Video Transcript
In this video, we will learn how to
describe various reactions of carboxylic acids and predict what products are
formed. We’ll focus on acid–base reactions,
esterification, and reduction.
Carboxylic acids are compounds that
contain a carboxyl group. A carboxyl group, represented in a
condensed formula by COOH, consists of a carbonyl group, a carbon atom double bonded
to an oxygen atom, and a hydroxy group. Carboxylic acids are weak acids due
to the resonance effect which is explained in more detail in another video. This means that the proton of the
carboxyl group can be donated to a base in an acid–base reaction. There are several common acid–base
reactions involving carboxylic acids that we will examine in this video.
The first reaction involves metal
hydroxides. When carboxylic acids are mixed
with metal hydroxides like sodium hydroxide, a neutralization reaction occurs
producing a salt and water. The specific salt produced by this
reaction is a metal carboxylate. Carboxylic acids also react with
metal carbonates to produce a salt, carbon dioxide gas, and water. Here is another way to write the
reaction between acetic acid and sodium carbonate using condensed organic formulas
and state symbols.
Metal bicarbonates, or metal
hydrogen carbonates, also react with carboxylic acids to produce a salt, carbon
dioxide gas, and water. This reaction can be used to test
if an organic substance contains a carboxylic acid. When sodium bicarbonate is added to
a carboxylic acid, a reaction occurs and carbon dioxide is produced. Visible effervescence or bubbles
when reacting a sample with sodium bicarbonate indicates that the sample contains a
carboxylic acid. Like mineral acids, carboxylic
acids can react with certain metals. This reaction produces a metal
carboxylate and hydrogen gas. The reactivity series can be used
to determine which metals react with carboxylic acids.
Listed here is a subset of the
reactivity series. Any metal more reactive than
hydrogen in this list should react with a carboxylic acid. Shown here is the reaction between
acetic acid and magnesium metal to produce magnesium acetate, a metal carboxylate,
and hydrogen gas. We’ve examined four reactions that
occur due to the acetic nature of carboxylic acids. Now let’s take a look at a
different type of reaction.
Esterification is a type of
chemical reaction where an ester is formed. Esters and carboxylic acids look
similar. The only difference in their
general structure is the substituent attached to the single-bonded oxygen atom. To transform a carboxylic acid into
an ester, we can perform a specific type of esterification called Fischer
esterification. In Fischer esterification, a
carboxylic acid is reacted with an alcohol in the presence of an acid catalyst such
as sulfuric acid. The products of this reaction are
an ester and water. Let’s take a look at the Fischer
esterification of propanoic acid with ethanol.
Over the course of this reaction,
the hydroxy group of the carboxylic acid is replaced by the alkoxide group of the
alcohol. This produces the ester ethyl
propanoate and water. Fischer esterification reactions
are actually in equilibrium with a reaction called ester hydrolysis. The details of ester hydrolysis are
beyond the scope of this video. To drive the equilibrium towards
the ester, excess of either the carboxylic acid or the alcohol can be used. The reaction is generally slow and
is usually carried out by heating under reflux.
In addition to being used as a way
to produce esters, Fischer esterification reactions can also be used to test if a
sample contains a carboxylic acid. To perform the test, ethanol is
added to a sample along with a few drops of sulfuric acid. The resulting mixture is then
gently heated. If the sample contains a carboxylic
acid, an esterification reaction occurs and then ester is produced. Many esters have a sweet, often
floral or fruity, odor. So if the mixture has a sweet smell
upon heating, the original sample likely contained a carboxylic acid.
We have one final reaction to
consider, reduction. In the reduction reaction of a
carboxylic acid, a reducing agent, represented by the capital H in brackets, reacts
with the carboxylic acid to produce an aldehyde and water. The aldehyde produced then reacts
rapidly with the reducing agent still present in the reaction vessel to form a
primary alcohol. The exact details of how the
reduction reaction proceeds are beyond the scope of this video.
Shown here is the overall generic
reaction equation for the reduction of a carboxylic acid to a primary alcohol. There are several reducing agents
and reaction conditions that can be used to perform this reaction. Hydrogen gas can be used as the
reducing agent in the presence of copper chromite, which acts as a catalyst. In addition to being defined as a
reduction reaction, this particular reaction can also be classified as a
hydrogenation reaction, a chemical reaction between molecular hydrogen and a
compound typically in the presence of a catalyst. Another common reducing agent is
lithium aluminum hydride. Notice that this reaction scheme as
written is unbalanced and only shows the starting material, reagents, and major
organic product.
We’ve now looked at acid–base
reactions, esterification, and reduction involving carboxylic acids. The reactivity of the carboxylic
acid used in each of these reactions can be affected by the carbon chain. Carboxyl groups that are joined to
aromatic rings have a slightly different reactivity than those that are not. Let’s consider benzoic acid and
cyclohexanecarboxylic acid. Benzoic acid is more acetic than
cyclohexanecarboxylic acid. As such, benzoic acid will react
faster with bases. In general, aromatic carboxylic
acids tend to react faster than aliphatic carboxylic acids when reacted with metals,
hydroxides, carbonates, and bicarbonates.
