Video Transcript
Consider the following
molecules. Which of these molecules has the
name 3-methylbutanoic acid?
Let’s begin by analyzing the IUPAC
name provided in the question. Firstly, IUPAC, which stands for
International Union of Pure and Applied Chemistry, has established a set of organic
chemistry nomenclature or naming rules to facilitate more effective communication
and collaboration. While we will not cover all of the
IUPAC rules for naming organic molecules here, we will review the ones pertinent to
answering this question.
The first rule we will highlight
consists of giving each type of functional group a different suffix in their
chemical name so that one can tell what type of organic molecule is being
discussed. In the name 3-methylbutanoic acid,
the presence of the suffix -oic acid indicates the molecule must be a carboxylic
acid. Recall a carboxylic acid consists
of a carbonyl group, signified by the C double bond O, which is singly bonded
through the carbon atom to a hydroxy group, indicated by the OH. As a unit, we call this a carboxyl
group.
The R group varies depending on the
carboxylic acid being discussed. If we take a look at molecules (A)
through (F), we can see they each have one carboxyl group, indicating that all of
these molecules are carboxylic acids. Since the suffix of the IUPAC name
indicates the molecule must be a carboxylic acid and molecules (A) through (F) are
all carboxylic acids, at this point, we cannot eliminate any of the answer
choices. It also means we can focus our
IUPAC review to carboxylic acid molecules.
To the left of the suffix -oic acid
in the name 3-methylbutanoic acid, we find the name stem butan. The second rule we will mention is
that the name stem is derived from the number of carbon atoms in the longest
continuous chain and how saturated the chain is. Utilizing the list of common
numerical prefixes from one to 10, the presence of the prefix but- indicates the
longest continuous carbon chain is four carbon atoms long. And the “an” that follows
demonstrates that the carbon chain is a saturated alkane and contains only single
bonds.
Recall that while the name stem
ends in “an” for a saturated molecule where the hydrocarbon portion of the molecule
only contains single bonds, unsaturated molecules that contain one or more double
bonds will end in “en.” And unsaturated molecules that
contain one or more triple bonds will have a name stem ending in “yn.”
To determine the longest continuous
carbon chain by numbering the carbon atoms within chemical structures (A) through
(F), first recall for carboxylic acids, the carbonyl carbon atom is labeled as the C
one carbon. From there, we can continue the
numbering process by labeling the carbon atom bonded to the carbonyl carbon as C
two, followed by the next carbon atom bonded to C two as C three, and continue this
numbering process for the longest continuous chain of carbon atoms within all the
molecules shown here. For molecules (A) and (D), we can
see the longest continuous carbon chain for both is four carbon atoms long. And their carbon chains solely
contain single bonds, not counting the double bond on the carboxyl group. This equates to their name stem
being butan, which matches the prefix of the IUPAC name provided in the
question.
While the longest continuous carbon
chains for molecules (B), (C), (E), and (F) are also saturated alkane carbon chains
containing only single bonds, they are eliminated as potential answer choices. Because their longest continuous
carbon chains do not contain four carbon atoms, which means their name stems cannot
be butan. One might wonder, but there are
more carbon atoms in each of the molecules. Why did we not include them when
numbering the carbon chains?
Using molecule (B) as an example,
it would also be correct to label the methyl group pointing down as the C three
carbon atom in the carbon chain, instead of the carbon in the methyl group to the
left. Numbering the molecule in this
fashion will still give the name stem propan in describing its longest continuous
carbon chain. This is because we are still
numbering carbon atoms that are bonded in sequence, meaning the C one carbon is
bonded to the C two carbon, which is bonded to the C three carbon.
On the other hand, if we tried to
include all four carbon atoms present in the compound and give the molecule the name
stem butan, this would be incorrect. Because in order for a fourth
carbon atom to be counted in terms of molecule (B)’s longest continuous carbon
chain, it would have to be bonded to the C three carbon atom to allow for sequential
numbering. Instead, the fourth carbon atom,
along with the three hydrogen atoms bonded to it in molecule (B), is considered a
methyl substituent that is branching off of the molecule, as opposed to being
counted as part of the longest continuous carbon chain.
The same thought process can be
used to confirm the prefixes we have assigned to the name stems for each of the
molecules listed. Now that we have narrowed it down
between molecules (A) and (D), we can focus on the beginning of the IUPAC name,
which indicates the name and position of any substituents present in the
compound. More specifically, the rule states
that all substituents are indicated in the chemical name by their position along the
continuous carbon chain.
The dash in between the number
three and the word “methyl” indicates the methyl group is found on the C three
carbon atom in the molecule. If we take a look at molecule (A),
it has a methyl substituent on the C three carbon, while molecule (D) has a methyl
substituent on the C two carbon and no methyl substituent on the C three carbon. Based on this information, the
IUPAC name provided in the question cannot be the chemical name of molecule (D). Because the C three carbon in
molecule (D) does not have a methyl substituent as the name indicates. Therefore, the answer is molecule
(A) has the name 3-methylbutanoic acid.
