Video Transcript
Which of the following molecules is
a positional isomer of C4H9OH? Five displayed formulas are shown
below.
In this question, we are given the
displayed formulas of five molecules. And we are being asked to determine
which molecule is a positional isomer of C4H9OH. Positional isomers are molecules
with the same molecular formula and functional groups but different positions of the
functional groups on the carbon chain. Let’s take a look at the molecular
formula provided in the question. The molecule contains four carbon
atoms, a total of 10 hydrogen atoms, and one oxygen atom. Positional isomers must have the
same molecular formula, so the correct answer must contain the same type and number
of atoms that we see here.
Now, the OH group that appears at
the end of the molecular formula is a functional group called a hydroxy group. The hydroxy group is the functional
group of an alcohol. The molecular formulas of alcohols
are often written so that the OH group stands alone as opposed to grouping all the
hydrogen atoms together in the formula. Let’s sketch out one possible
displayed formula for this alcohol molecule using the provided molecular
formula.
We can begin by connecting four
carbon atoms with single bonds. Next, we can draw a single bond
from the first carbon atom in the carbon chain to a hydroxy group. Finally, we can add in the
remaining nine hydrogen atoms, ensuring each carbon atom has a total of four single
bonds. The name of this alcohol is
butan-1-ol. The stem but- indicates that there
are four carbon atoms in the carbon chain, and the suffix -ol is used in the names
of all alcohols to indicate the presence of the hydroxy group. Finally, the number in the name
indicates the position of the hydroxy group on the carbon chain. In other words, the hydroxy group
is bonded to the first carbon atom in the carbon chain.
Positional isomers have the same
functional group, but the location of the functional group on the carbon chain
differs. The correct answer must not only
have the same molecular formula as butan-1-ol, but it must also contain a hydroxy
group bonded to the carbon chain. When looking at the answer choices,
we see that in answer choices (B) and (D), the molecules do contain a hydroxy group
bonded to the carbon chain, while the other three answer choices contain different
types of functional groups.
The molecule in answer choice (A)
contains the ether functional group, the molecule in answer choice (C) is a ketone,
and the molecule in answer choice (E) is an aldehyde. Because these three molecules do
not contain the alcohol functional group, they cannot be positional isomers of
C4H9OH. The last thing we need to do is
determine if choice (B) or choice (D) has the correct molecular formula.
Molecule (B) contains four carbon
atoms, 10 hydrogen atoms, and one oxygen atom. Therefore, its molecular formula is
C4H10O. Molecule (D) contains four carbon
atoms, eight hydrogen atoms, and one oxygen atom. So, its molecular formula is
C4H8O. As we discussed earlier, the
molecular formulas of alcohols are often written in a way to indicate the presence
of the hydroxy group. To do this, one of the hydrogen
atoms is written after the oxygen atom. Therefore, the molecular formula of
molecule (B) can be rewritten as C4H9OH. And the molecular formula of
molecule (D) can be written as C4H7OH.
We can eliminate answer choice (D)
because although it has the correct functional group, it does not have the correct
molecular formula. In conclusion, the displayed
formula in answer choice (B) contains the same functional group and has the same
molecular formula as C4H9OH. The hydroxy group is located on a
different carbon atom in the carbon chain.
Therefore, the positional isomer of
C4H9OH is the molecule in answer choice (B).
