Reactions of Esters
Carboxylic Acids Worksheet
1. Formation of Esters (Esterification)
Esters can be formed from carboxylic acids and alcohols. Write the equations for the following reactions using skeletal formulae.
a) The reaction between ethanol and methanoic acid.
b) The formation of propyl ethanoate.
c) The reaction between methylpropanoic acid and propan-2-ol.
d) The reaction between ethanedioic acid and ethanol.
e) The reaction between propanoic acid and butan-2-ol
2. Lab Preparation
You create ethyl ethanoate in a lab.
a) State the reagents and conditions for the reaction.
Concentrated sulfuric acid, heat under reflux.
b) Describe the process for purifying the product.
- Add sodium carbonate solution to remove any unreacted carboxylic acid (neutralisation) and wash out water-soluble impurities.
- Use a separating funnel to separate the organic ester layer from the aqueous layer.
- Add a drying agent, such as anhydrous calcium chloride, to remove traces of water.
- Filter to remove the solid drying agent.
3. Mechanism: Esterification
While the mechanism for the formation of esters is not on the A-level course, it is important that you can interpret and predict unfamiliar mechanisms. Draw out the following steps for the reaction between methanoic acid and methanol.
a) The lone pair on the carbonyl oxygen acts as a base, accepting a hydrogen from the sulfuric acid catalyst.
b) The following intermediary is formed and is attacked by a lone pair on the methanol’s oxygen.
c) The following intermediary is formed. The hydrogen sulfate formed earlier in the reaction acts as a base, accepting the hydrogen from the intermediary.
4. Hydrolysis of Esters
Esters can be hydrolysed via two different methods.
a) Write the equation for the acid hydrolysis of Propan-2-yl methanoate (isopropylmethanoate).
b) Write the equation for the basic hydrolysis of methyl-2-methylpropanoate.
c) Ethanedioic acid is being formed from Dimethyl Ethanedioate. It is boiled under reflux with NaOH and is then reacted with dilute HCl. Explain why this method is used rather than just reacting the Dimethyl Ethanedioate with an acid catalyst.
Basic hydrolysis is a forward reaction (goes to completion), whereas acid hydrolysis is a reversible reaction (equilibrium). This means you get a much better yield using NaOH (to form the salt) followed by HCl (to protonate the salt into the acid) compared to acid hydrolysis.
5. Mechanism: Hydrolysis
While the mechanism for the hydrolysis of esters is not on the A-level course, it is important that you can interpret and predict unfamiliar mechanisms.
a) Copy and complete the steps for the basic hydrolysis of methylmethanoate by adding in curly arrows to show the movement of pairs of electrons.
