Reactions of Esters

What are esters?

Ethyl acetate, systematically known as ethyl ethanoate, is one of the most common esters. If you give it a sniff, it smells characteristically fruity, like pear drops. We use it in a variety of different ways: as a solvent and diluent, to decaffeinate coffee and tea leaves, as a flavouring, and in toiletries. It is commonly found in perfumes as it is quite volatile and evaporates off the skin readily, leaving behind its pleasant scent.

Esters are derived from carboxylic acids and have the general formula RCOOR', as shown below:

Fig. 1 - The general structure of an ester. The R groups can be alkyl or aryl groups

Esters can be made in a variety of ways, but most commonly from carboxylic acids and alcohols. We name them using names based on these alcohols and carboxylic acids. The name derived from the alcohol comes first, followed by the name derived from the carboxylic acids. All esters end in the suffix -oate. For example, we call the ester made from propanol and methanoic acid propyl methanoate.

Fig. 2 - Propyl methanoate. Propyl is derived from propanol, shown in green, whilst methanoate is derived from methanoic acid, shown in purple

Let’s explore how we produce esters.

Esterification

Esterification is a reaction type that produces an ester. In this case, we react a carboxylic acid with an alcohol to produce an ester and water. This is a reversible reaction, meaning both the forward reaction and the backward reaction happen at the same time in a state of dynamic equilibrium.

Fig. 3 - Esterification. You’ll see this diagram again later in the article

Producing esters in a lab

Making esters is a common practical experiment that you might carry out in class.

To make esters at test tube scale, use a water bath to gently heat 10 drops of a carboxylic acid with 10 drops of an alcohol and 2 drops of a strong acid catalyst, such as sulfuric acid. You wouldn’t do this directly over an open flame because the organic liquids used are highly flammable.

Because this reaction is reversible, you’ll only produce a tiny amount of the ester. To smell it, pour the solution into a beaker of water. Longer chain esters are soluble, so will form a layer on top of the surface of the water, whilst the unreacted acid and alcohol will dissolve readily. If you waft the air over the top of the beaker, you should be able to smell the ester. Whilst short-chain esters such as methyl ethanoate, commonly known as methyl acetate, smell like solvents or glue, longer chain esters smell fruity and aromatic.

Fig. 4 - A diagram showing the production of an ester in a test tube

Let’s have a go at writing an equation. For example, reacting ethanoic acid with butanol produces butyl ethanoate, which smells like raspberry.

${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}\rightleftharpoons {\mathrm{CH}}_3{\mathrm{COOCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3+{\mathrm{H}}_2\mathrm{O}$

ethanoic acid butanol butyl ethanoate water

Fig. 5 - Butyl ethanoate. In this ester, R comes from ethanoic acid, shown in red, whilst R' comes from butanol, shown in green

Other methods of ester production

We can also make esters in other ways, such as:

• Reacting alcohols with acyl chlorides.
• Reacting alcohols with acid anhydrides.

We’ll look at both of these reaction types in more detail in Acylation.

Hydrolysis of esters

We can break down esters in two similar ways, using either an acid or a base as a catalyst. These reactions are known as hydrolysis reactions.

Acid hydrolysis

We mentioned above that esterification is a reversible reaction. If you mix a carboxylic acid and an alcohol with an acid catalyst, eventually the solution will reach a state of dynamic equilibrium. This just means that the molecules are constantly changing form, some combining into an ester and releasing water, and some returning back to an alcohol and carboxylic acid. When at equilibrium, the rate of the forward esterification reaction is the same as the backward reaction: we call this backward reaction hydrolysis.

Fig. 7 - Esterification and hydrolysis - two sides of the same reaction

To hydrolyse esters, mix them with a hot, aqueous acid under reflux conditions. In this case, the water from the aqueous acid acts as a nucleophile, which you’ll remember is an electron pair donor.

Favouring hydrolysis

You might know (see Equilibria) that we can change the conditions of a reversible reaction in order to favour one reaction or the other. Le Chatelier’s principle tells us that changing these conditions will cause the equilibrium to shift in the opposite direction to oppose the change. So how can we increase the rate of the backwards reaction, i.e., hydrolysis?

Well, one of the reactants is water. Therefore, by simply increasing the amount of water we use, we can favour the backward reaction. The equilibrium will move over to the left, to ‘use up’ the extra water we’ve added in. We do this by using an excess of the dilute acid catalyst. This is also why we use a concentrated acid to catalyse the forward reaction, esterification - using a minimal amount of water shifts the equilibrium to the right and increases the yield of the ester.

