Video Transcript
Using the IUPAC nomenclature, what
name does the following alcohol have? (A) Hexan-1-ol, (B) hexan-6-ol, (C)
hexan-3-ol, (D) hexan-4-ol, or (E) hexan-2-ol.
To name an alcohol following the
rules established by the International Union of Pure and Applied Chemistry,
abbreviated to IUPAC, the first rule consists of naming the longest continuous chain
of carbon atoms that contains the hydroxy group or the OH group. This involves using a prefix from
the prefix list that matches the number of carbon atoms in the longest continuous
chain and a suffix that indicates the type of carbon-carbon bonds present. This is known as the stem portion
of the name.
In this molecule, the hydroxy group
is bonded to a carbon atom that is part of a six-carbon atom continuous chain, where
the only type of bond present between carbon atoms is a single bond. We can therefore name this chain
hexane, the prefix hex- for the six carbon atoms of the chain and the suffix -ane
for alkane, meaning that the carbon atoms are singly bonded to one another.
Looking at the answer choices, we
notice the term hexan but not hexane, as we just determined should be the stem name
for this compound. This is because when there is only
one hydroxy group in the molecule, such as this one, the E in the stem name is often
removed. We will notice that all of the
answer choices list the stem name as hexan, which means at this point all of the
answer choices are possible answers for this question.
The second rule is to number the
carbon atoms of the chain so that the hydroxy group has the lowest possible numbered
position. Looking at the molecule, we could
number the carbon atoms from left to right or from right to left. Numbering the carbon atoms from
left to right places the hydroxy group on the second carbon atom of the chain. But if we number from right to
left, the hydroxy group is placed on the fifth carbon atom of the chain. This means that for this molecule,
we want to number the carbon chain from left to right rather than right to left. This is because numbering the
carbon chain from left to right places the hydroxy group on the carbon atom with the
lowest possible position number, which is the number two. Once we add the position number of
the hydroxy group to the name, we follow it up with the ending O-L to indicate it is
an alcohol.
Finally, we would want to name any
additional substituents that appear in the molecule. This molecule does not have any
additional substituents. So, the correct IUPAC name for the
structure pictured here is hexan-2-ol. Taking a look at the answer
choices, even though all the names begin with hexan and end with O-L, the only
answer choice that has the correct numbered position is option (E), which means
answer choices (A), (B), (C), and (D) are not the correct answers for this question,
because the numbers indicating the position of the hydroxy group for answer choices
(A), (B), (C), and (D) each place the hydroxy group on a carbon atom that does not
match the structural formula of the molecule in question. In addition, the IUPAC names in
answer choices (B) and (D) do not place the hydroxy group in the lowest possible
numbered position, which also makes these two answer choices incorrect with regard
to rule number two.
Therefore, what IUPAC name does the
following alcohol have? The answer is option (E)
hexan-2-ol.
