Video Transcript
In this video, we will learn how to
identify and name esters and describe their physical properties. We’ll examine a common reaction for
producing esters and learn some of the applications of esters.
Before we look at the structure of
an ester, let’s examine the structure of a carboxylic acid. A carboxylic acid consists of a
carbonyl and a hydroxy group. If we replace the hydrogen atom of
the hydroxy group with a carbon-containing group such as an alkyl or aryl group, we
get an ester, a derivative of a carboxylic acid. Esters contain a carbonyl group and
an alkoxy group. Now that we can recognize the
general structure of an ester, let’s examine how esters can be produced.
Esterification is a type of
chemical reaction where an ester is formed. In this video, we will focus on a
specific type of esterification called Fischer esterification. In a Fischer esterification
reaction, a carboxylic acid is reacted with an alcohol in the presence of sulfuric
acid, which acts as a catalyst. This produces an ester and
water. Let’s examine this reaction in more
detail.
Over the course of this reaction,
the hydroxy group of the carboxylic acid is replaced by the alkoxide group of the
alcohol, producing the ester. Fischer esterification reactions
are actually in equilibrium with the hydrolysis of an ester. The details of ester hydrolysis are
beyond the scope of this video. To drive the reaction towards the
ester, excess of the carboxylic acid or the alcohol can be used. And in some cases, the water
produced can be easily distilled to further increase the yield.
The esterification of propanoic
acid with ethanol is intermolecular, meaning that the reacting groups, in this case
a carboxylic acid and alcohol, are in different molecules. But Fischer esterification can also
be intramolecular, where both the carboxylic acid and alcohol are part of the same
molecule. Intramolecular esterification
typically occurs when the leaving hydroxy group and the replacing alkoxide group are
four or five carbon atoms away from one another. The overall reaction of
intramolecular esterification is the same as intermolecular esterification. The hydroxy group of the carboxylic
acid is replaced by the alkoxide group of the alcohol to produce an ester and
water.
It can be difficult to determine
the structure of the ester formed via intramolecular esterification. So let’s redraw the structure of
the starting material with the two reacting groups closer together. It may be helpful to number the
carbon atoms to make sure that the redrawn structure has the same backbone as the
original. We know that the alkoxide group of
the alcohol will replace the hydroxy group of the carboxylic acid. This means that a new single bond
will be formed between the carbonyl carbon and the alkoxide oxygen. This produces a structure with a
five-membered ring consisting of four carbon atoms and one oxygen atom. This product is a cyclic ester,
also called a lactone. Intramolecular Fischer
esterification will always produce a cyclic ester.
Let’s take a look at one more
Fischer esterification reaction, this time involving salicylic acid. Salicylic acid contains a
carboxylic acid functional group and a phenol functional group. The carboxylic acid and hydroxy
group of the phenol are unlikely to undergo intramolecular esterification because
the groups are too close to one another. But each functional group can
undergo intermolecular esterification under certain reaction conditions. When salicylic acid is reacted with
acetic acid, the phenol functional group can undergo esterification to produce
acetylsalicylic acid. This compound is more commonly
known as aspirin and is used to reduce pain, fever, and inflammation.
While we used acetic acid to
demonstrate this sample reaction, aspirin is more commonly prepared by reacting
salicylic acid with acetic anhydride, as this results in a higher yield. The details of how this reaction
proceeds are beyond the scope of this video.
When salicylic acid is mixed with
methanol in the presence of sulfuric acid, the carboxylic acid portion can undergo
Fischer esterification. This produces methyl salicylate and
water. Methyl salicylate is also known as
oil of wintergreen and is commonly used in topical muscle pain creams and as a minty
flavoring in candies, toothpaste, and mouthwash.
Understanding how an ester can be
produced via Fischer esterification can help us name ester molecules. Esters are named by first
identifying the carboxylic acid and alcohol from which they could be produced. To find these parent molecules, we
first locate the carbonyl carbon. This is the carbon atom
double-bonded to an oxygen atom. We then separate the ester by
breaking this carbon atom’s single bond to oxygen. If working from a condensed
formula, we can separate the molecule between the two adjacent oxygen atoms.
Here are the condensed and
structural formulas of the two separate pieces. We add a hydroxy group to the
carbonyl to complete the parent carboxylic acid, and we add a hydrogen atom to the
alkoxide to complete the parent alcohol. Next, we name the carboxylic acid
and alcohol following the rules established by the International Union of Pure and
Applied Chemistry, or IUPAC. This carboxylic acid is given the
name butanoic acid: but- meaning four carbon atoms, “an” for alkane, and “-oic acid”
indicating that the molecule is a carboxylic acid.
The alcohol is given the name
ethanol: eth- meaning two carbon atoms, “an” for alkane, and “ol” indicating that
the molecule is an alcohol. Next, we change the ending of the
carboxylic acid name to -oate, and we change the ending of the alcohol name to
“yl.” Finally, to complete the name of
the ester, we write the alcohol portion followed by the carboxylic acid portion with
a space in between. Thus the name of the original ester
is ethyl butanoate.
