Examples of Naming Cyclic Alkanes

Once again, I steal some problems from my textbook and do them here.
It's a ring made of six carbons, so it's a cyclohexane. Only one of the positions in the ring has any groups attached to it, and both groups are methyl groups, so it's 1,1-dimethylcyclohexane.
Another cyclohexane, obviously. This one has two groups at two different ring positions. The positions are across from each other in a 1,4 relationship. But which group gets numbered "1" and which one gets numbered "4"? Well, one is a methyl group and the other is a butyl group (four carbons in a straight chain). Alphabetical order determines which one comes first, so this is 1-butyl-4-methylcyclohexane.
The chain is bigger than the ring this time. So this compound is, as far as naming goes, defined as a five-carbon chain with a group attached at the first carbon, and the group that is attached is a cyclopropane ring. Therefore, we have 1-cyclopropylpentane.
The ring is a cyclopentane. Three groups this time, and all of them methyl groups, which makes naming this easy. Almost so easy that you could do it by yourself. But how do we number these groups? It doesn't matter which way we count, the smallest number we can start with is 1. This compound is 1,2,3-trimethylpentane.
This one is trickier. We definitely have a cyclohexane ring, but what are those groups attached to it. Well, let's start with the smaller one. It's an isopropyl group. See that? Probably not. Well, I told you, so now you know. Isopropyl group. The other one has four carbons. You might remember that there are four such groups possible. And if you have really been paying attention, it's clear that this is a sec-butyl group. Alphabetical order again, but the only prefix that matters for that is "iso-." That means the sec-butyl group is first. So this compound is 1-sec-butyl-2-isopropylcyclohexane.

Well, that's way that I learned to name this. And it's even the name that my solutions manual gives. But ChemSketch generated a different name that I am guessing is the true systematic name using proper IUPAC rules. The only difference is that the groups can't be written as isomeric forms of their straight-chain versions. This makes the nomenclature a bit messier (but it also scales up nicely, while the shortcut I'm using doesn't.

And that, children, is how to name cycloalkanes. I don't actually know what topic I'll cover next. You'll just have to wait to find out.

Nomenclature of Cycloalkanes

This will not cover all cycloalkanes. In fact, for now we're only dealing with compounds that have a single ring. But then I didn't really cover all acyclic alkanes either. But what you should have with this post is a basic idea of cycloalkane nomenclature.

Rings themselves are named by how many carbons they consist of. So, for example, this molecule...
...is cyclohexane. But you already know that, of course. I mean, you do, right? You'd better, seeing as I already told you that this is cyclohexane. Yes, it was back in October, but so what? I mean, you are supposed to read and remember everything I write here. You know, I'm getting the feeling that you're not being much of a team player here. Yeah, it sure seems like I'm the one doing all the work. Look, it's just cyclohexane. It's not complicated. It's a simple molecule with a simple name. Four syllables. That's not too many. Cyclohexane. Cyclohexane. Cyclohexane. And don't you forget it.

Speaking of earlier posts, in this one I showed cyclopropane. Unless you're as awful at geometry as you are at chemistry, you should be able to make the connection that if the triangle is cyclopropane and the hexagon is cyclohexane, a square is cyclobutane and a pentagon is cyclopentane. Yes, and a heptagon is cycloheptane and so on. All we're doing is using those chemical numeric prefixes I showed earlier and counting the numbers of carbon atoms making up the ring. You can count, right? You can at least do that much.

But watch out. Not everything in the molecule is necessarily part of the ring...
That is methylcyclohexane (in glorious 3-D). Seven carbons, but only six of them form a ring. The pesky seventh one is attached to the ring. And if you've already forgotten how skeletal structures work, the hydrogens attached to the carbons are not drawn in. All but one of the ring carbons has two hydrogens. One of them has only one hydrogen and is also bonded to that carbon outside the ring, which itself has three hydrogens. So it's a methyl group. Hence the name: methylcyclohexane.

Of course multiple groups could be attached to the ring. In that case, we use numbers. This compound...
...goes by the name 1-ethyl,3-methylcyclohexane. And if you think in terms of the rules you learned for acyclic alkanes, this makes sense. We have to number the positions on the ring somehow. So we're starting at first substituent alphabetically. Here, I'll even put the numbers in...
This system of numbering positions on rings will be used a lot in the future, so you should be comfortable with it. But it seems straightforward enough to me, so I'm not going to reiterate it further.

We might also end up with two groups attached at the same position on a ring. Not to worry...
That's 1,1-dimethylcyclopentane. The same general principles from naming acyclic alkanes still apply. This does run into limitations of course. I won't be covering those now. But I do think that I should to a follow-up post in which I name some examples from homework problems in the textbook. And just so that you can follow along, there is one more tiny little thing that you need to know about cycloalkanes. If a ring is attached to a hydrocarbon chain that is longer than the number of positions in the ring (like if a cyclopentane ring had an octane chain attached to it), the compound is named based on the chain (so that example I just made up would be 1-cyclopentyloctane).