Examples of Naming Acyclic Alkanes

As promised, here are some right out of the textbook.

The first one is in condensed notation: CH₃CH₂CH(CH₃)CH₂CH₃

Since I am so good, I immediately recognize that the third carbon has a one-carbon branch. Other than that, this is a straight chain. But let's not get ahead of ourselves. We are doing this the right way. We start with the last part of the name. With no heteroatoms, this is a hydrocarbon. With no multiple bonds, it's an alkane. With no rings, it's an acyclic alkane. We know the name must end in "-ane." Next, what's the longest carbon chain? Five. So the parent name is pentane. Branches? Yes, at the third carbon (counting either way). And the branch is a methyl group. Therefore, the name of this compound is...

...3-methylpentane. And you know what else? I checked the answer in the study guide and I was right! Woo hoo, Stephen got something right. Anyway...

(CH₃)₃CCH₂CH(CH₂CH₃)₂

This one is harder. First we have three methyl groups attached to one carbon. That carbon links to another that links to another, which is attached to two ethyl groups. Which methyl group and which ethyl group is considered part of the longest carbon chain does not matter because the groups are identical (that is, the methyl groups are identical to each other and the ethyl groups are identical to each other). So adding those three carbons to the rest of the chain, we find that the longest carbon chain is six carbons long, so this is a hexane.

Which group gets priority? In this case, we go in alphabetical order. "E" comes before "M." So this should be...

...3-ethyl-5,5-dimethylhexane. Or not. Oops. I started at the wrong end. It's actually 4-ethyl-2,2-dimethylhexane. It's that instead of the one I thought it was because 2 is lower than 5. It doesn't matter that 3 is lower than 4 because the method that gives the lowest number period is the one that gets priority, not the one that gives the lowest sum or anything like that. I hope you learned your lesson. Moving on.

CH₃(CH₂)₃CH(CH₂CH₂CH₃)CH(CH₃)₂

A propyl group? No, that's part of the longest carbon chain. They're trying to trick us. Starting from the left we have a carbon and then a string of three more, so that's four in a row. Then there's another (five) with that propyl group branching off. If we count going up the propyl group we get three more (eight). If we treat the propyl group as a branch, we get another carbon with two methyl groups, one of which would be a branch, making the total length seven. Sneaky textbook. This is actually an octane.

If we start from the end of what's being labeled as a propyl group (but is actually part of the chain) we get a branch at the fourth carbon. Starting from the left makes it at the fifth, so we start from the end of the propyl group instead. The branch consists of three carbons and two of them are attached to the other, which is where the branch connects, so it's an isopropyl group, meaning the compound is...

...4-isopropyloctane. And I'm right. I rule.

Enough of these condensed structures!
I used MS Paint because it was a small one and it's kind of hard to make them look less awful on ChemSketch. Anyway, this one seems easy to me. Five carbons long means pentane. Two methyl branches at the second carbon and two at the fourth. Therefore...

...2,2,4,4-tetramethylpentane. And I am right again. Excellent.
It's seven carbons long, but there are a couple of different ways to arrive at that. The one that give the lowest number to a branch is the one that simply starts on the far left, for a methyl group at the second carbon. There's another one at the fifth carbon and an ethyl group at the third, so this is...

...3-ethyl-2,5-dimethylheptane. And I'm right yet again. Three in a row! Let's do one more.
I moved back to MS Paint again when I perhaps should not have. But ChemSketch was being annoying (it kept trying to put rings into this). Obviously this one is larger than the other ones so far, but the principle is the same. The longest carbon chain is ten. The fastest we can get to a branch with it is on the second carbon, again counting from the far left. From there we label the other branches and put them in the proper order. About that, the branch on the fifth carbon is a sec-butyl group. When alphabetizing the branch names, this is treated as a "B" and not as an "S." The same would be true for tert-butyl but not for isobutyl. Unnecessarily confusing, I know. But in this case it does slightly affect the name, which is...

...5-sec-butyl-3-ethyl-2,7-dimethyldecane. And that's pretty much all there is to it. The study guide I used to check my answers breaks the process into three steps.

1. Name the parent chain by finding the longest C chain.
2. Number the chain so that the first substituent gets the lower number. Then name and number all substituents, giving like substituents a prefix (di, tri, etc.).
3. Combine all parts, alphabetizing the substituents, ignoring all prefixes except iso.

It takes some getting used to, but this is the basis for how other compounds, even ones with multiple functional groups, are named.

Nomenclature of Acyclic Alkanes: Prefix

I hope you have the other component of naming alkanes down, because I am never reviewing it again (just kidding). Now for the prefix. While the parent name identifies the longest carbon chain, the prefix tells us where on that chain branches occur and what the branches look like. Depending on how much branching (and what kind) is going on, the prefix may be anywhere from nonexistent (no branches, which we sometimes denote by using "n" as a prefix) to ridiculously long.

Firstly, the location of a branch is denoted using Arabic numerals. A branch at the second carbon in the longest carbon chain gets a "2" and a branch at the third carbon gets a "3" and so on. Some carbons in the longest carbon chain might have two branches. When that happens, its number gets used twice.

