Alkyl Groups

IUPAC naming is awesome because it’s pretty easy to master. But unfortunately there are some non-IUPAC names we need to know for this course. These apply mostly to weird looking substituents. Let's take a look.

Concept: Understanding Non-IUPAC Substituents 

Video Transcript

All right guys, so now that you know how to name simple alkanes, we're going to start increasing the difficulty level slowly. The first thing that I want to talk about is substituents. What happens if you have a substituent that you don't recognize the name for. That's what this next section is about.
It turns out that even though we try to use the IUPAC system as much as we can, there are a few common substituents that – remember common means that it's an old name. This is a name that comes from – predates 1919, predates the convention. These are names that have just stuck because they've been used for so long that even though they don't follow IUPAC convention, you're still supposed to know them.
Unfortunately, there's eight compounds, there's eight branches that you should just know the names for no matter what even though they're kind of random, but you just need to know them.
Let's talk about the first two and these are going to be really easy. The first one is just if I have a three-carbon chain where the R group, R meaning the actual main chain, is attached to just the first carbon of the chain, this would be called propyl. You already know this actually.
Actually, this part is the IUPAC part. This one right here is IUPAC, so don't worry about this one. This one you already know.
There's another one that you're supposed to know which is how about if I have a four-carbon chain. If I have a four-carbon chain and its attached right there, then that one would be called butyl. These are easy. Propyl and butyl, that has to do with the IUPAC nomenclature. But then there are a few different ways that we can arrange these substituents.
Imagine that I still have a three-carbon chain, but instead of being attached to the primary position, notice that would be primary because it's only attached to – if you don't count the R – it's only attached to one other carbon on that substituent, what if you attached it to the secondary position. Well, then this gets its own name because it's not truly a propyl group. Even though it has three carbons, it's attached in a different spot, so this one would be called isopropyl.
Why is it called isopropyl? Because it's an isomer of the propyl substituent. But still that does not follow – like I said, that's not an IUPAC name. That just a name that someone a long time gave it because they're like, “Oh, there's an isomer of propyl.”
So isopropyl, the way that I think about it is it looks kind of like a Y, so if you see that, you know it's an isopropyl group.
Now butyl is even more confusing because butyl can arrange in actually four possibilities and all of them are going to get their own names, which sucks. Here we go.
Butyl is normally attached at the primary position. But what if, instead of attaching at the primary position of this four-carbon chain, what if I attached at the secondary position. Then what would we call it? Well, what we would call this is actually just in italics sec-butyl.
Guess what the sec stands for. It stands for secondary, so what we're saying here is that this is a butyl group that is attached to the secondary position instead of the primary position like normal.
Then how about if instead of being attached to the secondary position, now it's attached to a tertiary position? Now remember in all of these substituents I'm talking about only the substituent, not the R group. So what if it's attached in a tertiary position? Well, then this is going to be called tert-butyl.
Those are three really important forms of butyl that you need to know. Butyl, sec-butyl and tert-butyl.
Now it turns out that butyl has four carbons so there's even one more possibility which would be what if it's still attached at the primary position like normal, but then it has a Y at the end, kind of like isopropyl. Well, then this is going to get its own name and that one's going to be called isobutyl. Isobutyl looks like isopropyl except it has this extra CH2 in the middle.
So those are our propyls and our butyls. They kind of make sense the way that I'm describing them. Hopefully, that will help you memorize them a little bit better, but at the end of the day, this is just simple recognition. You should be able to just know what is what and you will need to know this for many tests, well into orgo 2 even.
I also just want to say one thing which is that some textbooks and some professors will use a different letter or use a different notation for propyl and butyl. Instead of just saying propyl and butyl, sometimes they'll say n-propyl and n-butyl. Honestly, I have no clue what the n stands for. I don't know what it is. All I know is that n means that it's a straight chain.
Sometimes your professor will say butyl, sometimes maybe your professor says n-butyl. They're the same exact thing. N-butyl just says hey, this is not isobutyl, this is not sec-butyl, this is a straight-chain butyl. Cool Awesome.
Now there's just two more that I want you guys to know and those are with double bonds. What if you have a double bond coming straight off of your main chain. So your main chain is really long. You have a double bond coming right off of it. This gets its own unique name that's called vinyl. And you have heard of vinyl before. Vinyl is a plastic. That plastic is made out of these vinyl double bonds. The fact that old records are called vinyls, actually has to do with this molecule. But the molecule was named vinyl before the actual record was named a vinyl.
Then let's see. What if you have a double bond, but you're not attached directly to the R group, instead you have a CH2 in between. It's like a vinyl, but it has an extra CH2. This is going to be called – let me move out of the way – an allyl.
I understand these are weird words. These are not words that are fun to remember, but honestly, there's no way to get around it. You just need to know them.
What I want to do now is do some work practice with IUPAC nomenclature now knowing the names of these common substituents. What we try to do is, remember, we're still going to follow all the same rules as before. The only difference is that now, if we get a substituent that looks like that at the end, now we're going to know the name for it.
I want to do (a) as just a free response problem, like we're going to do it together. So go ahead and try and solve it and then when you're done, go ahead and go to the next video.  

