Nomenclature of Aromatics

Benzene’s structure fits as a stable structure when one thinks of a bond as being pairs of shared electrons. With benzene, there is one sigma bond making up a covalent bond as well as half of a pi bond between each of the two carbon atoms. Each bond has the same number of electrons and each is the same bond length. This resonance explains why benzene does not typically undergo addition reactions easily. It is basically because there are no simple pi bonds to connect to.

The other aspect of resonance is that the structure tends to be more stable than the molecule without resonance. This lowering of energy, which is about a third as much as in a typical covalent bond in the case of benzene, is important in the types of reactions associated with this molecule. It means that, when benzene does react, it usually means that the benzene ring persists.

According to Huckel rule, there is only automaticity when the number of pi electrons equals 4n + 2, where n = any integer at or above zero. Benzene has six pi electrons, in which n = 1. Remember that the number of pi electrons is two for every double bond. This is why cyclooctatetraene is not a stable resonance structure because there isn’t an integer that can be made into 8 using the 2n + 2 rule. The same is true of cyclobutadiene. In addition, there cannot be resonance if there is an interruption of the resonance feature.

The p orbitals that make up an unbroken ring of p orbitals can be associated with atoms that are not carbon. Furan, which contains five carbon atoms and an oxygen atom in a ring, and pyrrole, which is a five-carbon ring and an NH in the ring, are both stable molecules. This resonance can also be created be for a negatively charged carbon atom.

NOMENCLATURE OF AROMATICS

For all practical purposes, the term “aromatic” in organic chemistry refers to the benzene ring structure. There are other aromatic hydrocarbons, some of which involve more than one ring, which will be discussed later in this chapter.

Benzene is a planar molecule consisting of hexagonal rings of sp2-hybridized carbon atoms along with the unhybridized p orbitals, which stick up perpendicular to the ring. The sigma bonds contain three electrons (two with the neighboring carbon atoms and 1

with the hydrogen atom linked to it). The fourth valence electron is associated with the unhybridized p orbital making up the pi bonds.

Benzene has many derivatives. The hydrogen atom can be replaced by many different side chains. What you will see is that benzene and other aromatic compounds more readily participate in substitution reactions than in addition reactions. There are many possible aromatic compounds that can be made with benzene as the parent compound, some of which must be memorized. Figure 50 shows several of the most common benzene derivatives:

Figure 50.

Benzene has a zero-dipole moment when it has no side chains. The presence of a side chain can result in a dipole moment that will increase the intermolecular forces. This results in a greater melting point and boiling point for these molecules. An example is 1,4-dichlorobenzene, which has a melting point of 52.7 degrees Celsius, compared to a melting point for benzene, which is only 5.5 degrees Celsius.

Benzene is 150 kilocalories per mole more stable than would be expected if there were just three separate double bonds. This is because of the resonance factor of benzene, which is an arene molecule and not a true “alkene”. They are not good nucleophiles and instead have multiple types of electrophilic substitution reactions. This involves a process starting with the addition of an electrophile to the pi system of the benzene molecule, called “a carbo-cation”. What this looks like is depicted in figure 51:

Figure 51.

It would be difficult to describe all of the potent substances that can be made with a benzene ring. Substances as diverse as aspirin, amphetamine, ibuprofen, and adrenaline are ultimately benzene molecules with extensive side chains. These represent the common names of benzene derivatives, although there are scientific names associated with each of them.

Benzene is a ring that can have any number of substitutions. It does not have to be numbered if there is just one side chain associated with it. If there is more than one side chain of the same type, they are numbered and named as di, tri, etcetera, with the term “benzene” added as a separate word. Chlorobenzene or bromobenzene, for example, are simple substitutions that will not have to be numbered. NO2 added to the side chain gives the molecule the name of nitrobenzene.

The side chains start with the number 1 carbon atom and proceed so that the lower numbers are followed preferentially. In addition, they are listed alphabetically so that “chloro” precedes “ethyl” and not the other way around. There are side chains that take precedence by virtue of the type of side chain they are.

The terms ortho-, meta-, and para-, which have already been described, can be used to describe the side chains on these molecules. Incidentally, the ortho, meta, and para configuration does not have to apply to the same side chain. It is possible to have, for example, o-nitrochlorobenzene and m-nitrochlorobenzene.

Benzene isn’t the only name and aromatic compound that you need to memorize. An example is phenol, which is the benzene molecule with a hydroxyl group or OH group attached. An example would be o-chlorophenol, which is a chlorine side chain and a hydroxyl side chain attached in a 1, 2-carbon fashion. It can also be called 2-

chlorophenol, which gives the hydroxyl group the number one position. Figure 52 indicates some other common molecules based on benzene that you need to memorize:

Figure 52.

You should know for the purposes of nomenclature that, besides phenol, benzaldehyde and benzoic acid are retained in the IUPAC nomenclature. Others that can be used in the nomenclature system include styrene, toluene, phenanthrene, and naphthalene. The use of others can be recognized in the IUPAC nomenclature but is generally discouraged. One such term is TNT, which is trinitrotoluene—an explosive. The true IUPAC name is 2-methyl-1,3,5-trinitrobenzene.

The phenyl group is what benzene is called when it is a side chain. It is named “phenyl chloride” for example rather than chlorobenzene, etcetera. This is the preferred usage of the term when the phenyl group is attached to at least six alkane carbons, as in the molecule 3-phenyl hexane.

There is also the “benzyl group”, which is essentially toluene with a side chain on its methyl group, or the Phenyl-CH2-R molecule, also referred to by the initials “Bn-R”.