Often, there are multiple possible places to start from. With alkanes, the correct starting carbon is the one which, when started from, yields the lowest possible number being named first. If we start counting on one end of a chain and the first number that comes up is for a branch at the fourth carbon, but counting from the other end of the chain would make our first branch be at the second carbon, then it is the end that would make the first branch be at the second carbon that is the correct starting point.

Also, numerals are separated from each other by commas and from the rest of the name by hyphens. That's not just for alkanes. That's a universal rule. Commit it to memory, slave.

Anyway, to specify how long a branch is, we use the wonderful numerical prefixes I introduced in my last post. You know, the ones that are mostly Greek, but not really. A branch that is only one carbon is a "methyl" group. Two carbons is an "ethyl" group, etc. A branch that is seven carbons long is a "heptyl" group (and since it's not part of the longest carbon chain, that means the longest carbon chain must be really long). This all works nicely for branches that are themselves straight. But what about branches that have branches of their own? That's the hard part. Kind of. In order for considerable branching to occur, the molecule itself has to be pretty big. I've never had to deal with such compounds myself. The textbook is covering substituents with up to four carbons and that's always been good enough for what I've had to do. There are not very many. Here we go...

Methyl group: R—CH₃
Ethyl group: R—CH₂CH₃
Propyl group: R—CH₂CH₂CH₃
Isopropyl group: R—CH(CH₃)₂
Butyl group: R—CH₂CH₂CH₃
sec-Butyl group: R—CH(CH₃)CH₂CH₃
Isobutyl group: R—CH₂CH(CH₃)₂
tert-Butyl group: R—C(CH₃)₃

If you find the condensed structures confusing for those four-carbon groups, here are some links to pictures (off-site) for the butyl variations...

Butyl, sec-butyl, isobutyl, and tert-butyl.

And that's all. Now you know how to name acyclic alkanes. Oh, one more thing. If two or more of the same type of branch exists in a molecule, those branches get named together and get a Greek numerical prefix just to confuse you even more. But really, that's it. Stay tuned for next time, where I'll do a follow-up post with some examples of naming alkanes using problems from the textbook. Oh wait, this isn't a radio. You can't tune anything. Whatever.

Nomenclature of Acyclic Alkanes: Parent Name

I mentioned the IUPAC systematic nomenclature system before. I think I did, anyway. This project has been on hiatus for a while and I can't remember. But I'm back now! Really. I hope. Anyway, today we are going to learn how to name some alkanes. It's easy to do, and you need to know it to name other compounds. So learn it. I command you.

Let's start at the end. That's a good place to start, right? The last part of the name of any alkane is, get ready for this...

...it's "-ane." That should be quite easy to remember, even for you, because "alkane" itself ends in "-ane." If a compound is an alkane, its name ends in "-ane" and, conveniently enough, if a compound is not an alkane, its name will not end in "-ane." I know. Chemistry is so hard.

Next, we find the longest carbon chain. This is actually very easy, but teachers love trying to trick beginning students with odd drawings where they make part of the longest carbon chain look like a branch to people who are not paying attention. If this were a real chemistry class and I were the teacher (that would be bad), I would totally do this to you because I think it's hilarious. For now, I'll just give you the benefit of the doubt and assume that you are paying attention and can tell what the longest carbon chain in a molecule is.

Really? I shouldn't do that? Fine.
How long is the longest carbon chain? If you answered eight, congratulations, you did not fall for the dumbest trick in chemistry class. If you answered some other number, you were not paying attention or you cannot count or you're just a moron or something. I don't know. Shame on you anyway. You're bad (unless you got the right answer).

Once we know how long the longest chain is, we convert that into a numerical prefix, then attach it to our "-ane" suffix. Convert it into a numerical prefix? Yes, it's easy. No really. It is easy, just so long as you already know the Greek numerical prefixes—and use the Latin one for "nine" just to mess things up—and forget the first four prefixes and make up new special ones that are specific to chemistry. It was easy for me though! Here, I'll give you the first ten and we'll worry about going higher later.

1 = "meth"
2 = "eth"
3 = "prop"
4 = "but" (pronounced like the word "butte" just to confuse you even more)
5 = "pent"
6 = "hex"
7 = "hept"
8 = "oct"
9 = "non" (pronounced so that it rhymes with "tone" and not some other way)
10 = "dec"

Memorize them now. I command you. Done? Good. See, that wasn't so bad. Now, there's just one more tiny thing. Then we'll be all done and you'll know how to name acyclic alkanes. We have straight chains covered (unless they're longer than ten carbons long, but shut up). So a hydrocarbon that is a straight chain with five carbons would be "pentane" and one with nine would be "nonane" and so forth. Everything is fine, and then branches come and mess it all up. Not to worry: the IUPAC has an elaborate set of rules for us to denote where on a chain the branches lie and what the branches look like using prefixes and attaching them to the parent name (which simply describes the longest carbon chain. Well, it's elaborate enough that I'll save it for my next post, anyway. For now, just have the whole parent name part down.