Below is the written transcript of my YouTube tutorial video Naming Branched Substituents – Tert-Butyl, Isopropyl.
If you prefer to watch it, see Video HERE, or catch the entire Naming Organic Compounds Series.
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Leah here from Leah4sci.com and in this video, I will show you how to name branched substituents. In other words we’ll look out molecules that have a substituent coming off of another substituent.
We’ll use this molecule as our first example. We name the branched substituents using the same rules that we introduced in the first video. The first thing we want to do is identify the parent chain, or the longest carbon chain and highlights or mark it, and then we’ll number the parent chain. Given that I have 4 carbons from the right and 4 carbons from the left, it doesn’t matter where I start. I have a total of 9 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 substituents. If we only had the straight chain with only 3 carbons as a substituent, this would be a propyl. However, there’s a branch coming off the molecule which changes the rule 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 off by identifying the parent chain in the substituent where carbon number 1 is carbon attached to the parent chain. I have two methyl groups coming off the second carbon but it doesn’t matter which one I number as 3. I have a total of 3 carbons in the longest substituent chain which gives me a propyl group. Prop telling me 3 and yl telling me it’s a substituent. And then I have a methyl group coming off the second carbon, which gives me a 2-methylpropyl. However, since this entire substituent comes off of carbon 5 on the parent chain , I put the 5 infront of the name and 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 from this carbon we have only 2 on the left, but we have 3 carbons going down, which means my parent chain actually has the curve downward. I start number my chain from the bottom carbon because I reach my substituent group at 4, whereas if u started from the top right, I would reach my substituent at 5. Having 8 carbons in my parent chain, gives me a first name of “oct” having only single bonds, gives me a last name of “ane”. Ones again, we find that we have a branched chain substituent. We number the carbon coming of the parent chain as 1, and another of the carbons as 2. Since I have 2 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 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 in 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. 10 carbons on my parent chain gives me a first name of “dec” , only a single bonds gives me a last name of “ane”. This substituent is exciting, because in addition to a parent chain of 2 carbons, we have 2 substituents coming off my substituent. The parent chain of the substituents have 2 carbons, giving me the name “ethyl”, but coming off of the ethyl, I have 2 methyl groups on carbon number 1, and so I name them 1,1 dimethyl. Since the entire substituent comes off of carbon #4, we put 4 dash 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 they’re so much easier to name.
I’ll use R to represent the rest of the molecule, meaning the parent chain so that we focus on only the substituents. If you have a substituent of 3 carbons in a row, assuming all associated hydrogens are present, this is called propyl substituent. However, when you have a substituent that is 3 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 the shortcut for this name which is isopropyl. The prefix “iso” or isomer, tells you that we have the same molecular formula as the propyl substituent, however it’s connected in a different way. In other words, the isopropyl is a constitutional isomer of the standard or normal propyl substituent. When written out on line structure, the standard propyl is written as follows. The first end is attached to the R group or parent chain and doesn’t count so we just have 3 carbons coming off of the substituent while the Isopropyl looks as follows: ones again this represents the bond between the parent chain and the substituent and we have the 3 carbons attached at the second rather than the first carbon.
There are 4 isomers of a 4 carbon substituent. The first one having 4 carbons in a row with associated hydrogens is considered a normal butyl substituent , but meaning 4. However, you can have Isomers of this represented in a number of ways. When you have your 4 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 also 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 3 carbons in a row, but instead of the 4th carbon coming off the first substituent carbon, it comes off at 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 4 carbon substituent where 1 carbon is attached to the parent chain and the other 3 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 3 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 4 carbons coming off of a parent chain, for the secbutyl I have 3 carbons coming off of the parent chain with a branch coming off at first carbon, for the isobutyl, I have that same substituent of 3 carbons, but my branch comes from the second to last carbon. And finally for the tertbutyl, I have 2 carbons and 2 branches all originating at the same area. Many students often confused the isobutyl with secbutyl so I wanna show you a little trick.
Let’s compare the isobutyl to the isopropyl which look like this and I want you to notice that both the isopropyl and the isobutyl have the branch chain at the end just 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 2-(2-methylpropyl) nonane, if we analyze the substituent recognize that this is an isomer of butyl with a 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 ones again the branched tail helps you recognize that it’s an isopropyl group. Renaming this molecule, I get a 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 3 carbons coming off of it, we recognize it as 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 bi-cyclic alkanes.
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