Naming Benzene Rings

Benzene is by far the most common aromatic molecule that we're going to be using this semester. We're going to have to get really good at the naming of it. It turns out that since this is a really old molecule, it’s going to have some distinct and weird naming that most molecules wouldn’t have. Let’s just jump right into it.
Benzene was one of the first organic molecules to be identified way back in the early 1800s. What we find is that when we’re naming these guys, common names predominate. Meaning that there’s IUPAC and there’s common. A lot of common names are based on just weird Greek roots, Latin roots, people's girlfriends, naming them after their wives and stuff. It's weird. Most of these names are going to make sense thankfully but there are going to be some weird names that you don't have a really great way to understand where those names come from.
First of all, we have to know what the benzene derivatives are. Just so you know, your professor may not think that all of these derivatives are important. You're just going to have to calibrate this knowledge to whatever you learn in class. I'll go ahead and tell you all the names and then I’ll tell you which ones are the most important for you to know.
Definitely an alcohol attached to a benzene gets the name phenol, huge, very important. A methyl attached to a benzene gets the name toluene. An amino group attached to a benzene ring gets the name aniline. A carboxylic acid attached you a benzene ring gets the name benzoic acid.
This is actually called a methoxy group. Back when we talked about ethers, we learned that ethers can be named with an alkoxy substituents. Those would be like an ether, ROR, methoxy. If you see methoxy, that's called an anisole. Like I said, some of these are getting weird. If you see two methyl groups, that’s not toluene anymore. It’d be wrong to call this methyl toluene. Instead, we would call this xylene. See, I told you they’re weird.
What else? If you have an aldehyde on your benzene, that's going to be called benzaldehyde. That would make sense. It’s kind of like carboxylic acid is benzoic acid. A benzene aldehyde is benzaldehyde. That one was pretty easy. What if you have the most basic ketone, a methyl ketone? We just talked about benzaldehyde being an aldehyde. Methyl ketone on a benzene ring is actually a weird name. It’s acetophenone.
Finally, if you have 2 alcohols that are specifically in a 1,2 position next to each other, we're going to talk more about what those positions actually what we call there in benzene. But if you have those alcohols right next to each other, that's called a catechol.
Now that we've gone through all these crazy names, I'll go ahead and say that they’re all pretty important except for maybe the ones that I wouldn't necessarily memorize unless your professor really mentions them in class is anisole. I’m just going to take this one kind of off the list. Don't worry about it too much. Also catechol, I would probably take that off the list. Catechols become really important later in your medical studies where you talk about catechol amines which are neuro transmitters. They’re molecules that work in your body.
But for right now, just know that this is called a catechol. It's not something I need you to memorize at this moment. All these other ones you're going to see at some point this semester for sure. I would definitely memorize them.
Now let’s talk about locations. It turns out that the way that you number your substituents actually depends on the number of substituents you have. Monosubstituted benzene, no location necessary. If you have a methyl benzene, you just call that toluene. You should not call that 1-toluene. You don’t need to do that. Let’s say that you have a chlorine on the benzene. You should not call that 1-chlorobenzene. You should just name it chlorobenzene. I’m just going to write an example here of Cl and that would be chlorobenzene.
I wasn't planning on discussing this right here but this is a better time than any to say “Why the heck did I just put a circle inside of that molecule?” I have not done that yet this semester but just remember that benzene rings don't always have to be written with three double bonds. You could just write a circle and that means that those double bonds are in resonance with each other which we learned is actually part of the definition of aromaticity. It had to do with fully conjugated. If you get an aromatic molecule in the future, you can just write a circle in it if you want to be lazy like me. Notice there's no location there.
What if you have a disubstituted benzene? I just mentioned 1,2 hydroxyl groups. In that case, we don't use numbers if you only have 2. We’re going to use letter locations. 1,2 instead of being called 1,2, we actually call that ortho. Instead of 1,3 we’re going to use the word meta. Instead of 1,4, we're going to use the word para.
These even get shorter acronyms. Instead of ortho, we would say o, meta would be m, para would be p. These words, they don't all make tons of sense but para definitely means across from. That helps. Meta means middle. It’s something in the middle. Ortho doesn't technically mean next to. It didn't mean that before. But these days, we'll say that's what the definition is. Ortho would be next to each other. Para would be across from each other. Meta would be in the middle. Those would be the numbers that would correlate to them.
This is the way that you name a disubstituted benzene. But what happens if your benzene has three substituents on it? You can't use ortho, meta, para anymore because it’s going to be very confusing. Are you talking about that the first two groups are meta to each other or the first and the third group are meta to each other? It doesn't make sense. The system breaks down if you have more than two. That’s what we call multisubstituted benzene. That would be three or more substituents. If you have three or more, then numerical locations are necessary. You can't use ortho, meta, para. If you use it, you look like a dumbass because you look like you don't understand what o, m and p mean.
Let's take a really easy example. Throw three of the same atom on it. Let’s say that I was to throw three ethyl groups. Then you would name them literally 1,2,4 triethyl benzene. You would not try to use ortho or meta or para in that name because it's going to be very confusing.
That said, we’re going to do some practice. We're going to put all this together. Go ahead and try to do this first problem. Name it according to what we just learned and then I’ll walk you through the answer.