All organic compounds have a name. However, we cannot investigate a compound unless we learn the rules for naming them.
In such cases, there are rules that unify the naming of compounds. This is called the IUPAC nomenclature. Let’s understand how to name compounds according to the IUPAC nomenclature.
One of the first things you will learn in organic chemistry is IUPAC nomenclature. You will learn IUPAC nomenclature in organic chemistry, but it is less important. In organic chemistry, it is important to understand the reaction mechanism, and there is no organic chemist who cares about the names of compounds.
However, if you don’t understand the rules for naming compounds, you will have trouble when looking up the desired reagent. So, let’s learn the basic rules you need to follow when naming a compound.
Table of Contents
- 1 There Are Many Specific Names, but Understand the Official Name
- 2 The Order in Which the Compounds Are Named
- 3 The Parent Chain Is Altered by Alkenes (Double Bonds) and Alkynes (Triple Bonds)
- 4 Nomenclature of Cyclic Compounds (Cycloalkanes)
- 5 IUPC Nomenclature for E/Z Isomers (cis-trans)
- 6 Understand how to name compounds
There Are Many Specific Names, but Understand the Official Name
Organic compounds have many specific names; although they are not official names according to the IUPAC nomenclature of organic chemistry, many people use them because they are widely used as specific names.
A specific name describes a single substance. For example, the following compounds are well-known specific names for compounds.
- Ethyl acetate
All of us who study chemistry have heard of these names. These are all specific names and are not official names. However, they are used by so many people that no one calls these compounds by the IUPAC nomenclature.
Also, when searching for compounds (reagents), no one uses the IUPAC nomenclature to think about the names of compounds by themselves. In fact, naming a compound using IUPAC nomenclature based on its structural formula is very complicated. So many people write down the structural formula of the compound and then use a tool that will give you the name of the compound according to IUPAC nomenclature.
Or they often search for a compound by the number of atoms, such as a carbon atom, nitrogen atom or oxygen atom. After writing the structural formula, you can use the tool or software to give you the molecular formula in an instant. You can then search the database for a compound with the desired structural formula.
Furthermore, IUPAC nomenclature is almost never used for compounds with complex structures. IUPAC nomenclature is commonly used to find reagents for organic compounds, and many of these reagents are simple in structure. There is no need to look at a compound with a complex structure to be able to name it using IUPAC nomenclature.
Given this fact, IUPAC nomenclature is a low priority. However, if you don’t have a minimum of knowledge, you will be stuck searching for reagents. While it is not necessary to be able to fully master IUPAC nomenclature, it is necessary to have knowledge of it.
The IUPAC Nomenclature of Organic Chemistry Has Rules
So what are the rules for IUPAC nomenclature? The specific name applies to only one compound. If a compound does not have a specific name, the name is not known. On the other hand, using the IUPAC nomenclature, any compound can be named. This is the difference between specific names and IUPAC nomenclature.
IUPAC nomenclature follows a set of rules for naming compounds. The rules in this case are as follows.
- Prefix: position and name of the substituent.
- Parent structure: Carbon skeleton of the parent chain.
- Suffix (Suffix): the parent functional group.
Name the compounds in this order. For example, suppose you have the following compounds.
The IUPAC nomenclature for 2-chloropentane is broken down into the following.
- Prefix: 2-chloro (2-chloro)
- Parent structure: pent
- Suffix: ane
You should understand that it will always be divided into three parts. This is a simple rule when considering compound names in the IUPAC nomenclature.
The Order in Which the Compounds Are Named
How do we name our compounds using the IUPAC nomenclature? There is an order to this.
The first thing to consider is the prefix and the parent skeleton; in IUPAC nomenclature, the prefix and the parent skeleton are the most difficult to consider. For these, you should consider the name of the compound in the following order.
- Determine the parent structure (parent chain)
- Number the parent chain.
- If there is more than one substituent, use Greek numbers.
- Alphabetical Order of Substituents
We will explain the specific concept.
Look for the Longest Alkyl Chain First
When naming a compound according to the IUPAC nomenclature, the first thing you need to do is to find the parent structure of the compound. Organic compounds are made up of a carbon skeleton. Find the longest of these alkyl chains.
The longest chain of carbon is the parent skeleton. But in organic compounds, the carbon chains often branch out. So make sure you don’t mistake the main chain (parent chain).
For example, which is the longest carbon chain in the following compounds?
If you look at the structural formula, you would think that the five carbon chains are the parent chain, as shown on the left side of the figure. However, as you can see on the right, the six-carbon atom is the longest carbon chain. In other words, the parent skeleton of this molecule can be described as an alkyl chain with six carbon atoms.
