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.

Definition: Hydrolysis

It is a type of chemical reaction where water reacts, breaking one or more bonds.

Definition: Ester Hydrolysis

It is a type of chemical reaction where water reacts with an ester, breaking the carbon–oxygen single bond of the ester.

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 (HCl()aq)
  • Sulfuric acid (HSO()24aq)

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 (NaOH)
  • Potassium hydroxide (KOH)

The general equations are shown:

Hydrochloric acid (HCl()aq) 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.

Example 1: Identifying the Molecular Formula of the Ester That Is Hydrolyzed to Produce the Given Products from a Set of Molecular Formulas

Consider the reaction ACHCOOH+CHCHCHOH3322

What ester gives the products shown when hydrolyzed?

  1. CHCOOCHCHCH3223
  2. CHCHCOOCHCHCH32223
  3. CHCOOCHCH323
  4. CHCHCOOCHCHCHCH322223

Answer

Let’s examine the equation.

CHCOOH3 is a carboxylic acid and CHCHCHOH322 is an alcohol. These are the products we would expect for an acid-catalyzed hydrolysis of an ester.

When an ester (RCOOR) is hydrolyzed like this, it is the RCO group that will be converted into the carboxylic acid and the OR group that will be converted into the alcohol.

So, we can work backward.

This part will come from the RCOOR part of the original ester: ACHCOOH+CHCHCHOH3322

This part will come from the RCOOR part: ACHCOOH+CHCHCHOH3322

And these parts will come from the water molecule: ACHCOOH+CHCHCHOH3322

So, let’s add these bits together: CHCOOCHCHCH+HO32232

The formula of the original ester must have been CHCOOCHCHCH3223.

You can check by counting the carbon atoms: 2 carbon atoms end up in the carboxylic acid, and 3 carbon atoms end up in the alcohol; this is a total of 5 carbon atoms.

Therefore, the correct answer is A.

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

Definition: Soap

It is a type of chemical, a salt of a fatty acid (fatty acids are long-chain carboxylic acids).

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

Definition: Saponification

It is a type of chemical reaction involving the base-induced hydrolysis of fats, oils, or other lipids into soaps and alcohols.

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 R, R, and R 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 (NH3) in place of water.

Definition: Ammonolysis

It is a type of chemical reaction where ammonia reacts, breaking one or more bonds.

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

Definition: Ester Ammonolysis

It is a type of chemical reaction where ammonia reacts with an ester, breaking the carbon–oxygen single bond of the ester.

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 33.3C). 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:

++CH3CH2COOCH2CH3NH2CH2CH3?OHCH2CH3

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):

CH3CH2CNHCH2CH3O

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.

Definition: Transesterification

It is a type of chemical reaction where an alcohol reacts with an ester and displaces the existing OR group of the ester. The product is a different ester to the starting ester.

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:

2nOHCHCHHHOH(n+1)COHHHCOCOOCHHHnCHOCHCHHOOHCOOCHHCHOHHn++CH3OH

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 NaOH. Which alcohol product is formed from the hydrolysis of polytrimethylene terephthalate (PTT), the structure of which is shown below?

OCOCOOCH2CH2CH2n
  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:

......COOCOOCH2CH2CH2OCOCOOCH2CH2CH2

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

......COOCH2CH2CH2OCO

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

......COOCH2CH2CH2OCO

The product will be the equivalent diol:

OHCH2CH2CH2OH

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.

Key Points

  • Ester hydrolysis is where water reacts with an ester, breaking the carbon–oxygen single bond of the ester.
  • There are two common varieties of ester hydrolysis:
    • In acid hydrolysis, an acid catalyzes the hydrolysis reaction, producing a carboxylic acid and an alcohol.
    • In base hydrolysis, a strong base reacts with the ester, producing a carboxylate and an alcohol. The carboxylate can be converted to a carboxylic acid by treatment with a strong acid.
  • Saponification is the base hydrolysis of fats, oils, or other lipids into soaps and alcohols.
  • Ester ammonolysis is where ammonia reacts with an ester, breaking the carbon–oxygen single bond of the ester; the products of an ester ammonolysis are a primary amide and an alcohol.
  • Chemicals with multiple ester groups can be polymerized. For example, a diester and a diol can react to form a polyester.

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