Let’s take a look at another
ester. We locate the carbonyl carbon and
break the carbon-oxygen single bond. Then we complete the parent
carboxylic acid and alcohol by adding a hydroxy group to the carbonyl and a hydrogen
to the alkoxide. The names of these parent molecules
are methanoic acid and ethanol. We change the endings giving us
methanoate and ethyl. Then we write the names in the
correct order to give this ester the name ethyl methanoate. But this ester also has a common
name that is derived from the common name of the parent carboxylic acid. Methanoic acid is more commonly
called formic acid. When using the common name of the
carboxylic acid to name the ester, the ending of the name is changed to -ate. So the common name of this ester is
ethyl formate.
Common names for esters are
frequently used when naming esters that have methanoic acid or ethanoic acid as the
parent carboxylic acid. So methyl methanoate is commonly
called methyl formate and methyl ethanoate is commonly called methyl acetate.
We’ve learned how to make esters
and how to name esters. Now let’s take a look at some of
the properties of esters, starting with boiling point.
Shown here is an ester, an alcohol,
and a carboxylic acid. Each of these compounds has the
same molar mass, but methyl methanoate has a much lower boiling point. To understand why this is the case,
we need to examine the intermolecular forces that exist between the molecules. Alcohol molecules can form strong
hydrogen bonds with other alcohol molecules. This is also true of carboxylic
acid molecules. But ester molecules cannot form
strong hydrogen bonds with one another. The only electrostatic forces of
attraction between ester molecules are weak Van der Waals forces. As the forces of attraction between
ester molecules is weak, less energy is required to separate ester molecules from
one another. Thus, esters tend to have lower
boiling points and are also more volatile than comparable alcohols and carboxylic
acids.
While ester molecules cannot form
hydrogen bonds with other ester molecules, they can form hydrogen bonds with water
molecules. This means that small esters are
soluble in water, though not as soluble as comparable alcohols and carboxylic
acids. The solubility of an ester in water
decreases as either alkyl chain increases. This is because increasing either
carbon chain increases the overall nonpolar characteristics of the molecule. Thus, large esters tend to be
immiscible or insoluble in water.
A common example, used to
demonstrate the insoluble nature of large esters is vegetable oil. Vegetable oil, like other
vegetable- and animal-based oils and fats, consists of a variety of triglyceride
molecules. A triglyceride is also a triester,
a molecule containing three ester groups. While the three ester functional
groups can form hydrogen bonds with water, the large hydrocarbon chains cause the
triglyceride molecules to be significantly nonpolar. Thus, vegetable and animal oils and
fats are insoluble in water.
Another distinct property of esters
is their odor. While carboxylic acids often have
sour, disagreeable odors and alcohols have odors that range from herbal to rancid,
the odor of an ester is often sweet, floral, or fruity. In fact, some esters contribute to
the distinct odor and/or flavor of many fruits. Food chemists and perfumers can
extract or synthesize these esters to use as artificial flavorings and scents. Use as an artificial flavoring is
just one common use of esters.
Esters are also found in compounds
called polyesters that are used for a variety of purposes. Poly- means many. So a polyester contains many
esters. More specifically, a polyester is a
polymer that contains an ester functional group in each repeating monomer. One of the most common polyesters
is polyethylene terephthalate, abbreviated as PET or PETE. This polyester can be produced by
reacting terephthalic acid with ethylene glycol. Terephthalic acid has two
carboxylic acid functional groups. Each carboxylic acid group can
undergo esterification with one of the hydroxy groups of the ethylene glycol.
The second hydroxy group of the
ethylene glycol molecule can then undergo esterification with another molecule of
terephthalic acid. This repeating reaction creates the
polyester. Polyethylene terephthalate can be
used to make water-resistant, wrinkle-free clothing; recyclable water bottles and
packaging; and foil balloons. It is also used in the medical
industry as thread for stitches or in vascular grafts when repairing damaged blood
vessels.
Another common polyester is
polylactic acid, abbreviated PLA. Polylactic acid is a biodegradable
polyester and is made by linking lactic acid molecules. Polylactic acid is used to produce
disposable plastic cups and packaging and is one of the most common filaments for 3D
printers. PLA is also being used to create
medical implants that are meant to degrade inside of the body.
We’ve learned a lot about esters in
this video, so let’s review. Esters are derivatives of
carboxylic acids and contain a carbonyl group and an alkoxy group. Esters can be produced via Fischer
esterification, an equilibrium reaction between a carboxylic acid and an
alcohol. Esters are named from the parent
alcohol and carboxylic acid. The name of an ester will end in
-oate. Esters have lower boiling points
and are less soluble than comparable alcohols and carboxylic acids. Many esters have a sweet, floral,
or fruity odor and can be used as artificial flavorings. Polyesters are polymers that
contain an ester in each monomer and have many uses, from clothing production to
medical implants.