Look at the entire molecule and try to figure out where the longest carbon chain is. Here’s how the name of the parent skeleton changes depending on the number of carbon atoms.
|Number of carbon||Parent skeletal name||Name of alkane|
For clarity, I’ve also added the names of the alkane names to the table. Alkanes are prefixed with “ane” at the end. If you add ane to the name of the parent skeleton, it becomes the name of the alkane in the IUPAC nomenclature.
Look at the molecular skeleton and identify the longest carbon chain. For example, if you have a compound with seven carbons, you’ll know to use hept as the name of the carbon skeleton.
Assign Position Numbers to Substituents
The next step is to number the parent main chain. In many organic compounds, there are substitution groups. So, let’s number the positions where the substituents are located.
The rule is that the number should be as small as possible. For example, we can consider two different compound names in the following case.
In the IUPAC nomenclature, there is a rule to keep the numbers as small as possible. Therefore, the chloro group is in the second position, and 2-chloropentane is the correct name.
If There Is More Than One Substituent, Use Greek Numbers
So what if you have more than one substituent? If the same substituents are attached to the carbon skeleton, you should add the number. For each substituent, use a Greek number such as di or tri to indicate how many substituents are present.
Here are the Greek numbers to be used.
- Two: di
- Three: tri
- Four: tetra
- Five: penta
- Six: hexa
- Seven: hepta
Of course, make sure that the position numbers of the substituents are small. In addition to that, if there is more than one substituent for the same substituent, it will be represented by a Greek number. For example, the following.
For pentane, there are two chlorine groups attached in the second and third places, giving it the name 2,3-dichloropentane because of the two chlorine groups.
And what about the three fluorine groups in the second and third places for pentane? In this case, the compound shown in the above figure would be 2,3,3-trifluoropentane. In this way, the Greek numbers are used to give the compound its name.
Alphabetical Order of Substituents
In contrast, it is common for different substituents to be attached to a compound. If this is the case, how should you come up with a compound name? For this, let’s go in alphabetical order.
For this compound, 2-chloro-3-bromopentane is incorrect. Rather, it should be 3-bromo-2-chloropentane, which is in alphabetical order. In alphabetical order, b (bromo) comes first, rather than c (chloro).
You don’t have to think about the nature of the substituents, just arrange them in alphabetical order, so you don’t have to think particularly hard.
As a reminder, numbers that represent the number of substituents, such as di and tri, should not be included in the alphabetical order. You need to use the IUPAC nomenclature to name the substituents according to whether or not they are in alphabetical order, not by their numbers.
The Parent Chain Is Altered by Alkenes (Double Bonds) and Alkynes (Triple Bonds)
We have been thinking about IUPAC nomenclature in alkanes. In alkanes, the longest carbon chain is the main chain.
But the compound is not just a single bond. They can also be alkenes (double bonds) and alkynes (triple bonds). What happens in these alkenes and alkynes? In this case, the parent chain changes.
The IUPAC nomenclature considers the main chain to contain as many double (or triple) bonds as possible. Therefore, the longest carbon chain will not necessarily be the parent chain. It may not be the longest carbon chain, but it may be the parent chain because it contains a lot of double bonds.
For example, consider the names of the following compounds.
When considering the longest carbon chain, we must focus on the blue numbers. In this case, 4-ethenylheptane is the name of the compound.
However, this is not correct. You have to determine the main chain to contain more double (or triple) bonds. So, although the number of carbon chains is small, we need to focus on the red numbers to determine the parent chain. As a result, the compound name is 3-propyl-1-hexene.
In alkanes, ane is the last word. In alkenes, on the other hand, ene is the last word. In alkyn, it is yne. All you have to do is change the last word.
In alkenes and alkynes, it is necessary to note where the double or triple bond is located. So, for example, 3-propyl-1-hexene, the location of the multiple bond should be indicated by a number.
-Smaller Numbers Are Added to Double Bonds
Double and triple bonds should be written with as few numbers as possible. Focus on the number of the double bond (or triple bond), not the substituent attached to the parent chain, and gives it a smaller number.
For example, suppose you have the following compound.
In this alkene compound, if you reduce the number of the double bond, there is a double bond in the second position, which is called 4-chloro-2-hexene. On the other hand, if you try to reduce the number of substituents, the compound is called 3-chloro-4-hexene, which is a mistake.
