IUPAC Nomenclature

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.

• This article is about IUPAC nomenclature in organic chemistry.
• We'll start by defining IUPAC nomenclature.
• After that, we'll give an overview of the basic IUPAC nomenclature rules.
• We'll then look more closely at root names, prefixes and suffixes, saturation indicators and locants.
• Finally, we'll work through some IUPAC nomenclature examples so that you can practice naming organic molecules.

IUPAC nomenclature definition

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 nomenclature rules

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.

IUPAC nomenclature root name

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.

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.

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.

IUPAC nomenclature prefixes and suffixes

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.

Suffixes

A suffix is used to show the molecule's parent functional group.

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.

Prefixes

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.

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.

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.

Functional group table

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.

Substituent table

How about those substituents we mentioned earlier?

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.

You should note that the prefixes for halogens and alkyl groups vary, depending on the identity of the halogen atom and the length of the carbon chain respectively.

• 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.

IUPAC nomenclature saturation indicator

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.

Take a look at this next molecule.

An example of an unsaturated molecule.StudySmarter Originals

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-.

IUPAC nomenclature locants

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 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:

1. First, we look at the position of the parent functional group (which you'll remember gives the molecule its suffix). We then choose the numbering possibility which means that this functional group has the lowest-numbered locant.
2. If both numbering possibilities mean that the parent functional group is attached to the same number locant, we move on to look at the positions of any unsaturated bonds. We choose the numbering possibility which means that the unsaturated bond has the lowest-numbered locant.
3. If both numbering possibilities are still the same, we look at the position of any functional groups or substituents indicated by the prefixes. We choose the number possibility which means that this functional group or substituent has the lowest-numbered locant.

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.

Quantifiers

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.

Assembling the IUPAC nomenclature name

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.

• Organic molecule names are made up of a root name, a suffix, prefixes, saturation indicators, and locants.
• The root name tells you the length of the parent chain, which contains the parent functional group.
• The suffix tells you the identity of the parent functional group.
• Prefixes tell you about any other functional groups or substituents attached to the parent chain.
• Saturation indicators tell you if there are any unsaturated C=C double or C≡C triple bonds present.
• Locants tell you where the functional groups, substituents, and unsaturated bonds are positioned in relationship to the parent chain.
• If the molecule has multiple copies of the same component, locants are accompanied by quantifiers.

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.

The structure of an IUPAC nomenclature name.StudySmarter Originals

IUPAC nomenclature examples

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.

1. Identify the parent functional group and its suffix.

2. Find the parent chain and respective root name.

3. Identify any other functional groups or substituents and their prefixes.

4. Determine the saturation indicator.

5. Assign locants to each prefix, suffix and saturation indicator, adding in quantifiers if necessary.

6. Assemble the name as described above.

Not too tricky, right? Here's another example.

Here's one final molecule for you to consider. This one is a little harder to name.