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Would you like a biscuit? In many countries, this means a sweet treat like a ginger snap or shortbread. But if you're from the US, you'll take this to mean a small piece of quick bread, almost like a savoury version of the classic UK scone. When you bring out the tin of wafers and crumbly round oat slices studded with chocolate chips, your US friend smiles - "Oh, you mean cookies!"
You see, names can be misleading. One name can be used to represent two very different things. On the other hand, the same item is often known using two very different names. This makes life a little tricky. How can we be sure that we know what someone else is talking about if we don't necessarily use the same word to mean the same thing?
For much of the time leading up to the 20th century, scientists around the world experienced these problems. Discussing certain reactions, molecules, and chemical advancements was time-consuming and over-complicated because nobody quite agreed on a universal name. This is where IUPAC stepped in. Thanks to their unique system of nomenclature, we now have a way to not only positively and uniquely identify a molecule's structure from its name, but also to name a molecule from its structure.
IUPAC nomenclature is a standard way of naming organic compounds and molecules, invented by the International Union of Pure and Applied Chemistry (IUPAC). It was designed so that every chemical structure has its own unique, unambiguous name.
IUPAC nomenclature assigns a name to each molecule or compound based on its structure. The names are useful because they give you information not only about the species' atoms and functional groups, but also about how these groups are arranged. Once you know a molecule's name, you should be able to confidently draw its structure without any confusion or chance of a mix-up. Likewise, once you know its structure, you should be able to arrive at a name that scientists worldwide can recognise.
IUPAC names are based on four ideas:
Root names tell you the number of carbon atoms in the molecule's parent carbon chain.
Prefixes and suffixes give you information about the functional groups and other substituents present.
Saturation indicators let you know if the molecule has any unsaturated C=C double or C≡C triple bonds.
Locants tell you where the molecule's functional groups, substituents, and unsaturated bonds are found.
Let's look at the rules concerning these four components in more detail.
The root name of an organic compound tells you the number of carbon atoms in its parent chain.
A molecule's parent chain is its longest carbon chain that contains the parent functional group. The parent functional group is the highest priority functional group and gives the molecule its suffix; we'll look at this in more detail in the next section.
Some molecules have multiple copies of the parent functional group. In this case, the parent chain is always the carbon chain that contains as many copies of this functional group as possible - even if other chains are longer.
The root name varies depending on the number of carbon atoms within the parent chain. Here's a handy table that gives you the root names of molecules with parent chains of up to eight carbon atoms in length.
|Length of parent chain (# of carbon atoms)||Root name|
If the parent chain is a cyclic chain, we add the prefix -cyclo- in front of the root name.
The parent chain can sometimes be a little tricky to spot. It might not be obvious - it could snake off in different directions. Here are a couple of examples.
The molecule on the left is an alkane - it doesn't have a parent functional group. To find its parent chain we simply look for the longest carbon chain. Here, the parent chain (shown in blue) is 4 carbon atoms long, giving this molecule the root name -but-.
The parent functional group in the molecule on the right is the hydroxyl group (-OH). The longest carbon chain (shown in red) containing the hydroxyl group is once again 4 carbon atoms long, giving this molecule the root name -but-.
The two examples above, along with all of the other examples you'll meet in this article, use skeletal formulae. If you aren't sure how to interpret skeletal formulae, check out Organic Compounds for a detailed explanation.
That's the root name covered - let's move on to the next part of organic nomenclature. This involves using prefixes and suffixes to show a molecule's functional groups and substituents.
A suffix is used to show the molecule's parent functional group.
A molecule's parent functional group is the functional group that takes the highest priority.
Many molecules have more than one functional group. We label one of their functional groups as the most important according to an order of priority produced by IUPAC. This particular functional group becomes the parent functional group and forms the basis of the parent functional chain, which we looked at earlier. The suffix always goes at the end of the molecule's name.
Note that an organic molecule can only have one suffix!
So, an organic compound's suffix tells us about the parent functional group. But what if a molecule has multiple functional groups? How do we show these?
Well, instead of using additional suffixes, we use prefixes. Prefixes are also used to show any substituents branching off from the parent chain. Prefixes go at the start of the molecule's name.
