Video Transcript
Consider the following. What name is given to the
trisubstituted benzene? (A) 2-Nitro-4-bromobenzene, (B)
4-bromo-2-nitrotoluene, (C) 1-bromo-3-nitrotoluene, (D) 1-nitro-3-bromotoluene, (E)
2-bromo-4-nitrotoluene.
In this question, we need to
determine which of the answer choices is the correct name for the trisubstituted
benzene molecule shown.
Let’s start by discussing the rules
for naming monosubstituted benzene compounds first, which are relatively simple. We can see from these three
examples that the stem of the name is typically benzene. The stem refers to the parent
molecule. We can also see that the prefix in
the name relates to the composition of the single substituent. However, this rule does not apply
to all monosubstituted benzene molecules. For example, when the single
substituent is a hydroxy group, the name of the molecule is phenol. When the substituent is a methyl
group, the name of the molecule is toluene. There are other exceptions, which
we will not discuss in this video.
The molecule in this question is a
multisubstituted benzene compound. And the rules for naming a molecule
like this are more complicated. We can start by identifying the
prefix terms for all of the substituents. The CH3 group uses the prefix
methyl-, the Br group uses bromo-, and the NO2 group uses nitro-.
Now, we need to identify the stem
or parent molecule. Just like in monosubstituted
benzene compounds, the methyl group has a special status. While we might’ve thought that the
stem of this molecule would be benzene, the stem will be toluene instead. Now, when looking at the answer
choices, we can see that numbers are used before each prefix term. We will need to assign numbers to
substituents in the following way.
First of all, since the parent
molecule is toluene, the methyl group automatically gets a benzene ring position
number of one. Next, we need to number the
remaining substituents so that they have the lowest possible benzene ring position
numbers. If we number the benzene ring in a
counterclockwise direction, the nitro group is at position four and the bromo group
is at position six. If we number the benzene ring in a
clockwise direction, the bromo group is at position two and the nitro group is at
position four. Since the bromo group has a lower
position number, the clockwise numbering is correct.
Finally, when constructing the
name, we should put the prefixes in alphabetical order before the stem. This means that bromo- will appear
before nitro- in the name because B is before N in the alphabet. So the correct name of the molecule
is 2-bromo-4-nitrotoluene. Dashes should be used between
numbers and letters in the name. And the last prefix should be
combined with the stem into a single word.
In conclusion, the name given to
the trisubstituted benzene molecule in this problem is 2-bromo-4-nitrotoluene, or
answer choice (E).
