Video Transcript
Considering correct nomenclature
and isomerism, which of the following is not a correctly named structural isomer of
C3H6Cl2? (A) 2,2-Dichloropropane, (B)
1,1-dichloropropane, (C) 1,2-dichloropropane, (D) 1,3-dichloropropane, (E)
2,3-dichloropropane.
First of all, structural isomers
are molecules with the same molecular formula that differ from one another by how
atoms are connected. A structural isomer of C3H6Cl2 must
have the same molecular formula, which means the molecule must contain exactly three
carbon atoms, six hydrogen atoms, and two chlorine atoms. To determine which of the molecules
in the answer choices is a structural isomer of C3H6Cl2 that is named incorrectly,
we first need to draw the displayed formulas so that we can see how the atoms are
connected and then we can check that the IUPAC name is correct.
The five molecules are haloalkanes,
which is a type of organic molecule that contains only carbon, hydrogen, and one or
more halogens. In this case, the halogen is
chlorine. The names of haloalkanes contain
three main parts: the stem, which is the name of the parent alkane, the prefixes,
which tell the number and type of halogen atoms present, and the indexes, which tell
us which carbon atom or atoms in the carbon chain the halogen atoms are bonded
to. Now, we notice that all of the
names have the same stem, which is propane. Prop- indicates the presence of
three carbon atoms, and the suffix -ane tells us that the bonds between the carbon
atoms are all single bonds.
Let’s make some space to sketch a
carbon chain composed of three carbon atoms connected with single bonds for all of
the molecules. For now, let’s exclude the hydrogen
atoms from our sketches. Now, we’ve probably also noticed
that all of the names include the prefix dichloro-, which indicates that there are
two chlorine atoms bonded to the carbon chain. What differs among the names are
the indexes, which tell us to which carbon atoms the chlorine atoms are bonded. According to IUPAC nomenclature,
the carbon chain should be numbered so that the chlorine atoms get the lowest
possible numbers. We will need to check that this is
true for each molecule.
For molecule (A), let’s number the
carbon chain from left to right and draw two chlorine atoms singly bonded to the
second carbon atom. If we had numbered the carbon chain
from right to left, the two chlorine atoms would still be located on the second
carbon atom. So the name of the molecule would
not change. Therefore, this molecule is named
correctly. Let’s go ahead and add in the
hydrogen atoms. After adding in enough hydrogen
atoms so that each carbon atom is surrounded by a total of four bonds, we can see
that this molecule has a molecular formula of C3H6Cl2. Therefore, answer choice (A) is a
correctly named structural isomer of C3H6Cl2. And we can eliminate it because we
are looking for the incorrectly named structural isomer.
For molecule (B), after numbering
the carbon chain from left to right, we can draw two chlorine atoms singly bonded to
the first carbon atom. Now, if we number the carbon chain
from right to left, these chlorine atoms would be located on carbon number
three. We know that the chlorine atoms
must be located on the lowest possible carbon numbers. Therefore, based on what we have
drawn, the carbon chain should be numbered from left to right and the correct IUPAC
name is 1,1-dichloropropane. After adding in the correct number
of hydrogen atoms, we can see that the molecular formula of this molecule is
C3H6Cl2. So this molecule is a correctly
named structural isomer and can be eliminated.
As before, let’s number the carbon
chain for molecule (C) from left to right. Then, we can draw two chlorine
atoms singly bonded to the first and second carbon atom. If we number the carbon chain from
right to left instead, the chlorine atoms would be located on the second and third
carbon atoms of the chain instead of the first and second. Therefore, the correct numbering is
from left to right and the correct IUPAC name is 1,2-dichloropropane. After adding in the correct number
of hydrogen atoms, we can see that the molecular formula of this molecule is
C3H6Cl2. So we can eliminate molecule (C)
because it is a structural isomer and it has a correct IUPAC name.
After numbering the carbon chain
for molecule (D) from left to right, we can draw two chlorine atoms singly bonded to
the first and third carbon atom. And if we number the chain from
right to left instead, the chlorine atoms are still located on the first and third
carbon atom. Therefore, 1,3-dichloropropane is
the correct IUPAC name for this molecule. Finally, after adding in the
correct number of hydrogen atoms, we see that the molecule has the molecular formula
C3H6Cl2. So molecule (D) is a structural
isomer with a correct IUPAC name and can be eliminated.
If we number the carbon chain for
molecule (E) from left to right, then we would need to draw two chlorine atoms
singly bonded to the second and third carbon atom. If we now numbered the carbon chain
from right to left, we’d notice that the chlorine atoms are now located on the first
and second carbon atom. Because this numbering gives the
chlorine atoms the lowest possible carbon numbers, the correct IUPAC name should be
1,2-dichloropropane, not 2,3-dichloropropane. After adding in the correct number
of hydrogen atoms, we see that the molecular formula of this molecule is
C3H6Cl2. So molecule (E) is a structural
isomer; however, it was not named correctly.
In conclusion, the incorrectly
named structural isomer of C3H6Cl2 is 2,3-dichloropropane, or answer choice (E).