If we have multiple prefixes, we arrange them in alphabetical order. Priority doesn't matter here; it is only relevant when choosing the parent functional group.
Although a molecule can only have one suffix, it can have multiple prefixes!
Let's now look at how we look at the priorities of different functional groups and decide which becomes the parent functional group using an IUPAC nomenclature table.
We find a compound's parent functional group by looking for the functional group that takes the highest priority according to IUPAC's official rules. This functional group determines the molecule's suffix; all other functional groups are instead shown using prefixes. To make nomenclature a little easier, IUPAC has produced a handy table that ranks all the functional groups from highest to lowest priority.
Here are some of the main functional groups, alongside their respective prefixes and suffixes. The highest priority functional group is at the top of the table, whilst the lowest priority is at the bottom.
|Family name||Functional group name||Functional group formula||Prefix||Suffix|
|Carboxylic acid||Carboxyl||-COOH||carboxy-||-oic acid|
For your exams, you are unlikely to encounter molecules with multiple high-priority functional groups. In fact, it is safe to say that you don't have to worry about prefixes such as alkoxycarbonyl- or amido-! However, it is good to know the suffixes and prefixes for more common functional groups such as the hydroxyl group (-OH) in alcohols and the amine group (-NH2) in amines.
Consider the following molecule.
It contains a carboxyl (-COOH) functional group, found in carboxylic acids, and a hydroxyl
(-OH) functional group, found in alcohols. Looking at the table above, we can see that the carboxyl group has a higher priority than the hydroxyl group. Therefore, the carboxyl group becomes the parent functional group. We use a suffix to indicate the carboxyl group and a prefix to indicate the hydroxyl group. Hence, this molecule ends with the suffix -oic acid and starts with the prefix hydroxy-.
How about those substituents we mentioned earlier?
A substituent is an atom or group of atoms that has replaced a hydrogen atom in a hydrocarbon chain.
We also need to show that they are part of the molecule. Substitutents never take suffixes - instead, we always show them using a prefix. Here's a table of a few common substituents and their respective prefixes.
To form a halo- prefix, simply remove the final three letters (-ine) from the halogen's name. Then, add the letter -o- to what is left. For example, we show fluorine atoms using the prefix fluoro- and chlorine atoms using the prefix chloro-.
To form an alkyl- prefix, you first count the number of carbon atoms present in the alkyl group's carbon chain. You then take the matching root name and add the letters -yl- to the end. For example, an alkyl group with a chain that is 3 carbon atoms long uses the prefix propyl- whilst an alkyl group with a chain that is 5 carbon atoms long takes -pentyl-.
If the alkyl group is cyclic, we add in the prefix cyclo-. For example, a cyclic alkyl group made up of 4 carbon atoms takes the prefix -cyclobutyl.
You might remember from the article Functional Groups that halogen atoms can be a functional group. In particular, they're the functional group found in halogenoalkanes. However, when it comes to nomenclature, halogen atoms are considered to be substituents, not functional groups. This simply means that they never take a suffix - instead, we always show them using a prefix.
You'll notice that in the functional group table above, we didn't include anything that shows a molecule's saturation. Saturation refers to the type (or types) of carbon-carbon bonds. You should already know that saturated molecules contain just C-C single bonds, whilst unsaturated molecules contain one or more C=C double or C≡C triple bonds.
In some cases, we show saturation using a suffix, as with the other functional groups that we looked at above. But in the majority of cases, we instead use a saturation indicator. Here are the rules that you need to know.
If the molecule has no other functional groups, we use a suffix to show its saturation. This suffix takes the place of the parent functional group.
If the molecule does have other functional groups, we instead have to use a saturation indicator. The saturation indicator goes in between the root name and the suffix.
We show the position of saturated C=C double and C≡C triple bonds using locants in the same way as we'd show the position of any other functional group. You'll find out how to do this in just a second.
We assume that all of the remaining, unlabelled carbon-carbon bonds are C-C single bonds. This means that you don't need a locant for every single bond - only the unsaturated ones!
Here are the saturation indicators and suffixes for different levels of saturation.
|Saturated (contains just C-C single bonds)||-an-||-ane|
|Unsaturated (contains one or more C=C double bonds)||-en-||-ene|
|Unsaturated (contains one or more C≡C triple bonds)||-yn-||-yne|
Take a look at this next molecule.