Base hydrolysis

We mentioned above that acid hydrolysis never goes to completion - it is a reversible reaction. We can instead hydrolyse esters using a base as a catalyst. This reaction goes to completion. Heating a hot, aqueous base such as a hydroxide with an ester under reflux produces a carboxylate salt and an alcohol.

For example, reacting methyl ethanoate with sodium hydroxide solution produces methanol and sodium ethanoate:

${\mathrm{CH}}_3{\mathrm{COOCH}}_3+\mathrm{NaOH}\to {\mathrm{CH}}_3\mathrm{COONa}+{\mathrm{CH}}_3\mathrm{OH}$

Sodium ethanoate is our carboxylate salt. It is made up of positive sodium ions and negative ethanoate ions ionically bonded together.

Fig. 8 - The structure of sodium ethanoate

But what if we want a pure carboxylic acid instead of a carboxylate salt? We can do the following:

• Distill the alcohol off.
• Add an excess of strong acid such as sulfuric acid or hydrochloric acid. This gives up an excess of ${\mathrm{H}}^{+}$ ions.
• The ${\mathrm{H}}^{+}$ ions are collected by carboxylate ions in solution, forming a carboxylic acid.
• Separate the carboxylic acid from the solution by further distillation.

Base hydrolysis is also known as saponification. Take a look at that term, and you’ll be able to guess what some specific carboxylate salts are used for - soap! We make soaps out of animal fats and vegetable oils, which we’ll explore later.

Comparing acid and base hydrolysis

The following table should help you summarise your knowledge of acid and base hydrolysis.

Fig. 9 - A table comparing acid and base hydrolysis of esters

Predicting the products

Have a go at the following question.

First, let’s draw out propyl ethanoate. This will help us see its structure and figure out its component parts.

Fig. 10 - Propyl ethanoate

Remember that esters are derived from carboxylic acids. Ethanoate tells us that the part of the molecule that comes from the acid is based on ethanoic acid. The other part of the name, propyl, tells us that the remainder of the molecule is based on a propyl chain. When the ester is broken down, this forms propanol.

Let’s now consider how propyl ethanoate is broken down using an acid. Remember that this is a reversible reaction - it doesn’t go to completion. The products are ethanoic acid and propanol.

${\mathrm{CH}}_3{\mathrm{COOCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{{\mathrm{H}}_2{\mathrm{SO}}_4}{\rightleftharpoons }{\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}\left(\mathrm{aq}\right)$

Acid hydrolysis of propyl ethanoate produces ethanoic acid, right, and propanol, left. StudySmarter Originals

If we use a base, the reaction does go to completion - but it produces a carboxylate salt instead of a carboxylic acid. Because we used sodium hydroxide, the salt formed is sodium ethanoate.

${\mathrm{CH}}_3{\mathrm{COOCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3\left(\mathrm{aq}\right)+\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{CH}}_3\mathrm{COOH}\left(\mathrm{aq}\right)+{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}\left(\mathrm{aq}\right)$

Saponification

We mentioned above that we can make soaps out of different fats and oils. This is known as saponification.

Fats and oils, collectively known as lipids, are also called triglycerides. This is because they are based on the alcohol glycerol. Glycerol has three -OH groups. In a triglyceride, each -OH group forms a bond with a carboxylic acid that has a long hydrocarbon tail. Hydrolysing a triglyceride using a base breaks it back down into glycerol, which we use in medication or to improve the performance of athletes; and carboxylate salts, which we use as soaps.

Fig. 11 - A triglyceride, left, breaks down in saponification into glycerol, above, and a carboxylate ion, below. The carboxylate ion forms a salt

How do soaps work?

Carboxylate salts are ionic. In solution, they dissociate to form a positive metal ion and a negative carboxylate ion. The carboxylate ion contains a polar end with the -COO group, and a nonpolar hydrocarbon tail. The polar end bonds with water whilst the nonpolar end bonds with other nonpolar molecules such as lipids, helping fats like scum and grease mix with water and be washed away.

Fig. 12 - A carboxylate salt

Biodiesel

In 2006, a shuttle bus operating at Yale University was successfully converted to run on 100 percent biodiesel - a form of renewable fuel derived from plants. This was a major step towards lowering the environmental impact of the transport industry. Before we finish, let’s quickly explore what biodiesel actually is.

Biodiesel is made from triglyceride esters from plant crops, such as rapeseed oil. When we react them with methanol using an alkali catalyst, we get long-chain methyl esters. We can burn these instead of fossil fuels. In fact, up to 10 percent of the diesel or petrol used in cars can be replaced by biodiesel without affecting their engines. Because biodiesel comes from quick-growing plant matter, it is carbon neutral, and a much more sustainable choice than fuels derived from crude oil.

Fig. 12 - A methyl ester, like those found in biodiesel.