The Name of the Main Chain Depends on the Functional Group
An organic compound is not just has a double or triple bond in the carbon chain. There are various functional groups such as hydroxy groups (-OH) and amino groups (-NH2). Also, not just one of these groups is present, but a number of functional groups can be attached to a single carbon chain.
In this case, how should we think about it? Functional groups have a priority. The rule is that when this functional group is present, the compound name should be based on the parent chain of the functional group.
These functional groups take precedence over alkenes and alkynes. In other words, the carbon chain to which the functional group, such as -OH and -NH2, is attached takes precedence over the alkene and alkynes, and becomes the parent chain. The order of priority of the functional groups is as follows.
|1||carboxylic acid||-COOH||carboxy-||-oic acid|
There is no point in remembering these priorities; almost no organic chemist can accurately name them using the IUPAC nomenclature. And as mentioned above, the software can tell you the names of the compounds. Just as a matter of knowledge, you just need to understand that the priority is determined by the functional groups in this way.
For example, let’s say you have the following compounds.
When comparing the double bond and the hydroxy group (-OH), the -OH has a higher priority. Therefore, the parent chain is the carbon chain to which the -OH is attached. As a result, the compound name is 3-ethenylpentane-1,4-diol.
We think that the ethenyl is attached to the third pentane, and the -OH is attached to the first and fourth. Hence the name.
Nomenclature of Cyclic Compounds (Cycloalkanes)
However, some cyclic compounds also exist. Cycloalkanes and cycloalkenes fall into this category. But if you understand what we’ve been explaining, naming cyclic compounds is simple. Just add the cyclo- at the beginning. For example, consider the following.
As a reminder, if only one double bond or substituent is present, there is no need to number it. This is because they are symmetrical and will be the same compound without numbering.
However, if there are two or more double bonds or substituents, they should be position numbered. The number of the double bond and substituent should be a small number. For example, consider the following.
It’s the same as what we’ve been describing in terms of prioritizing and numbering substituents. Think of cyclic compounds as just adding cyclo- first.
Note that cyclic compounds are mostly related to cyclohexane (hexagonal) or cyclopentane (pentagonal) compounds. Other cyclic compounds are unstable in structure, so think of them as the main cyclic compounds.
IUPC Nomenclature for E/Z Isomers (cis-trans)
This is how we come up with the parent chain based on the number of hydrocarbons and functional groups, and then we write the names of the substituents in alphabetical order. However, the presence of a double bond often produces a cis-trans. These have to be identified because cis-trans result in completely different compounds.
But cis-trans, which we learn about in high school chemistry, are powerless. It is not possible to identify the compounds. For example, it is unclear which of the following compounds are cis-trans.
This is why cis-trans nomenclature is powerless. So, we treat it as an E/Z isomer. For the atoms attached to the double bond, we make a decision based on whether they have high priority. It is as follows.
- Z (cis): the atoms of higher priority are on the same side
- E (trans): the atoms of higher priority are on the opposite side
The way the substituents are ranked is simple. The higher the atomic number bonded to an atom, the higher the priority for the E/Z isomer. In addition, if the same atoms are bound to each other, priority is determined by referring to the functional group (atomic number) bound to that atom.
As described above in the IUPAC nomenclature, the priority of the parent chain is determined by the order of the atomic number alone for E/Z isomers. For this reason, it is easy to judge. For example, suppose that the following compounds are available.
When comparing chlorine and bromine, the bromine atom has a higher atomic number.
Also on the right side, the ethyl group has a higher priority than the methyl group, although the same carbon atoms are bonded to it. For this reason, it is determined to be an E (trans); in IUPAC nomenclature, it is necessary to determine whether it is E or Z.
Understand how to name compounds
There are many specific names for organic compounds. However, we cannot use specific names for all compounds, because there are an infinite number of compounds. Therefore, we came up with the idea of naming them according to the rules, which is the IUPAC nomenclature.
In the IUPAC nomenclature, it is very difficult to predict the name of a compound with a complex structural formula. In addition, there is a priority order depending on the functional group, and even double and triple bonds must be included to determine the priority. For this reason, many people draw the structural formula and then use tools to give the name of the compound.
Therefore, there is no point in learning the IUAPC exactly and understanding it completely. However, it is essential to understand the basics and then be able to give simple IUPAC names of compounds. Otherwise, you will not be able to search for reagents in organic synthesis.
While there is no need to work hard to learn at all, you must learn at least some knowledge of IUPAC nomenclature.
What we have described here is the basis of the IUPAC nomenclature. There is no need to become able to say the names of complex compounds. Instead, you should be able to use IUPAC nomenclature to describe compounds with simple structural formulas.