It contains two amine (-NH2) functional groups and a C=C double bond. The amine group is the molecule's parent functional group and is responsible for the molecule's suffix. Because we can't use a suffix to show the unsaturated C=C double bond, we need to use a saturation indicator instead. Therefore, this molecule ends with the suffix -amine and contains the solubility indicator -en-.
So, we've learned that we use a root name to show the length of an organic molecule's parent chain. We've also found out that we use a suffix to tell us about the molecule's parent functional group and prefixes to tell us about any other functional groups or substituents found in the species. However, these pieces of information alone aren't quite enough to confidently draw a molecule from just its name. We need some further details such as where the functional groups and substituents are found. For this, we use locants.
Locants are numbers showing the position of a functional group or substituent within an organic molecule.
Locants specifically tell us the position of functional groups or substituents with regards to the different carbon atoms in the parent chain. In particular, they tell us which carbon atom the component is bonded to. For example, if a functional group has a locant of 5, you know that it is attached to the fifth carbon atom in the parent chain.
To assign locants, we first number the carbon atoms in the parent chain in both directions: from left to right and from right to left. We then consider the locants given by both possible numbering possibilities in terms of the lowest number rule:
Some functional groups, such as the carboxyl group (-COOH), always go at the end of the hydrocarbon chain. If this functional group is the parent functional group, it therefore always has a locant of 1. In this case, we don't need to bother writing the locant - as long as there is no ambiguity, everyone else should know what we mean. For example, we can simplify propan-1-oic acid to propanoic acid. Here, locant 1 is implied.
Quite commonly, a molecule will have several unsaturated bonds or multiple additional functional groups and substituents. In this case, we add each of their locants up and choose the numbering possibility which gives the lowest total sum of locants.
Sometimes, in steps 2 or 3, both numbering possibilities might still give the same total sum of locants. In this case, we consider the components involved in the step alphabetically. We then choose the numbering possibility which means that the prefix or saturation indicator which comes first in the alphabet has the lowest-numbered locant.
This molecule has a bromine atom substituent and a chlorine atom substituent. Its parent chain is 5 carbon atoms long. Let's consider how we'd assign locants to these two substituents.
If we number the parent chain from left to right, the bromine atom is attached to carbon 2 whilst the chlorine atom is attached to carbon 4. The sum of the locants is 2 + 4 = 6. If we number the parent chain from right to left, the chlorine atom is attached to carbon 2 whilst the bromine atom is attached to carbon 4. The sum of the locants is still 2 + 4 = 6. According to the lowest number rule, we choose the numbering possibility that gives us the lowest total sum of locants. However, for this molecule, both possibilities give us the same sum. We therefore need to consider the substituents alphabetically and choose the numbering possibility which means that the prefix which comes first in the alphabet has the lowest-numbered locant. bromo- alphabetically precedes chloro-, and so we choose to number the parent chain from left to right. This gives the bromine atom a locant of 2 and the chlorine atom a locant of 4.
We've illustrated the two numbering possibilities in the diagram above, with the correct numbering in a larger font and the incorrect numbering in brackets.
If there are multiple copies of the same unsaturated bond, functional group, or substituent present in the same molecule, we also have to add in a quantifier. This makes it clear that you have more than one copy of the component.
Here's a table showing the number of copies of a particular component present and the quantifier used.
Remember how we said that we list prefixes in alphabetical order? When we do this, we ignore any quantifiers.
We've got all of our constituent parts: root name, suffix, prefixes, saturation indicators, locants, and quantifiers. Here's a quick summary of all that we've learned so far.
But how do we put the different parts together to make a complete name?
We start with prefixes, arranged in alphabetical order.
We follow the prefixes with the root name and the saturation indicators.
We end with the suffix.
Locants go before their respective prefix, saturation indicator or suffix.
If there are multiple copies of one component, we don't bother repeating the prefix or suffix. Instead, we group the locants together, separated by commas, and add in the quantifier after the locants but before the prefix or suffix.
Finally, we separate all numbers from letters using dashes.
