Video Transcript
In this video, we will learn how to
describe the reactions of esters and predict what products are formed. We’ll focus on hydrolysis,
saponification, aminolysis, and polytransesterification.
An ester is a derivative of a
carboxylic acid. It contains a carbonyl group and an
alkoxy group. Esters are commonly prepared via
Fischer esterification, a chemical reaction between a carboxylic acid and an alcohol
in the presence of an acid to produce an ester and water. Over the course of this reaction,
the hydroxy group of the carboxylic acid is replaced by the alkoxy group of the
alcohol, producing an ester and water. Fischer esterification reactions
are actually in equilibrium with the hydrolysis of an ester.
Hydrolysis is a type of chemical
reaction where water reacts with a compound breaking one or more bonds. In ester hydrolysis, the carbonyl
carbon’s single bond to oxygen is broken. Over the course of this reaction,
the alkoxy portion of the ester is replaced by the hydroxy group of the water,
producing a carboxylic acid and alcohol. To shift the equilibrium towards
the carboxylic acid and alcohol, excess water should be used. The ester hydrolysis reaction that
is in equilibrium with Fischer esterification uses sulfuric acid as a catalyst. So, this reaction is an
acid-catalyzed ester hydrolysis.
Ester hydrolysis can also be
base-induced. Despite being classified as a
hydrolysis reaction, a strong base is used as the reactant instead of water. The strong base is usually aqueous
sodium hydroxide or potassium hydroxide. This reaction is induced rather
than catalyzed because the base reacts with the ester and is not regenerated. In base-induced ester hydrolysis,
the carbonyl carbon’s single bond to oxygen is broken, just like in the
acid-catalyzed hydrolysis reaction. An alcohol is produced from the
alkoxy group and a carboxylate salt is formed.
Notice that base-induced ester
hydrolysis does not produce a carboxylic acid, but rather its conjugate base. To produce a carboxylic acid, a
second reaction with hydrochloric acid is required. Unlike acid-catalyzed ester
hydrolysis, base-induced ester hydrolysis is not in equilibrium reaction. So, base-induced ester hydrolysis
will produce higher yields than acid-catalyzed ester hydrolysis.
Now that we’ve examined the general
reaction of base-induced hydrolysis, let’s take a look at this reaction using a
large ester molecule. Shown here is a sample triglyceride
found in coconut oil. Triglycerides are triesters that
are found in vegetable- and animal-based fats and oils. When a triglyceride is reacted with
a strong base, a hydrolysis reaction occurs. The carbon-oxygen single bonds of
the three esters break, producing glycerol and three carboxylate salts.
Carboxylate salts that have long
hydrocarbon chains are also called fatty acid salts, more commonly known as
soaps. They can be used to remove nonpolar
fats and oils from skin, dishes, or clothing. The reaction of a triglyceride with
a strong base is a base-induced ester hydrolysis, but it is more commonly called
saponification. Saponification is a type of
chemical reaction involving the base-induced ester hydrolysis of fats, oils, or
other lipids into soaps and alcohols.
We’ve examined how esters can be
hydrolyzed in acids and bases. Now, let’s take a look at a
different reaction that is similar to hydrolysis. Ammonolysis is a chemical reaction
where ammonia reacts with a compound breaking one or more bonds. In ester ammonolysis, the bond that
is broken is the carbon-oxygen single bond of the ester, just like in
hydrolysis. The products of ester ammonolysis
are a primary amide and an alcohol. A primary amide is a functional
group that contains a carbonyl bonded to a nitrogen atom that has two hydrogen
substituents.
Let’s examine the ester ammonolysis
reaction involving methyl propanoate. Over the course of this reaction,
the alkoxy group of the ester is replaced by the ammonia to produce propanamide, a
primary amide, and methanol in alcohol. The ammonia used in this reaction
can be added to the reaction vessel as a gas, as a liquid, or dissolved in
water.
The final reaction we want to
consider involves the formation of polyesters. A polyester, as the name implies,
contains many esters. More specifically, a polyester is a
polymer that contains an ester functional group in each repeating monomer. One common polyester is
polyethylene terephthalate, abbreviated as PET or PETE. Polyethylene terephthalate can be
used to produce wrinkle-free water-resistant clothing and recyclable water bottles
and packaging in addition to numerous other products. This polyester can be produced by
reacting terephthalic acid with ethylene glycol.
Terephthalic acid has two
carboxylic acid functional groups. Each of the carboxylic acid groups
can undergo esterification with one of the hydroxy groups of ethylene glycol. The other hydroxy group of ethylene
glycol can then undergo esterification with another molecule of terephthalic
acid. This repeating reaction creates the
polyester. The overall reaction of
terephthalic acid with ethylene glycol is shown here. Notice that this reaction requires
an acid catalyst, and water is produced as a byproduct.
Polyethylene terephthalate can also
be produced by starting with ethylene glycol and the ester dimethyl
terephthalate. The ester and alcohol will react in
a similar fashion as the carboxylic acid and alcohol. In this reaction, the alkoxy group
of the alcohol will replace the alkoxy group of the ester. The reaction can occur with both
ester groups of dimethyl terephthalate and both hydroxy groups of ethylene
glycol. This reaction is an example of
transesterification, a chemical reaction where an alcohol reacts with an ester
displacing the alkoxy group and producing a different ester. In this case, the new ester
produced is the polyester poly(ethylene terephthalate). Notice that the reaction of
dimethyl terephthalate and ethylene glycol requires an acid catalyst, just like the
reaction involving terephthalic acid. But the byproduct of this reaction
is methanol instead of water.
We’ve examined several reactions
involving esters in this video. Before we summarize what we’ve
learned, let’s take a look at a few questions.
