CH 106 - Lesson 2
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Nomenclature

Now let's deal with how to name simple aromatic compounds. Aromatic compounds are what we call compounds derived from benzene. Actually, as noted earlier, that is too narrow a definition for aromatic compounds but it will serve for this course.

Examples

At this point I need to introduce a new functional group, one that we have not dealt with before, but one that is very important with respect to aromatic compounds. That group is the nitro group which consists of a nitrogen atom bonded to the ring and bonded to two oxygen atoms, so the nitro group is an -NO2 group that can be bonded to carbon. This is shown in the nitrobenzene.

In many cases when only one group is attached to a benzene ring, you simply name that group and follow that name by the name benzene. This is the case for these two compounds (which are also shown in Example 15 in your workbook). The compound with a nitro group attached to a benzene ring is called nitrobenzene. A benzene ring with a chloro group attached is called chlorobenzene.
Diagram of nitrobenzene structure. [62arom01.JPG] Diagram of chlorobenzene structure. [62arom02.JPG]
nitrobenzene chlorobenzene

 

Special Names

Many simple aromatic compounds, however, have their own unique name. These are names that are not derived from the word benzene. The only way you would become familiar with them is to memorize them or use them frequently.

Two such compounds are shown here (and also in Example 15 in your workbook). These are toluene and phenol. Toluene is the one that could be called methylbenzene. Phenol could be called hydroxybenzene.
Diagram of toluene structure. [62arom03.JPG] Diagram of phenol structure. [62arom04.JPG]
toluene phenol

 

Multiple Side Groups

When more than one group is attached to the benzene ring, there are two methods that can be used to identify where those groups are attached. One of those methods is a numbering system that is an extension of the IUPAC method and similar to what we have used before with cyclic alkanes. In this system, one of the groups that is attached to the benzene ring is considered to be attached to the number one carbon atom. The other groups are indicated (as they were before) by a prefix and a number to indicate where they are attached on the benzene ring.

Some examples are given here. Here we have 1,2-dichlorobenzene, 1,3-dichlorobenzene, and 1,4-dichlorobenzene. Again, the numbers are chosen in such a way as to give the smallest combination of numbers possible. I should point out that although the #1 carbon atom is usually shown at the top of the structural formula, it does not have to be that way.
Diagram of o-dichlorobenzene structure. [62arom05.JPG] 1,2-dichlorobenzene
Diagram of m-dichlorobenzene structure. [62arom06.JPG] 1,3-dichlorobenzene
Diagram of p-dichlorobenzene structure. [62arom07.JPG] 1,4-dichlorobenzene

 

There is also another type of notation that is often used. This notation is to use the prefix ortho- to indicate that the two functional groups are on adjacent carbon atoms within the ring. So 1,2-dichlorobenzene can also be called ortho-dichlorobenzene. Sometimes the ortho- prefix is abbreviated just with a small letter o-.

If the functional groups are attached to carbon atoms on the ring which have one carbon between them, then they are referred to as being meta to one another. Thus, 1,3-dichlorobenzene is sometimes called meta-dichlorobenzene. Quite often the meta- is abbreviated by simply using the small letter m-.

When the two functional groups are on opposite ends of the ring of carbon atoms, they are referred to as being para to one another, and the third compound here can be referred to as para-dichlorobenzene. The para- is sometimes abbreviated, using small letter p-.
62arom05.JPG (977 bytes) ortho-dichlorobenzene
o-dichlorobenzene
62arom06.JPG (985 bytes) meta-dichlorobenzene
m-dichlorobenzene
62arom07.JPG (969 bytes) para-dichlorobenzene
p-dichlorobenzene

The ortho, meta, and para prefixes are generally used when only two groups are attached to the benzene ring. This method pretty much falls from use when more than two functional groups are attached. The number method, on the other hand, can be used for any number of functional groups attached to the benzene ring.

Some more examples are shown here (and  also in Example 17 in your workbook).

The first of these is 4-chlorotoluene. It can also be called para-chlorotoluene.
Diagram of p-chlorotoluene structure. [62arom08.JPG] 4-chlorotoluene
para-chlorotoluene
The next compound, 2,4,6-trinitrotoluene, is usually referred to as TNT. Notice that there are three nitro groups attached; and since it is the nitrogen atom that is attached to the ring, the NO2 group in the upper left corner is written in the opposite direction just to emphasize that it is the nitrogen atom that is attached to the ring.
Diagram of TNT structure. [62arom09.JPG] 2,4,6-trinitrotoluene
TNT
The third compound shown here is called p-xylene. We will use it in the lab instead of benzene because it is safer to work with.
Diagram of p-xylene structure. [62arom10.JPG] p-xylene

 

Practice

You can get some additional practice working with the names of aromatic compounds by figuring out the structural formulas for these compounds for which the formulas are shown below. (These are also shown in Exercise 18 in your workbook.) Please take some time to draw structures for those now.

bromobenzene
o-dibromobenzene
3-bromo-5-chloro-1-nitrobenzene
m-ethylisopropylbenzene

 

Answers

bromobenzene Structure of bromobenzene. [62arom11.JPG]
o-dibromobenzene Structure of o-dibromobenzene. [62arom12.JPG]
3-bromo-5-chloro-1-nitrobenzene Structure of 3-bromo-5-chloro-1-nitrobenzene. [62arom13.JPG]
m-ethylisopropylbenzene Structure of m-ethylisopropylbenzene. [62arstr7.JPG]

 

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