Naming Benzene Rings: Videos & Practice Problems

Benzene is a key aromatic molecule with unique naming conventions. Important derivatives include phenyl (alcohol), toluene (methyl), aniline (amino), benzoic acid (carboxylic acid), and benzaldehyde (aldehyde). For disubstituted benzene, use ortho (O), meta (M), and para (P) instead of numbers. For three or more substituents, numerical locations are necessary. Understanding these names and structures is crucial for mastering aromatic compounds and their reactions in organic chemistry.

Benzene Nomenclature

Benzene is by far the most common aromatic molecule that we're going to be using this semester. So we're going to have to get really good at the naming of it. And it turns out that since this is a really old molecule, it's going to have some distinct and weird namings 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 and 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. Now just so you guys know, your professor may not think that all of these derivatives are important, so 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 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 to 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 substituent. This would be like an ether, right? R-O-R, methoxy. If you see methoxy, that's called anisole. Like I said, some of these are getting weird. If you see 2 methyl groups, that's not toluene anymore. It would be wrong to call this methyl toluene. Instead, we would call this xylene. See, I told you they're weird. What else? Well, if you have an aldehyde on your benzene, that's going to be called benzaldehyde. That one makes sense. It's kind of like carboxylic acid is benzoic acid. A benzene aldehyde is benzaldehyde. That one was pretty easy. Now, what if you have the most basic ketone, a methyl ketone? We just talked about benzaldehyde being the most simple aldehyde or not the most simple but an aldehyde. Methyl ketone on a benzene ring is actually a weird name. It's acetophenone. Acetophenone. Then 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 the 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 when we talk about catecholamines which are neurotransmitters and all you know, their molecules that work in your body. But for right now, just know that this is called a catechol. It's not something I'd 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. Mono substituted benzene, no location necessary. If you have a methyl benzene, you should just call it toluene. You should not call that 1 toluene. You don't need to do that. Or 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. Okay? Now guys, 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 3 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 the definition. Part of the definition of aromaticity 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 disubstituted benzene like I just noticed, 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. And 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. Now these words, they don't all make tons of sense but para definitely means across from, so 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. Next to. Ortho would be next to each other. Para would be across from each other and 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 guys if your benzene has 3 substituen

Name the following benzene derivative

• A. m-toluene
• B. o-xylene
• C. m-xylene
• D. p-xylene

Answer: (B)

Name the following benzene derivative

• A. p-iodoaniline
• B. m-iodoaniline
• C. p-aminoidobenzene
• D. o-aminoiodobenzene

Answer: (A)

Name the following benzene derivative

• A. o-cyclohexylethylacetophenone
• B. 1-cyclohexyl-2-ethyl-4-acetophenone
• C. 1-cyclohexyl-6-ethyl-4-acetophenone
• D. 4-cyclohexyl-3-ethyl-1-acetophenone

Answer: (D)

Here’s what students ask on this topic:

What are the common names for benzene derivatives with different functional groups?

Benzene derivatives have specific common names based on the attached functional groups. An alcohol group attached to benzene is called phenol. A methyl group attached to benzene is known as toluene. An amino group attached to benzene is called aniline. A carboxylic acid group attached to benzene is named benzoic acid. An aldehyde group attached to benzene is called benzaldehyde. These names are crucial for understanding aromatic compounds and their reactions in organic chemistry.

How do you name disubstituted benzene compounds?

For disubstituted benzene compounds, we use the terms ortho (O), meta (M), and para (P) instead of numerical positions. Ortho (O) indicates substituents on adjacent carbons (1,2-positions), meta (M) indicates substituents separated by one carbon (1,3-positions), and para (P) indicates substituents on opposite sides of the benzene ring (1,4-positions). For example, a benzene ring with two hydroxyl groups in the 1,2-positions is called ortho-dihydroxybenzene or o-dihydroxybenzene.

When do you use numerical locations for naming benzene compounds?

Numerical locations are used for naming benzene compounds with three or more substituents. The ortho, meta, and para system becomes confusing with more than two substituents. For example, a benzene ring with three ethyl groups at positions 1, 2, and 4 is named 1,2,4-triethylbenzene. This system ensures clarity in identifying the exact positions of the substituents on the benzene ring.

What is the significance of the circle inside a benzene ring structure?

The circle inside a benzene ring structure represents the delocalized π-electrons in the ring, indicating resonance. Instead of drawing alternating double and single bonds, the circle simplifies the representation of benzene's aromaticity. This notation highlights that the electrons are evenly distributed across the ring, contributing to benzene's stability and unique chemical properties.

Why is it important to learn the common names of benzene derivatives?

Learning the common names of benzene derivatives is important because these names are frequently used in literature, exams, and practical applications. Common names like phenol, toluene, aniline, benzoic acid, and benzaldehyde are widely recognized and used in organic chemistry. Understanding these names helps in identifying compounds quickly and facilitates communication among chemists.