Fill in the blanks. The hydrolysis of esters using
sodium hydroxide produces blank and alcohols, while the ammonolysis of esters
produces blank and alcohols.
An ester is a compound that
contains a carbonyl group bonded to an alkoxy group. Esters undergo a variety of
reactions. One such reaction is hydrolysis, a
chemical reaction where water reacts with a compound breaking one or more bonds. Ester hydrolysis may be
acid-catalyzed or base-induced. In this question, we are told that
the hydrolysis of esters uses sodium hydroxide, which has the chemical formula
NaOH. Sodium hydroxide is a strong
base. So, let’s examine a base-induced
ester hydrolysis reaction.
Hydrolysis is a reaction where
water reacts with a compound, but water does not appear as a reactant in a
base-induced hydrolysis reaction. This is because in base-induced
ester hydrolysis, it is actually the hydroxy group of the base that reacts with the
ester rather than water. The bond that is broken during this
reaction is the carbon-oxygen single bond of the ester. The alkoxy group of the ester
becomes an alcohol, and the carbonyl group of the ester becomes part of an ionic
compound that contains a carboxylate anion and a metal cation. Since sodium hydroxide was used in
this reaction, this compound is a sodium carboxylate.
The other reaction mentioned in the
question is ammonolysis. Ammonolysis is a chemical reaction
where ammonia reacts with a compound, breaking one or more bonds. When ammonia reacts with an ester,
the carbon-oxygen single bond of the ester is broken, just like in hydrolysis. Over the course of this reaction,
the ammonia replaces the alkoxy group of the ester. This produces an alcohol and a
compound that contains a carbonyl group bonded to a nitrogen atom. This functional group is called an
amide.
We now know that the hydrolysis of
esters using sodium hydroxide produces sodium carboxylates and alcohols, while the
ammonolysis of esters produces amides and alcohols. With this in mind, we should fill
in the first blank with sodium carboxylates and the second blank with amides.
Aspirin is one of the most widely
used medicines globally. The structure of aspirin is
shown. In moist conditions, aspirin can
undergo hydrolysis to form salicylic acid and another acid. Which of the following structures
is that of salicylic acid?
We’ve been given the structure of
aspirin, also called acetylsalicylic acid. A molecule of aspirin contains a
benzene ring which has a carboxylic acid substituent and an ester substituent. We are told that aspirin can
undergo hydrolysis. Hydrolysis is a chemical reaction
where water reacts with a compound breaking one or more bonds. Benzene rings and carboxylic acids
cannot undergo hydrolysis, but esters can.
Ester hydrolysis may be
acid-catalyzed or base-induced. The question does not specify if
the hydrolysis of aspirin is occurring in an acid or a base. But as aspirin is an acid, it could
act as both a reactant and a catalyst in the hydrolysis reaction. So, we should take a look at what
occurs during acid-catalyzed ester hydrolysis. During this reaction, the
carbon-oxygen single bond of the ester is broken and the hydroxy group of a water
molecule replaces the alkoxy group of the ester. This produces a carboxylic acid and
an alcohol.
Looking at the structure of
aspirin, we can identify the carbon-oxygen single bond of the ester. This is the bond that will be
broken during hydrolysis. The alkoxy portion of the ester
shown in blue can be replaced by the hydroxy group of a water molecule. Two molecules will be produced when
aspirin is hydrolyzed. One molecule will contain a
carboxylic acid group that was present in the original aspirin molecule and a newly
formed hydroxy group. The second molecule will contain a
newly formed carboxylic acid group. We know from the question that the
products of the hydrolysis of aspirin are salicylic acid and another unnamed
acid. We need to determine which of the
answer choices is salicylic acid.
We can see that each of the answer
choices contain a benzene ring. This indicates that the product of
the hydrolysis of aspirin that contains a benzene ring is salicylic acid. Comparing the structure to the
answer choices, we see that the structure that is the same is answer choice (C). Thus, the answer to the question
“Which of the following structures is that of salicylic acid?” is (C).
For the next part of the question,
we’ll keep only the relevant information on screen.
What is the name of the other acid
produced? (A) Butanoic acid, (B) methanoic
acid, (C) benzoic acid, (D) ethanoic acid, (E) propanoic acid.
The question is asking us to name
this molecule, which is a carboxylic acid. To name a carboxylic acid, we first
name the longest continuous chain of carbon atoms that contains the carboxyl
carbon. The carboxyl carbon is the carbon
of the carboxylic acid. The longest continuous chain of
carbon atoms that contains the carboxyl carbon is two carbon atoms long. This carbon chain is given the name
ethane, eth- meaning two carbon atoms and -ane for alkane.
Next, we drop the letter e from the
end of the name and add the suffix -oic acid, indicating that the molecule is a
carboxylic acid. This gives us the name ethanoic
acid. Thus, the answer to the question
“What is the name of the other acid produced?” is answer choice (D), ethanoic
acid.
Now, let’s summarize what we’ve
learned. Ester hydrolysis is a reaction
between an ester and water that breaks the carbon-oxygen single bond of the
ester. This reaction, may be
acid-catalyzed or base-induced. Acid-catalyzed ester hydrolysis
produces a carboxylic acid and an alcohol. This reaction is in equilibrium
with the stratification of a carboxylic acid. In base-induced ester hydrolysis,
the ester reacts with the hydroxy group of a strong base like sodium hydroxide
rather than with water. The products of this reaction are a
metal carboxylate and in alcohol.
Saponification is the base-induced
ester hydrolysis of fats, oils, or other lipids to produce soaps and alcohols. Ammonolysis is the reaction of an
ester with ammonia to produce a primary amide and an alcohol. Diesters like dimethyl
terephthalate and diols like ethylene glycol can undergo transesterification to form
polyesters.
