Nomenclature of Heterocycles - Online Tutor, Practice Problems & Exam Prep

Heterocycles are cyclic compounds containing carbon and heteroatoms like oxygen, sulfur, or nitrogen. Saturated heterocycles are named using prefixes: "oxa" for oxygen, "thia" for sulfur, and "aza" for nitrogen, with the heteroatom at position 1. Unsaturated heterocycles, such as furan, thiophene, and pyrrole, use common names. Substituents are numbered to give the lowest possible numbers, prioritizing heteroatoms based on the mnemonic "open snacks nightly." Understanding these naming conventions is crucial for identifying and working with various heterocyclic compounds in organic chemistry.

Nomenclature of Heterocycles Concept 1

Everyone, in this video, we're going to take a look at naming heterocycles. Now, what exactly are heterocycles? Well, they are cyclic compounds composed of carbon and one or more heteroatoms. The heteroatoms that we're going to look at are oxygen, sulfur, or nitrogen.

When it comes to heterocycles, we say that saturated and unsaturated heterocycles are named differently. We're going to look at the naming convention for the saturated ones first. We'll say here that the parent chain is named as a cycloalkane with a prefix to indicate the type of heteroatom within it. Now we have oxygen, sulfur, and nitrogen. Their prefixes are: for oxygen, it's "oxa"; for sulfur, it's "thia"; and for nitrogen, it's going to be "aza".

Substituents are assigned location numbers and the heteroatoms are given the location of 1. Now, what happens if you have more than one heteroatom within your heterocycle? Well, we can follow a priority. We're going to say the priority if more than one heteroatom is present is oxygen before sulfur before nitrogen. And the memory tool that we can use to remember this order is that we open snacks nightly.

Now, if we take a look here at our heterocycle, it's a six-membered ring with oxygen within it. Our naming convention is the location of the substituent, then we have the prefix of the heteroatom followed by the parent chain name. In this example, we don't have a substituent on the ring; we just have the ring with oxygen within it. So it's a six-membered ring, so it would be cyclohexane, and we've replaced one of the carbons with an oxygen. Remember, for oxygen, its prefix would be "oxa".

So the name of this heterocycle would just be "oxa cyclohexane" all as one word. Right? So these are the rules that we'd follow when naming saturated heterocycles.

Nomenclature of Heterocycles Example 1

Hey, everyone. So here it says, name the following saturated heterocycle. Remember when we're numbering this heterocycle, we start out with the heteroatom. Since there's only one, we're going to start out with the sulfur atom. So this would be 1.

We then number the ring towards the next substituent. We're going to say 1, 2, 3. Our substituent is an ethyl group. So we're going to say here it's on carbon number 3. So this will be 3-ethyl.

Now remember, if our heteroatom is a sulfur, that means that the prefix will be thia. And then it's a 5-membered ring, and we know with a 5-membered ring, it would be a cyclopentane. So with this heterocycle, we say that the name is 3-ethylthiacyclopentane.

Provide the name for following structure. 

• A. 2-nitro-4-methylazacyclohexane
• B. 4-methyl-2-nitroazacyclohexane
• C. 4-methyl-2-nitro-1-azacyclohexane
• D. 4-methyl-1-nitroazacyclohexane

Nomenclature of Heterocycles Concept 2

Hey, everyone. In our continued discussion on the nomenclature of heterocycles, we now take a look at the unsaturated ones. Here we're going to say that unsaturated heterocycles use common names for the parent chain and when it comes to these common names we're going to have to first memorize these initial three. Knowing these three will help us to remember the following ones. So here we have Furan, Thiophene, and Pyrrole.

Now, Furan is a 5-membered ring with oxygen replacing one of the carbons and we have 2 double bonds. For Thiophene, it's sulfur that replaces the carbon and again, 2 double bonds. And then finally, we have Pyrrole which has a nitrogen replacing our carbon with 2 double bonds. And remember, nitrogen ideally wants to make 3 bonds, so it has to be connected to a hydrogen as well. Here our memory tool would be, furious, thieves, petrol.

Furious for Furan, Thieves for Thiophene and petrol for Pyrrole. The next 3 are oxazole, thiazole, and imidazole. They're very similar to these first three that we learned, except now we have a nitrogen in the top right corner. The hint here is that they all have an imine involved if you really think about it. It's a carbon double bonded to a nitrogen.

So think about a carbon double-bonded to a nitrogen to form this side of these 3 heterocycles. Next, we have these next 2. And if you pay close attention, you can see that this portion of it resembles Furan and this portion here represents Thiophene. Now, benzo is just referring to the benzene ring attached to these rings. So that's why our names here would be Benzofuran.

And could you guess this one? Yes. That's a Thiophene. So this would be Benzothiophene. Again, remembering these 2 is based on us remembering the first three that we looked at.

Next, we have indole. Indole has this portion of itself which resembles pyrrole, And the memory tool is that pyrrole is indole. Alright. So indole has a portion of it that looks like pyrrole. Next, we have this structure here.

And if we look, we're going to say that this portion resembles imidazole. So here we're going to say this is Benzimidazole. Next, our last 3 we have pyridine, pyrimidine, and quinoline. With pyrimidine or pyridine actually, pyridine, we're going to see iodine means a cyclic compound with nitrogen. So think of pyridine as a benzene ring that has a nitrogen that has replaced one of our carbons.

Here, pyrimidine is just one step more. Here, we can think of it as an imine involved right here. So it's related to pyrimidine in that way. And then, finally, here we have quinoline, which we're going to say queen in line for a parade. Quinoline.