Quick note: "n" stands for normal :)

Pro Tip: For the butyl groups, you can use the number of substituent carbons attached to it to determine the prefix on the name. (i.e. Butyl with 2 carbons branching off of it is a secondary, or sec-butyl)

Example: Name the following alkane


Remember to always apply IUPAC rules 1-5 before deciding if a substituent is common or not! Let’s try another. 

Example: Name the following alkane


Iso, just like cylo and neo should be considered when alphabetizing. However, prefixes such as tert-, tetra-, sec-, tri-, di- and so forth can be ignored

Awesome. Now you know how to name pretty much any branched alkane. Party time!

Alkyl Groups Additional Practice Problems

What is the IUPAC name for this alkane? 

(a) 2-ethyl-3-methylpentane 

(b) 4-ethyl-3-methylpentane 

(c) 3, 4-dimethylhexane 

(d) 2, 3-diethylbutane 

(e) octane

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What is the IUPAC name for 

(a) 1-methylbutane. 

(b) 4-methylbutane. 

(c) pentane. 

(d) butane. 

(e) hexane. 

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What is the IUPAC name for the compound shown below? Spelling and punctuation count!

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Branches within branches.

The IUPAC name of this compound is _____________. (omit italics)

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What is the IUPAC name for the compound shown below? 

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Select the correct name for the following compound. 

1, 1, 3-triethyl-2-methylbutane 

3-ethyl-4, 5-dimethylheptane 

2, 4-diethyl-3-methylhexane 

1, 1 -diethyl-2, 3-dimethylpentane 

none of the above

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What is the name of 

3, 3-dimethylbutane 2, 2-dimethylbutane 

dimethylbutane 2-dimethylbutane, hexane

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What is the correct name of the alkane shown below?

A. 2-Ethyl-3-isopentyl-3-propylhexane

B. 5-sec-Butyl-6-ethyl-2-methylnonane

C. 2-Methyl-5-sec-butyl-6-ethylnonane

D. 5-Ethyl-4-isopentyl-3-methyloctane

E. None of the above

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Draw a line–angle structure for 5–sec–butyl–3,3–diethyl–4–isopropylnonane.

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Draw a line–angle structure for 5–tert–butyl–8–isobutyldodecane.

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For the two questions below, an incorrect name has been given for an alkane. To answer these questions, you will need to draw the structure described by the incorrect name. Once you have drawn the structure, use the nomenclature rules to assign the correct name to the alkane.


The incorrect name is: 2-Methyl-5-tert-butylheptane What is the correct name of this alkane?

A. 5-Ethyl-2,6,6-trimethylheptane

B. 3-Ethyl-2,2,6-trimethylheptane

C. 5-tert-Butyl-2-methylheptane

D. 4-tert-Butylisooctane

E. None of the above


The incorrect name is: 4-Butyl-2,2-dimethyloctane What is the correct name of this alkane?

A. 5-Butyl-7,7-dimethyloctane

B. 2,2-Dimethyl-4-butyloctane

C. 5-Neopentylnonane

D. 5-tert-Butylnonane

E. None of the above

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Give the IUPAC name for the following hydrocarbon.

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Draw the line structure of the following compound.


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The name sec-butyl alcohol defines a specific structure but the name sec-pentyl alcohol is ambiguous. Explain.

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Give systematic IUPAC names for each of the following:

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Grandisol is one component of the sex attractant of the boll weevil. Write a structural formula for grandisol given that R in the structure shown is an isopropenyl group.

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Alkyl isothiocyanates are compounds of the type RN=C=S. Write a structural formula for allyl isothiocyanate, a pungent-smelling compound isolated from mustard.

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Write a structural formula for each of the following compounds:

(a) 6-Isopropyl-2,3-dimethylnonane

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Provide a structural formula for  5-(sec-butyl)-8-ethyl-3-methyldecane. Please show a complete structural formula showing all atoms and all bonding valence electrons.

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