Here's an example of an organic molecule name, fully assembled so you can see how it all fits together.
We've covered all that you need to know about IUPAC nomenclature. If you're feeling confident, let's now try naming some organic molecules from their structures. But don't worry if you get stuck - we'll walk you through the solutions in our handy worked examples.
Here's the process that you should follow when naming organic molecules.
Identify the parent functional group and its suffix.
Find the parent chain and respective root name.
Identify any other functional groups or substituents and their prefixes.
Determine the saturation indicator.
Assign locants to each prefix, suffix and saturation indicator, adding in quantifiers if necessary.
Assemble the name as described above.
Give the full IUPAC name of this molecule.
We saw this molecule in one of the earlier examples. We know that it contains a carboxyl
(-COOH) and a hydroxyl (-OH) functional group, and that the carboxyl group has a higher priority. This means that the carboxyl group is the parent functional group. The molecule takes the suffix -oic acid and the prefix -hydroxy.
Now let's find the parent chain. This is the longest carbon chain that contains the parent functional group. In our case, the parent chain is 3 carbon atoms long, giving the molecule the root name -prop-.
When it comes to saturation indicators, this molecule is completely saturated. We simply need to use the saturation indicator -an-.
Moving on, let's consider locants. We can number the parent chain from left to right or right to left. The first step of the lowest number rule tells us to choose the numbering possibility which means that the parent functional group is attached to the carbon atom with the lowest-numbered locant. For this molecule, that means numbering the parent chain from right to left, giving the carboxyl group a locant of 1 and the hydroxyl group a locant of 3. We've illustrated this in the diagram below, once again showing the correct numbering possibility in a larger font and the incorrect numbering possibility in brackets.
Now we're ready to assemble our name. We start with prefixes; we then add in the root name and saturation indicator. We end with the suffix. After that, we add in locants and quantifiers before any prefixes, saturation indicators, or suffixes. Finally, we separate numbers from letters with dashes. This particular molecule has the final name 3-hydroxypropanoic acid.
Not too tricky, right? Here's another example.
Give the full IUPAC name of this molecule.
This molecule is similar to one that we looked at earlier. However, this time it has a ketone parent functional group, as well as bromine atom and chlorine atom substituents. We need to use the suffix -one and the prefixes bromo- and chloro-.
The parent chain is 5 carbon atoms long, giving the molecule the root name -pent-.
As before, this molecule is saturated and so takes the saturation indicator -an-.
Both numbering possibilities give the parent functional group a locant of 3. We move on to the next step of the lowest number rule, which involves looking at prefixes. As we discovered in an earlier example, both numbering possibilities give us the same total sum of locants, and so we focus on the alphabetical order of the prefixes. Because bromo- comes before chloro- in the alphabet, we want the bromine atom to have a lower-numbered locant than the chlorine atom and so number the parent chain from left to right. This gives the bromine atom a locant of 2 and the chlorine atom a locant of 4.
Our final name? 2-bromo-4-chloropentan-3-one.
Here's one final molecule for you to consider. This one is a little harder to name.
Give the full IUPAC name of this molecule.
Once again, we've already met this molecule. The parent functional group is the amine (-NH2) group, giving us the suffix -amine. The parent chain is the chain that contains as many copies of this functional group as possible. For this molecule, you can see that the parent chain is 4 carbon atoms long, giving it the root name -but-.
The molecule doesn't have any other functional groups. However, it does have an alkyl group substituent. The group is 2 carbon atoms long and so uses the prefix -ethyl.
The molecule also has an unsaturated C=C double bond. We show this using the saturation indicator -en-.
Finally, let's look at locants. As in all of our examples, we number the carbon atoms in the parent chain from left to right and right to left then consider both possibilities.
First, we focus on the parent functional group. There are two amine groups, and so no matter their locants, we also need to include the quantifier -di-.
If we number from left to right, one amine group is found on carbon 1 and the other on carbon 4. If we number from right to left, one amine group is again found on carbon 1 and the other on carbon 4. Both numbering possibilities give us the same total sum of locants, and so we must move on to the next step of the lowest number rule and instead consider the position of the double bond. The lowest number rule tells us that we choose the numbering possibility which means that the unsaturated bond has the lowest-numbered locant. Here, we have to number from left to right, as that gives the double bond a locant of 2. As a consequence, the ethyl group has a locant of 3.
