Leah here from Leah4Sci.com. And in this video, I will show you how to name branched substituents.
In other words, we'll look at molecules that have a substituent coming off of another substituent.
We'll use this molecule as our first example. We name the branched substituent using the
same rules that we introduced in the first video.
The first thing that we want to do is identify the parent chain or the longest carbon chain
and highlights to mark it and then we'll number the parent chain. Given that I have four carbons
from the right and four carbons from the left, it doesn't matter where I start. I have a
total of nine carbons giving me a first name of non. Only single bonds in the chain giving
me a last name of ane.
This molecule gets tricky when you look at the substituent. If we only had the straight
chain with three carbons as a substituent, this would be a propyl. However, we have a
branch coming off the molecule which changes the rules slightly.
When you're naming a substituent, you treat it almost as if it's a new molecule that you
have to name and you start out by identifying the parent chain on the substituent where
carbon number one is the carbon that is attached to the parent chain. I have two methyl groups
coming off the second carbon so it doesn't matter which one I number as three.
I have a total of three carbons in the longest substituent chain which gives me a propyl
group. Prop telling me three and yl telling me that it's a substituent. And then I have
a methyl group coming off of a second carbon which gives me a 2-methylpropyl. However,
since this entire substituent comes off of carbon 5 on the parent chain, I put a 5 in
front of the name and then put the entire substituent in parenthesis. This gives me
a final name for this molecule of 5- (2-methylpropyl) nonane.
Let's try another example and this one is slightly tricky. At first glance, this appears
to be my parent chain. However, when you look closer, coming off of this carbon, we have
only two on the left but we have three carbons going down which means my parent chain actually
has to curve downward. I start numbering the chain from the bottom carbon because I reached
my substituent group at four whereas if I started from the top right, I would have reached
my substituent at five. Having eight carbons in my parent chain gives me a first name of
oct, having only single bonds gives me a last name of ane.
Once again, we find that we have a branch chain substituent. We number the carbon coming
off the parent chain as 1 and another of the carbons as 2. Since I have two carbons in
my substituent chain, this gives me an ethyl. But coming off of the first carbon, I have
a methyl group which gives me a 1- methyl. The entire substituent is coming off of carbon
4 which gives me a 4- 1-methylethyl, for a final name of 4- (1-methylethyl) octane.
We'll use this method on one more example and then I'll show you a shortcut. If we highlight
our longest carbon chain, we recognize that we have a total of 10 carbons. I have to number
this chain from the left because I hit my substituent at carbon number 4. Ten carbons
on my parent chain gives me a first name of dec, only single bonds gives me a last name
of ane.
This substituent is exciting because in addition to a parent chain of two carbons, we have
two substituents coming off my substituent. The parent chain of the substituent has two
carbons giving me the name ethyl. But coming off of the ethyl, I have two methyl groups
on carbon number 1 and so I name them 1,1-dimethyl. Since the entire substituent comes off of
carbon number 4, we put 4- parenthesis for a substituent named 1,1-dimethylethyl, for
a final name of 4- (1,1-dimethylethyl) decane.
If naming those branched substituents were a little tedious, I have good news. There
are certain substituents that have an accepted abbreviation and are so much easier to name.
I'll use R to represent the rest of the molecule meaning the parent chain so that we can focus
on only the substituents. If you have a substituent of three carbons in a row, assuming all associated
hydrogens are present, this is called a propyl substituent.
However, when you have a substituent that is three carbons attached to the parent chain
by the second instead of the first carbon, you have the option to name the substituent
1-methylethyl or you can use a shortcut for this name which is isopropyl.
The prefix iso or isomer tells you that we have the same molecular formula as a propyl
substituent however it's connected in a different way. In other words, the isopropyl is a constitutional
isomer of this standard or normal propyl substituent. When written out in line structure, the standard
propyl is written as follows, the first n is attached R group or parent chain and doesn't
count so we just have three carbons coming off of the substituent while the isopropyl
looks as follows. Once again, this represents the bond between the parent chain and the
substituent and we have the three carbons attached at the second rather than the first
carbon.
There are four isomers of a four carbon substituent. The first one having four carbons in a row
with associated hydrogens is considered a normal butyl substituent, "but" meaning four.
However, you can have isomers of this represented in a number of ways. When you have your four
carbons in a row, however the group is connected to the parent chain by the second rather than
the first carbon, this would be named as a 1- methylpropyl substituent but can only be
named secbutyl. Sec meaning secondary because the carbon attached to the parent chain has
two carbon atoms coming off of it within the substituent alone.
If I have a similar chain where an isomer of this substituent has three carbons in a
row but instead of the fourth carbon coming off the first substituent carbon, it comes
off of the second one. The long name for this would be 2-methylpropyl or you can name this
isobutyl given that this is an isomer of the butyl substituent.
And last but not the least, you can have a four carbon substituent where one carbon is
attached to the parent chain and the other three carbons are attached to this first carbon.
The long name for this substituent would be 1,1-dimethylethyl but you can also call this
substituent tertiary or simply tertbutyl. And that's because the carbon that is directly
attached to the parent chain has three carbons coming off of it within the substituent.
Now, let's see how each of this would look in line structure. For the normal butyl, I
simply have four carbons coming off of a parent chain. For the secbutyl, I have three carbons
coming off the parent chain with the branch coming off that first carbon. For the isobutyl,
I have that same substituent of three carbons but my branch comes from the second of the
last carbon. And finally for the tertbutyl, I have two carbons and two branches all originating
at the same area.
Many students often confused the isobutyl with the secbutyl so I'm going to show you
a little trick. Let's come here at the isobutyl to the isopropyl which looked like this and
I want you to notice that both the isopropyl and isobutyl have the branch chain at the
end which is how you can recognize the iso group.
And now that we know how to name the special isomers, let's go back to our original problems
and see if we can make those names shorter. For our first example of 5- (2- methylpropyl)
nonane, if we analyze the substituent, recognize that this is an isomer of butyl with that
fork or branch at the end which means this is an isobutyl group. This gives me the simpler
name of 5-isobutylnonane.
Our second example of 4- (1-methylethyl) octane can be recognized as an isomer of a propyl
substituent. And once again, the branch tail helps you recognize that it's an isopropyl
group. Renaming this molecule, I get 4-isopropyloctane.
Our final example of 4- (1,1-dimethylethyl) decane looks a lot less scary when you recognize
that this is simply an isomer of the butyl substituent. Since we have a central carbon
with three carbons coming off of it, we recognize it as a tertbutyl giving me a final name of
4-tertbutyldecane.
Be sure to join me in the next video where I show you how to name cyclic and bicyclic
alkanes.
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