is left open so that pressure cannot build up. Anti-bumping granules give smooth boiling. An electric heater or water bath is used, not a naked flame.
Distillation separates a liquid from a mixture by its boiling point. The flask is heated and the condenser slopes downwards, so vapour condenses and runs into a collecting vessel. The thermometer bulb is level with the side arm, to measure the temperature of the vapour that is distilling.Which one to use
Primary alcohol to aldehyde: distil as the aldehyde forms, so it is removed before it can be oxidised further.
Primary alcohol to carboxylic acid: heat under reflux, so the aldehyde stays in the flask and is fully oxidised. The acid is then separated by distillation.Worked example: ethanal boils at 21 °C, ethanol at 78 °C and ethanoic acid at 118 °C. On warming, ethanal has by far the lowest boiling point, so it distils off first and is collected before it can be oxidised further.
The reaction mixture is heated in a round-bottomed flask with a condenser fitted vertically above it. Vapour rises, condenses in the condenser and drips back into the flask. So volatile reactants and products cannot escape, and the mixture can be heated for a long time until the reaction is complete. Flammable vapours are also kept in.
Cold water enters the condenser at the bottom and leaves at the top. The top of the condenser is left open so that pressure cannot build up. Anti-bumping granules give smooth boiling. An electric heater or water bath is used, not a naked flame.
Distillation separates a liquid from a mixture by its boiling point. The flask is heated and the condenser slopes downwards, so vapour condenses and runs into a collecting vessel. The thermometer bulb is level with the side arm, to measure the temperature of the vapour that is distilling.Which one to use
Primary alcohol to aldehyde: distil as the aldehyde forms, so it is removed before it can be oxidised further.
Primary alcohol to carboxylic acid: heat under reflux, so the aldehyde stays in the flask and is fully oxidised. The acid is then separated by distillation.Worked example: ethanal boils at 21 °C, ethanol at 78 °C and ethanoic acid at 118 °C. On warming, ethanal has by far the lowest boiling point, so it distils off first and is collected before it can be oxidised further.