When we put everything together, we get 3-ethylbut-2-en-1,4-diamine.
Before we finish, let's explore one example of drawing a molecule from a name. We'll use a name that we saw earlier in the article.
Draw the structure of 2-bromo-3,3-dimethylbutan-1-ol.
First, we look for the root name. It is -but-, meaning the parent chain is 4 carbon atoms long.
Next, find the parent functional group as given by the suffix. Here, the suffix is -ol, indicating a hydroxyl (-OH) group. The locant before the suffix tells us that the hydroxyl group is bonded to carbon 1 in the parent chain.
What other functional groups and substituents do we have? This molecule has the prefixes bromo- and methyl-; these refer to a bromine atom and a methyl group respectively. methyl- is preceded by the quantifier -di- and so we know that we actually have two methyl groups. The locants tell us that the bromine atom is found on carbon 2 in the parent chain whilst the two methyl groups are both attached to carbon 3.
Finally, what's the saturation indicator? It is -an-, and so we know that the molecule is saturated. There aren't any C=C double or C≡C bonds.
If we combine all of our inferences and draw the molecule out, we end up with the following final structure:
Organic nomenclature is a way of naming organic compounds. It is recommended by IUPAC (the International Union of Pure and Applied Chemistry) to ensure that every chemical structure has its own unique, unambiguous name.
To name an organic compound using IUPAC nomenclature, you need to find the following:
Here's a brief summary of the rules of IUPAC organic nomenclature. However, we'd recommend reading the rest of this article for a more in-depth look.
There are many types of nomenclature, all designed to classify items in various different fields. For example, binomial nomenclature is used in biology to name and organise different species. In chemistry, the most commonly used type of nomenclature is the system created by IUPAC. It was designed so that every chemical structure has its own unique and identifiable name.
IUPAC nomenclature can be a tricky topic. Here's a brief summary of some of its rules. However, we'd recommend reading the rest of this article for a closer look.
What is IUPAC nomenclature?
IUPAC nomenclature is a way of naming organic compounds and molecules, invented by the International Union of Pure and Applied Chemistry (IUPAC).
Which of the following are parts of a IUPAC nomenclature name?
A root name
A molecule's root name tells you _____.
The length of its parent chain.
How do you identify a molecule's parent chain?
A molecule's parent chain is its longest carbon chain that contains the parent functional group, which is its highest priority functional group. If the molecule contains multiple copies of the parent functional group, the parent chain is always the carbon chain that contains as many copies of this functional group as possible - even if other chains are longer.
Give the root names for parent chains that are up to 4 carbon atoms long.
Which of the following statements are correct?
A molecule's suffix tells you about its parent functional group.
How do you decide which functional group is the parent functional group?
You look for the functional group with the highest priority according to the order produced by IUPAC.
Some molecules require multiple prefixes. How do you order these prefixes according to IUPAC rules?
In alphabetical order.
True or false? We always show alkyl groups using a prefix.
True or false? A molecule can have more than one suffix.
Saturation indicators show _____.
The molecule's saturation: whether it contains any unsaturated C=C double or C≡C triple bonds.
Which of the following statements are true?
If a molecule has no other functional groups, we show saturation using a suffix.
What saturation indicator would you use for:
What are locants?
Locants are numbers showing the position of a functional group or substituent within an organic molecule. In particular, they tell us which carbon atom the component is bonded to.
Describe how you assign locants using the lowest number rule.
You number the parent chain in both directions, from left to right and right to left. You then decide on the correct numbering possibility using the following three rules in order:
True or false? The parent functional group should always have the lowest possible numbered locant.
When do we use quantifiers in IUPAC nomenclature?
We use quantifiers to show that a molecule contains multiple copies of the same functional group.
Give the quantifiers used if a molecule has the following number of copies of a particular functional group.
Which of the following shows the correct arrangement of parts in a IUPAC nomenclature name?
Prefixes, root name, saturation indicators, suffix.
Which of the following statements are true about IUPAC nomenclature names?
Numbers are separated from letters using dashes.
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