Let's dive into the world of esters and their reactions. First off, let's talk about esterification and hydrolysis. By altering reaction conditions, we can change the outcome of these reactions. Esters have many uses, such as being used in biodiesel, oils, and soaps. They can be formed by combining an alcohol and a carboxylic acid or by acylation with acyl chlorides or acid anhydrides. They can also be broken down through hydrolysis with a base or an acid catalyst. The process of base hydrolysis is also called saponification. As you can see, there are a variety of ways in which esters can react!

What are esters?

Have you ever smelled something fruity and thought of pear drops? Well, that scent you're picking up might be ethyl acetate, also known as ethyl ethanoate. This ester is one of the most commonly used and has many different purposes. It's used as a solvent and diluent, to decaffeinate coffee and tea leaves, as a flavouring, and even in toiletries. Ethyl acetate is often found in perfumes because it evaporates off the skin easily, leaving behind its lovely scent.

Now, let's talk about the science behind esters. Esters are organic molecules that have a functional group called -COO-. They're made by combining carboxylic acids and have a general formula of RCOOR'. So, next time you're enjoying a fruity scent or using a scented product, remember that esters are what give it that lovely aroma.

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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.

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Let’s explore how we produce esters.

Esterification

Let's talk about a reaction called esterification. This type of reaction produces an ester by combining a carboxylic acid and an alcohol, resulting in the production of both an ester and water. It's important to note that this reaction is reversible, which means that the forward reaction and the backward reaction happen simultaneously in a state of dynamic equilibrium. To learn more about reversible reactions, take a quick peek at our section on Equilibria. Check out the diagram below to get a better understanding of how esterification works. We'll revisit this diagram later on 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.

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

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Large-scale ester production is a little different to test-tube ester production, and depends on the type of ester you want to create. To make short-chain esters such as ethyl ethanoate, CH3COOCH2CH3, heat ethanol and ethanoic acid with a strong, concentrated acid catalyst and distill off the product, i.e., the ester. The ester has the lowest boiling point out of all the substances involved because it cannot form hydrogen bonds with itself, unlike alcohols and carboxylic acids. Distilling off the product also shifts our equilibrium to the right, increasing the yield of the reaction. However, if we want to make longer chain esters we have to use reflux. Reflux involves heating a reaction mixture in a sealed container. This means that any volatile components that evaporate condense and fall back into the reaction mixture, preventing them from evaporating before they can react.

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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.

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.

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.

Although we can shift the position of the equilibrium, acid hydrolysis will never give us a 100 percent yield because it is one half of a reversible reaction.

Base hydrolysis

To hydrolyze esters, we can use a base as a catalyst instead of acid hydrolysis, which is a reversible reaction. The hydrolysis of esters using a base as a catalyst is a complete reaction. When an ester is heated with a hot, aqueous base such as hydroxide under reflux, it produces a carboxylate salt and an alcohol. A salt is formed when negatively charged ions are ionically bonded to positively charged cations, producing a giant lattice structure.

For instance, reacting methyl ethanoate with a solution of sodium hydroxide produces methanol and sodium ethanoate, which is our carboxylate salt. It is composed of positive sodium ions and negative ethanoate ions ionically bonded together, resulting in the structure of sodium ethanoate.

But what if we want to obtain a pure carboxylic acid instead of a carboxylate salt? Here's how we can do it:

• First, distill the alcohol off.
• Then, add an excess of strong acid such as sulfuric acid or hydrochloric acid, which gives up an excess of ions.
• The ions combine with carboxylate ions in solution, forming a carboxylic acid.
• Finally, separate the carboxylic acid from the solution by further distillation.

Base hydrolysis is also known as saponification. Some specific carboxylate salts are used for making 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.

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Predicting the products

To break down propyl ethanoate, we can use either sulfuric acid or sodium hydroxide. Here are the equations for each reaction and the products formed:

1. Acid hydrolysis:
   Propyl ethanoate + sulfuric acid → ethanoic acid + propanol

The products formed are ethanoic acid and propanol.

2. Base hydrolysis:
   Propyl ethanoate + sodium hydroxide → sodium ethanoate + propanol

The products formed are sodium ethanoate and propanol. However, it's important to note that this reaction goes to completion, producing a carboxylate salt (sodium ethanoate) instead of a carboxylic acid (ethanoic acid). 

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. 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. 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.

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Here are the key takeaways about reactions of esters:

• Esters are organic molecules with the functional group -COO-.
• Esters are made in esterification reactions, which are reversible reactions between an alcohol and a carboxylic acid, using a strong acid as a catalyst.
• Esters can be hydrolysed using an acid or a base.
• Acid hydrolysis is the reverse of esterification, and as a result doesn't go to completion.
• Base hydrolysis produces a 100 percent yield, but produces a carboxylate salt instead of a carboxylic acid. Base hydrolysis of esters is also known as saponification.
• When triglycerides are hydrolysed, they produce glycerol and long-chain carboxylate salts, which are commonly used as soaps.
• Biodiesel is made from plant triglycerides and methanol. It is a sustainable fuel choice compared to crude oil.

Reactions of Esters

What kind of reaction forms an ester?

Esters are formed in esterification reactions between carboxylic acids and alcohols, and in acylation reactions between alcohols and either acyl chlorides, or acid anhydrides.

Why is the acid hydrolysis of esters a reversible reaction?

Ester acid hydrolysis is reversible because it forms a carboxylic acid and an alcohol, which are able to react with each other to form an ester again.

What are esters commonly used for?

Esters have fruity scents and so are commonly used as flavourings in foods. Esters are also used in the manufacture of soaps and drugs like aspirin. Another use of esters is as a polyester polymer, used to make fabrics and plastics. 

What reaction breaks esters?

Esters are broken down in hydrolysis reactions. These use either an acid or a base as a catalyst. Acid hydrolysis is a reversible reaction and produces an alcohol and a carboxylic acid. Base hydrolysis goes to completion and produces an alcohol and a carboxylate salt.

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