Unit A23 4.9 Derivatives of carboxylic acids

Question 1

4.9.5 - Prepare a liquid ester from a carboxylic acid and an acohol

Answer 1

• Place a mixture of the alcohol and concentrated sulfuric acid in a pear shaped/round bottomed flask.
• Add a mixture of the alcohol and carboxylic acid slowly from a dropping funnel.
• Swirl the mixture
• Add some anti-bumping granules and heat under reflux
• Arrange the apparatus for distillation, heat the mixture gently and collect the distillate around 2° C either side of the boiling point of the ester
• Place the crude ester in a separating funnel and shake with sodium carbonate solution. Invert the funnel and open the tap occasionally to release pressure. Remove the stopper, allow the layers to separate and discard the aqueous layer; (to decide which layer is the aqueous one,
add 5 cm3 of water to the separating funnel, and the aqueous layer is the one which increases in volume)
• Add a spatula measure of anhydrous calcium chloride/sodium sulfate/magnesium sulfate (drying agent) to the organic layer in a flask, stopper and shake. Repeat until the ester changes from cloudy to clear.
• Filter or decant to remove calcium chloride.
• Redistill to remove any remaining organic impurities, collecting the fraction at the boiling point.

Note that for some small esters such as ethyl ethanoate, distillation can be used rather than reflux - the ester can be distilled off as soon as it is formed as it has the lowest boiling point of the substances present.