Video Transcript
What is the name of this
molecule? (A) 2-Bromopentane, (B)
5-bromopentane, (C) 1-bromopentane, (D) 2-bromohexane, or (E) 5-bromohexane.
The diagram drawn here is what’s
known as a skeletal formula. A skeletal formula represents
carbon-carbon bonds as lines. In other words, we can assume that
there is a carbon atom at the intersection or end of each of the lines in the
diagram. Hydrogen atoms are also assumed to
be present but not directly indicated by the skeletal formula. The benefit of a skeletal formula
is that by minimizing the visual presence of carbon and hydrogen atoms, it allows us
to clearly and quickly visualize which atoms and functional groups are attached to
the carbon chain. In this case, we can see that a
bromine atom is attached to the carbon chain.
In this question, we are being
asked to name the compound that appears in this skeletal structure. The five answer choices give us
five possible names that each follow a similar template: a number followed by the
prefix bromo- followed by pentane or hexane. Let’s take a look at the three
individual parts of the name to figure out which one is represented in the diagram
here.
The part of the name that ends in
-ane describes the molecule’s carbon chain. Is this alkane, pentane, or
hexane? Well, the diagram shows that there
are six carbon atoms in the structure. A six-carbon chain is called
hexane. Hexa- means six, just like a
hexagon has six sides. On the other hand, pentane would
have only five carbons. So we can write in hexane as the
first part of the final answer.
The prefix bromo- appears in all
five of the answer choices. This prefix represents the bromine
atom attached to the carbon chain. We can include it in our final
answer. The last part of the name to figure
out is the number that goes in front of it.
Eliminating the answer choices that
use pentane instead of hexane, we’re left with two options. Is this 2-bromohexane or
5-bromohexane? The number in this name will
indicate the position of the attached atom or functional group. In this case it’s the attached
bromine atom. The bromine atom is attached to
this carbon right here. So the number that appears in the
name will be the position of this carbon atom one, two, three, four, five, or six
along the chain.
We’ve already numbered the carbons
right to left, but we could do it another way. We could number them left to
right. How do we know which way is
correct? As a tiebreaker, we can say that
the carbon atoms are numbered to minimize the values that appear in the name. Numbering the carbons from right to
left, like we’ve done in pink here, makes the carbon of interest carbon number two,
a lower number than if we numbered the carbons from left to right.
Since the bromine atom is attached
to carbon number two, the number we include in the name of this compound is two,
giving us the full name 2-bromohexane. This corresponds to answer choice
(D). The names of compounds like this
reveal important information about the carbon chain, the attached atoms or
functional groups on the carbon chain, and the location of those attachments. So what is the name of this
molecule? That’s choice (D)
2-bromohexane.
