Naming Substituted-Benzene Compounds
A Crystal Clear Chemistry Nomenclature Tutorial
Monosubstituted Benzene
Singly substituted benzene is relatively easy, but the main problem is that there are many nicknames and alternate names for various compounds. For example, the following compound can be called phenyl bromide or bromobenzene. Phenyl is a common synonym for a benzene ring.
There are usually two equivalent names for each benzene-based compound, one with phenyl as the root and one with benzene. Again, for example, benzaldehyde and phenyl aldehyde both refer to the compound pictured below.
The simplest guideline to follow is to always use the "phenyl" naming convention. This is the most consistent and systematic way to name these compounds, whereas benzene-based names often involve various arrangements of roots and suffixes or prefixes that are mostly learned by experience.
One additional thing to realize is that benzyl bromide and phenyl bromide are different compounds. When "benzyl" or "benzyllic" is used, this denotes that whatever comes afterwards is bound, not to the phenyl ring, but to a methylene carbon that comes off of the ring. It illustrate, phenyl bromide is pictured on the left, and benzyl bromide is pictured on the right. Notice that the difference between the two structures is that for benzyl bromide, the bromine atom is one more carbon away from the ring.
Thus, use phenyl as the root when the substituent is on the ring. Use benzyl as the root when the substituent is one carbon away from the ring.
Disubstituted and Polysubstituted Benzene
There are two different ways of naming compounds that have more than one substitution on benzene: the numbering system, and what we call the "OMP" system (ortho-meta-para). The OMP is older and more widespread, but it can get a bit hard to use when you're dealing with more than two substituents. The numbering system is the new standard, since it is more systematic and can be used for any number of substituents. You'll probably have to know how to do both, since organic chemists switch between them often.
The OMP System
There are essentially three unique different ways to stick two different substituents on a benzene ring; all the other ways you can think of are equivalent to these three unique ways (try rotating and flipping the molecule to line up the substituents with the three configurations). The substituents can be right next to each other, separated by one corner, or completely across from each other on the ring. The first configuration is called ortho, the second configuration is called meta, and the last configuration is called para. You can see the three examples of the names below.
This system works pretty well, as long as you can remember which name goes with which configuration. I find it helpful to remember it as the "OMP" system, with each letter going with the configuration in which the substituentss are one more carbon apart.
As an example, the compound below has the chlorine and the aldehyde one carbon apart, and so by counting across the OMP, we see this is the second of the OMP configurations. Thus, this is a meta configuration, and so this compound is called meta-chloro phenyl aldehyde.
The Numbering System
With more substituents, however, the ortho-meta-para system can get pretty hairy. How, for example, would you name the following?
In the numbering system, we pick one substituent and designate its place on the benzene ring as position 1. Then we count around the ring and assign each substituent a number in order. This is very similar to the system of naming linear hydrocarbon chains that you may have learned in general chemistry, except that because we have a ring, we pick a point to start with (instead of starting at an end). We can, of course, start anywhere on the ring and come up with alternate numbering systems. Usually, the point is picked on the ring that gives the smallest numbers for most of the substituents. Then, we name the benzene ring as if none of the other substituents were there except for the one in position 1, and then attach the other names in front as if we were naming a linear hydrocarbon chain.
Let us go back to the complicated-looking example directly above. Let's pick a starting place and go around the ring to come up with a preliminary name. Start with the ethyl group in the position corresponding to 2 o'clock. Let us call this the 1 position, and count up in the clockwise direction (so the amine group is in position 2, the methyl is in position 3, the iodine is in position 5, and the bromine is in position 6). Since the ethyl group is in position 1, the root name is phenyl ethane, and then we attach the other names in front with the numbers, like you do when naming hydrocarbon chains. Thus, the full name is 2-amino 3-methyl 5-iodo 6-bromo phenyl ethane.
Let's try this again with the same molecule, except we pick a different position now. Pick iodine as position 1, and then count in the clockwise direction once more. Then, the molecule's root name is phenyl iodide and the whole name is 2-bromo 3-ethyl 4-amino 5-methyl phenyl iodide.
Although this system may seem quite difficult, it is relatively easy once you get the feel for it, which can only come with practice. Read back through this article and try to work out each example yourself once more, and practice, practice, practice. 