Video Transcript
Which of the following is not an
example of a monobasic carboxylic acid? (A) Benzoic acid, (B) oxalic acid,
(C) butyric acid, (D) acetic acid, or (E) formic acid.
Let’s begin by highlighting an
important word in the question that if missed can cause some confusion. The question is asking which of the
answer choices is not a monobasic carboxylic acid. In order to determine which of the
following molecules is not a monobasic carboxylic acid, it would be helpful to know
what a monobasic carboxylic acid is.
First off, a carboxylic acid is an
organic molecule that contains a carboxyl functional group. A carboxyl group consists of a
carbonyl group signified by the C double bond O. And coming from the same carbonyl
carbon atom is a single bond to a hydroxy group signified by the OH. Together, this unit is called a
carboxyl group, which is the signature functional group for a carboxylic acid
molecule. The R group varies depending on the
carboxylic acid being discussed.
Now that we have an understanding
of what a carboxylic acid is, let’s take a look at the term monobasic as it refers
to a carboxylic acid. Firstly, the term monobasic is
usually used to illustrate that an acid has one proton or one ionizable hydrogen
atom in the molecule that can disassociate and be donated to a base in an acid–base
reaction. Hence, the prefix mono- is used
because it means one. An example of a monobasic acid in
an acid–base reaction is aqueous hydrochloric acid reacting with aqueous sodium
hydroxide, where hydrochloric acid is a monobasic acid because it has one proton
that dissociates and is donated to the base to produce water.
Now, one might wonder, if we’re
talking about acids, why is the word “basic” used in the name? When we are looking at the number
of protons present in a molecule of acid, we are looking at what is called the
basicity of an acid. In other words, the basicity of an
acid describes how many protons the molecule can donate in an acid–base
reaction. More specifically to the topic at
hand, a monobasic carboxylic acid is a molecule that has one carboxyl group with one
proton to participate in an acid–base reaction. Therefore, when we have a molecule
with one carboxyl group, we call it a monobasic carboxylic acid, hence the prefix
mono-, which means one.
In order to determine which of the
answer choices is not a monobasic carboxylic acid, it will be helpful to draw the
chemical structure for each acid listed. Let’s begin by using the names
provided in the question or an alternative name they are known by to determine their
structural formulas. Starting with benzoic acid, since
its name contains the word benz- in it, its structure must have a benzene ring as
the R group and one carboxyl group because of the -oic acid at the end of the name,
making it the simplest aromatic carboxylic acid.
Moving on to oxalic acid, to help
us in remembering its chemical structure, we can look at its other name, ethanedioic
acid. This lets us know that while it is
a two-carbon atom chain with no double or triple bonds present between the two
carbon atoms, based on the presence of the word “ethane,” the prefix di- in front of
the -oic acid at the end of the name indicates the presence of two carboxyl groups,
which means the only two carbon atoms in the molecule are left to make up the two
carboxyl groups. This chemical structure is what we
would call a dicarboxylic acid.
Next is butyric acid, which from
the presence of the prefix but- lets us know its structure consists of a carbon
chain four carbon atoms long. It may be difficult to determine
the exact structure using the remainder of its common name. So let’s take a look at its IUPAC
name, butanoic acid. Its prefix butan- also indicates a
four-carbon atom chain due to the presence of the prefix but- and also lets us know
that the carbon chain is saturated or doesn’t contain any double or triple
bonds. The presence of the -oic acid in
the suffix of the name indicates one carboxyl group on the end of the molecule.
The next chemical structure we will
draw is acetic acid, the most well-known carboxylic acid among the answer choices
listed. We may recall that acetic acid has
the IUPAC name ethanoic acid, which from the ethan- portion of the name indicates a
two-carbon atom molecule with no double or triple bonds between the carbon atoms,
and the -oic acid indicating the presence of one carboxyl group.
The final structure we will draw
for this question is formic acid, the simplest carboxylic acid, which if we’ll
recall is also called methanoic acid. From the prefix methan-, we can
determine it is a one-carbon atom molecule, and the -oic acid indicates it contains
one carboxyl group.
Notice the only difference between
benzoic acid, butyric acid, acetic acid, and formic acid is they each have a
different R group. We can see from the chemical
structures we’ve drawn for choices (A), (C), (D), and (E) that they all contain one
carboxyl group within the molecule, making each one of these a monobasic carboxylic
acid. Therefore, the answer to this
question cannot be choices (A), (C), (D), or (E). Choice (B) oxalic acid is the only
molecule listed that is not a monobasic carboxylic acid but rather is a dibasic
carboxylic acid because it contains two carboxyl groups, which contains two protons
to donate to a base in an acid–base reaction. Therefore, our final answer is
choice (B) oxalic acid.