Benzoic acid is also easier to
esterify than cyclohexanecarboxylic acid due to the resonance effect. As benzoic acid is easier to
esterify, it will react faster and give higher yields when reacted with
alcohols. The exact details of how the
resonance effect increases the rate of the reaction and the yield are beyond the
scope of this video. In general, aromatic carboxylic
acids tend to esterify more quickly and give higher yields than aliphatic carboxylic
acids. The carboxyl group of benzoic acid
and cyclohexanecarboxylic acid can be reduced in a similar fashion. But when using molecular hydrogen,
there is the risk that the aromatic ring will be hydrogenated. Special catalysts or lithium
aluminum hydride can be used to prevent the hydrogenation of the aromatic ring.
Before we summarize what we’ve
learned about the reactions of carboxylic acids, let’s take a look at a few
questions.
Which of the following esters will
be obtained when methanoic acid reacts with methanol? (A) HCOOC2H5, (B) HCOOCH3, (C)
CH3CH2COOCH3, (D) CH3COOCH3, (E) CH3COOC2H5.
Let’s start by drawing the
structures of methanoic acid and methanol. The suffix -oic acid indicates that
methanoic acid is a carboxylic acid. Carboxylic acids are composed of a
carbonyl group, a carbon atom double bonded to an oxygen atom, and a hydroxy
group. The meth- term of the name
indicates that this carboxylic acid only contains one carbon atom. As the carboxyl functional group
already contains one carbon atom, this means that the R group must be a hydrogen
atom. The name methanol also contains the
meth- term, indicating that the molecule only contains one carbon atom. The suffix -ol indicates that
methanol is an alcohol and contains a hydroxy group.
As carbon atoms tend to form four
bonds, we complete the structure of methanol by adding three hydrogen atoms. The question tells us that these
two molecules react to produce an ester. This type of reaction is called
esterification. More specifically, the
esterification of a carboxylic acid with an alcohol is called Fischer
esterification. This reaction is typically carried
out in the presence of an acid catalyst such as sulfuric acid. Over the course of this reaction,
the hydroxy group of the carboxylic acid is replaced by the alkoxide group of the
alcohol. This produces an ester and
water. This ester only contains two carbon
atoms.
Looking at the answer choices, we
can eliminate answer choice (A), (C), (D), and (E), as each of these formulas
contains more than two carbon atoms. To verify that answer choice (B) is
the correct answer, let’s write the condensed formula of the ester we produced. The condensed formula is
HCOOCH3. This formula matches answer choice
(B). Thus, the ester obtained when
methanoic acid reacts with methanol is answer choice (B), HCOOCH3.
The reduction of a carboxylic acid
with hydrogen and a CuCr2O4 catalyst gives the following product. What structure did the original
carboxylic acid have?
In order to answer this question,
we need to recognize what occurs during the reduction of a carboxylic acid. A carboxylic acid contains the
carboxyl functional group. When a carboxylic acid is reacted
with a reducing agent represented here by a capital H in brackets, the carboxyl
functional group is reduced to a primary alcohol, and water is produced. This is the general equation for
the reduction of a carboxylic acid. In this question, the reducing
agent is hydrogen, typically in the form of hydrogen gas. And copper chromate is added as a
catalyst. We are given the structure of the
alcohol produced. Using this information, we need to
determine the structure of the original carboxylic acid.
Let’s take another look at the
general equation for guidance. Notice that the R group of the
carboxylic acid remains unchanged when forming the alcohol, as does the carbon atom
bonded to the R group. In addition, both molecules have a
hydroxy group bonded in the same position. Thus, this portion of the
carboxylic acid and alcohol, shown here in pink, are the same in both molecules. The difference between the two
structures is that the double-bonded oxygen atom of the carboxylic acid is replaced
by two hydrogen atoms to form the alcohol. This is not exactly what occurs
during the reaction but can help us see how the two molecules are related to one
another.
Now let’s look at the alcohol given
in the question. We know that the alcohol and the
original carboxylic acid will both have a hydroxy group bonded to a carbon atom
which is bonded to the same R group. We saw in the general equation that
the double-bonded oxygen atom of the carboxylic acid was replaced by two hydrogen
atoms when forming the alcohol. Working backwards, we can see that
the two hydrogen atoms in the alcohol must have originally been a double-bonded
oxygen atom in the carboxylic acid. This structure for the molecule
3-methylbutanoic acid is the structure of the original carboxylic acid used in this
reduction reaction.
Let’s review what we’ve learned
about carboxylic acid reactions. Carboxylic acids are weak acids
that can react with metals to produce a salt and hydrogen gas, metal hydroxides to
produce a salt and water, and metal carbonates and metal bicarbonates to produce a
salt, water, and carbon dioxide. Fischer esterification is the
reaction of carboxylic acids with alcohols in the presence of an acid catalyst to
produce an ester and water. Carboxylic acids can be reduced
using a variety of reducing agents to produce a primary alcohol and water. Aromatic carboxylic acids are more
acetic than aliphatic carboxylic acids. So they will react faster with
metals, hydroxides, carbonates, and bicarbonates. They are also easier to esterify
due to resonance effect.
