Lesson Explainer: Reactions of Esters Chemistry

In this explainer, we will learn how to describe the reactions of esters and predict what products are formed.

We will look specifically at the hydrolysis (including saponification), ammonolysis, and polymerization of noncyclic esters.

An ester is a type of chemical. Esters contain the ester functional group.

Ester groups can react with a number of different chemicals, including water, ammonia, and alcohols. These are the reactions we are going to be looking at.

In the lab, there are two types of chemicals commonly used to hydrolyze esters: acids and bases.

An acid-catalyzed ester hydrolysis will produce a carboxylic acid and an alcohol. The acid is regenerated in the process.

Ester hydrolysis using a base will produce a carboxylate salt and an alcohol. The carboxylate can be converted to a carboxylic acid by adding an acid.

In both cases, the reactions are usually carried out under reflux. During a reflux reaction, the reaction mixture is heated to boiling point, but a condenser connected to the reaction vessel cools and condenses the vapors. Liquid drips back into the reaction vessel and is not lost to the surroundings; this allows for high temperatures that accelerate the reaction without the reactants, products, or solvent escaping.

Acids that are commonly used for acid-catalyzed ester hydrolysis are dilute forms of the following:

• Hydrochloric acid ()
• Sulfuric acid ()

The general equation for acid-catalyzed ester hydrolysis is shown:

Acid hydrolysis of an ester is a reversible reaction, so an excess of water can be used to increase the equilibrium yield.

Bases that are commonly used for base-induced ester hydrolysis are strong bases such as the following:

• Sodium hydroxide ()
• Potassium hydroxide ()

The general equations are shown:

Hydrochloric acid () is commonly used in step 2.

In an acid-catalyzed ester hydrolysis, the acid acts solely as a catalyst. However, in the base-induced variety, the base is a reactant and 1 equivalent of the base is required per equivalent of the ester group.

Reacting an ester with a strong base goes to completion, so the yield will be higher than for an acid-catalyzed ester hydrolysis.

Saponification is the name for the process of converting fats, oils, and other lipids into soaps.

Soap in water is very effective at pulling fats and oils into solution. Soapy water is used to clean dirt and grease off surfaces.

The most common type of fat in your body are triglycerides.

Triglycerides are triesters. Triglycerides can be saponified using strong bases like metal hydroxides, producing metal carboxylate salts and glycerol.

This is the saponification of a triglyceride:

For example, stearin (a triglyceride where all , , and groups are the same) can be saponified with sodium hydroxide.

This is the saponification of stearin with sodium hydroxide:

Ammonolysis is similar to hydrolysis, with ammonia () in place of water.

When an ester is ammonolyzed, a primary amide is formed.

The general equation for the ammonolysis of an ester is shown:

Ammonia can be introduced as a gas into the reaction vessel, or the mixture can be cooled so that ammonia is in liquid form (the boiling point of ammonia is ). Alternatively, ammonia can be dissolved in water first.

Example 2: Determining the Structure of the Reaction Product of an Amine with an Ester given Their Structures

Esters can react with amines in the same way they react with ammonia. Consider the following reaction:

What product, aside from an alcohol, is formed from this reaction?

Answer

Esters can react with ammonia in what we call an ammonolysis reaction. The carbon–oxygen single bond in the ester group breaks, and the products are an amide and an alcohol.

So, let’s run this reaction with ammonia first.

The main product, the amide, has parts of the ammonia molecule incorporated in it.

This particular amide is a primary amide (the nitrogen has only one carbon-based group attached to it).

When an amine reacts with an ester, something very similar happens. This reaction is an example of an aminolysis:

The main product here is a secondary amide (the nitrogen has two carbon-based groups attached to it):

Not all esters can be polymerized, but some members of a certain group of esters, called diesters, are used to make plastics on an industrial scale.

Diesters can react with diols (chemicals containing two alcohol groups) in a polycondensation reaction to produce polyesters. These reactions could also be considered poly-transesterification reactions.

Poly(ethylene terephthalate) (PET) is a commonly used polymer that can be used, for instance, to make drink bottles. It can be made by reacting two different monomers together, which are ethane-1,2-diol (ethylene glycol) and dimethyl terephthalate (its IUPAC name is dimethyl benzene-1,4-dicarboxylate).

Here is a condensation reaction that produces poly(ethylene terephthalate) and methanol:

In this particular condensation reaction, the small-molecule by-product is methanol.

Example 3: Identifying the Name of the Alcohol Produced by the Hydrolysis of Polytrimethylene Terephthalate from a Set of Chemical Names

Polyesters can be hydrolyzed by . Which alcohol product is formed from the hydrolysis of polytrimethylene terephthalate (PTT), the structure of which is shown below?

1. Propan-1-ol
2. Ethane-1,2-diol
3. Ethanol
4. Propane-1,3-diol
5. Butane-1,4-diol

Answer

Let’s break down the structure of polytrimethylene terephthalate (do not be intimidated by the name; it is not important for this question).

There are two ester groups:

When ester groups are hydrolyzed, one of the products is an alcohol.

Here is the acid hydrolysis of an ester:

The key here is to remember that we are dealing with a polymer; the alcohol monomer we start with must have more than one alcohol group.

We can see that this is a straight-chain polymer, so let’s extend the polymer chain a little to include two whole units:

In the middle, we can see a chain of three carbon atoms between two ester linkages, which will have come from the alcohol monomer:

When these ester linkages are hydrolyzed, these bonds will break:

The product will be the equivalent diol:

This diol has three carbons in its carbon backbone, with a hydroxyl group on both ends. This is propane-1,3-diol.

The correct answer is D.