Naming aromatic compounds is a topic you will likely come across somewhere in the middle of your organic chemistry course. This may come up during the topic of aromaticity and conjugation, or perhaps during EAS – Electrophilic Aromatic Substitution. In this article I will give you a quick overview of the ortho, meta, and para naming system to help you recognize these compounds and apply them to your reactions
Since benzene is a conjugated 6-member ring, it is much easier to use a designation system for identifying the relationship of substituents on a molecule compared to the standard numbering.
If you have just 2 substituents on your aromatic ring you can use the prefixes ortho, meta, or para to identify the relationship of the 2 groups around the ring
Ortho substituents are located just 1 carbon away from the primary functional group. For example, when you have a methyl group and a chlorine atom sitting on neighboring carbons of your benzene ring, the molecule can be named as follows:
2-chloro-1-methylbenzene or simply 2-chlorotoluene
However, it is much simpler to name the molecule:
ortho-chlorotoluene or simply o-chlorotoluene
When you have 1 carbon in between the methyl and chlorine so that they are located on carbons 1 and 3, you can use the meta designation
3-chloro-1-methylbenzene or 3-chlorotoluene becomes meta-chlorotoluene or simply m-chlorotoluene
And finally, when the methyl and chlorine groups are directly opposite each other on the ring so that they are located on positions 1 and 4, you can use the ‘para’ designator.
Using the numbering system you get 4-chloro-1-methylbenzene or simply 4-chlorotoluene. Using para you simplify the name to para-chlorotoluene or simply p-chlorotoluene