Naming Molecules
Introduction
In our opinion, the best way to communicate the structure of an organic molecule is through a drawing. These are instantly recognizable and are unambiguous (if drawn well. See the post on drawing molecules). The alternative is using a molecule’s unique systematic name. For simple molecules it is relatively easy to understand the name, and in this summary, we present the basics of chemical nomenclature (naming). A frustration for students studying chemistry is that many common molecules have trivial names that bear no relation to their structure. If in doubt, draw the molecule!
The Structure of a Systematic Name
The name of all organic molecules can be divided into the same three parts:
Parts of an organic molecule’s name
The parent is the longest carbon chain containing a functional group. It also tells us if the molecule has an alkene or alkyne in it. The suffix is based on the most important functional group. All other information is contained within the prefix.
Below is a simple introduction to naming organic molecules. This works with most small molecules. The full naming system has to account for all possibilities and is more complex with many more rules, but it starts with the same general concepts.
Naming a molecule
There are four steps to naming an organic compound:
Identify the major functional group
Identify the longest carbon chain or parent
Number the carbon atoms of the chain
Identify the substituents
1. Identify the major functional group
If there is one functional group in the molecule this is the major functional group (except if it is a halide or a nitro group). The major functional group will determine the suffix. The suffix will replace –ane at the end of the name. The major functional group also determines the longest chain (parent), and the numbering of the molecule.
The major functional groups and naming suffix
If there are multiple functional groups then they follow the list of priorities below. As an approximation, the higher the oxidation state of a functional group the higher the priority.
Order of priority of the functional groups
Examples found in the gallery at the bottom of the page.
2. Identify the longest carbon chain or parent
The parent, the backbone of the molecule from which we build its name, is the longest continuous chain of carbon atoms that contains a functional group. If there are are no major functional groups, then it is the chain with the most multiple bonds (alkenes or alkynes). If there are no functional groups at all, then it is the longest chain with multiple substituents coming off it.
The number of carbon atoms gives the molecule its parent name according to the table below. If the molecule contains a functional group (except halide or nitro group), then replace the ‘e’ of –ane with the suffix above. If there is an alkene or alkyne present this replaces –ane with -ene or -yne and proceeds the suffix.
Parent name based on number of carbon atoms
Examples found in the gallery at the bottom of the page.
3. Number the carbon atoms of the chain
The longest chain is numbered from one end to the other so that the major functional group has the lowest possible number. If there is no functional group in the molecule, number the chain so that the substituents have the lowest possible numbering.
The chain is only numbered once. It is consistent for the naming of a molecule.
Examples found in the gallery at the bottom of the page.
4. Identify the substituents
All minor functional groups (except alkenes & alkynes) and all other substituents (other carbon chains coming off the parent) are lumped together in the prefix. They will have a number identifying which carbon of the parent they are on and will be listed in alphabetical order. The name of a functional group changes depending on whether it is the a prefix or suffix. The prefixes are given in the table below.
Minor functional groups and their prefixes
Carbon chains that sprout from the parent - other alkyl groups - use the same naming system as the parent except the -ane is replaced with -yl.
Examples found in the gallery at the bottom of the page.
Putting it all together
The prefix is listed in alphabetical order then there is the parent (normally there is no space or hyphen between the prefix and the parent). If there are any alkenes or alkynes in the molecule they come next and finally the suffix is added. The position of each substituent or functional group is given by a number that is separated from the prefix or suffix by a hyphen.
The example below shows how to name a molecule:
The steps to name an organic compound
Special Cases
Alkenes and alkynes
These are always included in the parent and replace the -ane of the alkane name. If they are the only functional group in the molecule then the ending will be -ene or -yne. If there is a major functional group, the suffix replaces the -e.
The position/number of an alkene or alkyne refers to the first carbon in the chain, the one with the lowest number.
If there is both an alkene and an alkyne, then they are listed in alphabetical order (-enyne).
Guess what? There is an example in the gallery at the bottom of the page.
Alkyl halides and nitro groups
These groups are always prefixes, even if they are the only functional group in the molecule. Don’t ask why, I have no idea.
Multiple identical substituents
If there is more than one example of the same functional group or substituent within a molecule, it is unnecessary to list them multiple times. Instead, add the following terms:
di = 2; tri = 3; tetra = 4; penta = 5; hexa = 6
These ‘multipliers’ are not counted when considering the alphabetical order of groups. Again, don’t ask why, I don’t make the rules.
There are examples in the gallery.
Aromatic rings
Monosubstituted benzene derivatives (compounds common at undergraduate level) are named after benzene. Most functional groups are considered prefixes to benzene with the exception of aldehydes and carboxylic acid derivatives.
Benzene derivatives are common compounds and many have trivial names that are used instead of the systematic names.
Naming monosubstituted benzene derivatives
For disubstituted benzene derivatives, the functional group with the highest priority is found on carbon 1. The numbering then goes around the ring (2 - 6) to give the substituents the lowest numbers. Sometimes, the numbers are replaced by the prefixes ortho- (positions 2 & 6), meta- (positions 3 & 5) or para- (position 4).
Positions on a multiply substituted benzene ring
Non-aromatic cyclic molecules
The parent chain
Normally this is the ring. It is named according to the number of carbon atoms in the ring with addition of cyclo– as a prefix.
Identify the major functional group
As before this will be the suffix. It will also be carbon 1 when we come to number the ring.
Number the carbon atoms of the ring
Start with the major functional and then proceed around the ring so that the smallest numbers indicate the position of any substituents or multiple bonds.
Examples of naming cyclic molecules given in gallery below.
Going from a name to a structure
Another important skill is to be able to take a systematic name and draw the correct structure from it. This is effectively the reverse process to above except you should start with the parent as everything builds off this central carbon chain.
Draw the parent carbon chain
Add the major functional group (the suffix) to appropriate carbon. If there is no number associated with the suffix then it is likely to be on carbon 1 (the exception is with ketones, they must be on an internal carbon).
Number the chain so that the major functional group is in the correct place
Add multiple bonds and substituents
The diagram below shows how to draw the structure of 4-amino-5-methylhex-5-en-2-one.
Draw the structure of 4-amino-5-methylhex-5-en-2-one.
Conclusion
The naming system is useful but can get complex very quickly. This is further exasperated by the common use of trivial names. We recommend that you start to keep a note book of common chemicals, their names and their structures. They are common for a reason and will keep cropping up. But, if you want to rapidly and unambiguously communicate a molecule’s structure, you are much better off drawing it.
Some worked practice questions can be downloaded HERE.