Queen in line, Quinoline. For parade, we have a portion of it that kind of resembles pyridine. And then we have an additional ring attached to it that also has alternating double and single bonds. Right? So here, remember what's in purple represents our most common ones that you need to commit to memory.

The other ones are common types of heterocycles. They're just seen less regularly. But again, we kind of remember how to name them if we can remember the first three heterocycles we reviewed. So we have Furan, Thiophene, and Pyrrole.

Nomenclature of Heterocycles Concept 3

Hey, everyone. We're going to take a look at the rules for naming substituted unsaturated heterocycles. Here we're going to say that substituents are assigned location numbers with some additional rules. Remember, our naming convention is the location of substituent and then our parent. If we take a look at rule 1, rule 1 says that the heteroatom gets location 1.

So here, 1 and it gives substituents the lowest possible number. In this example, there are no substituents, so you just number it from either side: 1, 2, 3, 4, 5, or you could go the other way: 1, 2, 3, all the way around. Rule 2, the higher priority heteroatom gets location 1.

Remember, "open snacks nightly." Oxygen has higher priority than sulfur. Sulfur has higher priority than nitrogen. The second heteroatom gets the lowest or the next lowest number. So it's a 1, 2, 3. We're going this way towards the other heteroatom: 4, 5. So if more than one nitrogen is present, you start with the amine nitrogen. So, basically, the one that has a hydrogen attached. So here, rule 3, in bicyclic molecules, carbons at ring junctures or ring junctions are not numbered.

We're going to say the heteroatom gets location number 1. If we take a look here, we have two nitrogens within this bicyclic compound. And we're going to say that this is the amine nitrogen since it's the one with the hydrogen. And we're going to say 1, 2, 3. Now, our junctions are this carbon and this carbon.

Basically, think of them as the carbon bridges that connect the two rings together. We don't count them. So we go 1, 2, 3, skip, 4, 5, 6, 7. So this would have 7 in terms of the number. And these would be the three rules that we have to employ when dealing with these substituted, unsaturated heterocycles.

Nomenclature of Heterocycles Example 2

Here in this example, it says name the following heterocyclic molecule. Now, if we ignore the bromine for right now, we know that this heterocycle, this portion from what we've seen up above from our example and discussion of how to name these, we know that its name is thiazole. And we know following the rules that we'd start out numbering the higher priority heteroatom, which would be sulfur. Remember, open snacks nightly. Oxygen has a higher priority than sulfur.

Sulfur has a higher priority than nitrogen. Once we've numbered sulfur as location number 1, we count to the next heteroatom as quickly as we can. So, we'd have to go this way, 2, 3, 4, 5. Alright. This named thiazole is already discussing the sulfur and the nitrogen within our cyclic molecule.

All we have to do now is talk about the location of the substituent. Here the bromine. Here we're going to say it's on carbon number 5, so it's gonna or position 5. We're going to say 5-bromothiazole. So, this will be the name of this particular heterocyclic molecule.

Provide name of given molecule.

• A. 3-methoxypyridine
• B. 3-ethoxypyrimidine
• C. 2-ethoxypyridine
• D. 3-ethoxypyridine

Provide structure for benzimidazole-6-carboxylic acid.

• A.
• B.
• C.
• D.

Provide structure for N-propylindole.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What are heterocycles in organic chemistry?

Heterocycles are cyclic compounds that contain carbon atoms and one or more heteroatoms such as oxygen, sulfur, or nitrogen. These heteroatoms replace one or more carbon atoms in the ring structure, leading to a variety of chemical properties and reactivities. Heterocycles are classified into saturated and unsaturated types, with different naming conventions for each. Saturated heterocycles use prefixes like 'oxa' for oxygen, 'thia' for sulfur, and 'aza' for nitrogen, while unsaturated heterocycles often use common names like furan, thiophene, and pyrrole.

How are saturated heterocycles named?

Saturated heterocycles are named by identifying the parent cycloalkane and using a prefix to indicate the type of heteroatom present. The prefixes are 'oxa' for oxygen, 'thia' for sulfur, and 'aza' for nitrogen. The heteroatom is always assigned the location number 1. For example, a six-membered ring with one oxygen atom is named 'oxacyclohexane.' If there are substituents, they are numbered to give the lowest possible numbers, with the heteroatom taking priority.

What are the common names for unsaturated heterocycles?

Common names for unsaturated heterocycles include furan, thiophene, and pyrrole. Furan is a five-membered ring with one oxygen atom and two double bonds. Thiophene is similar but has a sulfur atom instead of oxygen. Pyrrole has a nitrogen atom and two double bonds, with the nitrogen also bonded to a hydrogen atom. These names are essential to memorize as they form the basis for naming more complex heterocycles.

What is the priority order for heteroatoms in heterocycles?

The priority order for heteroatoms in heterocycles is oxygen first, followed by sulfur, and then nitrogen. This order can be remembered using the mnemonic 'open snacks nightly.' When naming heterocycles with multiple heteroatoms, the heteroatom with the highest priority is assigned the location number 1, and the numbering continues to give the next heteroatom the lowest possible number.

How are substituted unsaturated heterocycles named?

Substituted unsaturated heterocycles are named by assigning location numbers to substituents, ensuring the heteroatom gets the location number 1. The substituents are numbered to give the lowest possible numbers. If multiple heteroatoms are present, the one with the highest priority (oxygen > sulfur > nitrogen) gets the lowest number. In bicyclic molecules, carbons at ring junctions are not numbered. For example, in a substituted furan, the oxygen gets position 1, and substituents are numbered accordingly.