Benzene Chemistry

This can lead to molecules like benzyl chloride or benzyl alcohol. In such cases the CH3 side chain is part of the molecule and an R side chain is attached to it. Figure 53 shows benzyl chloride and benzyl alcohol:

Figure 53.

This can lead to a number of side chains that are also attached to the benzyl ring. This can lead to 2,4-fluorobenzyl chloride. The numbering system leads to the lowest number assigned to side chains as possible on the benzene ring.

BENZENE CHEMISTRY

Benzene is considered relatively unreactive because of the stability of its aromaticity, making it inert to compounds like bromine gas and hydrochloric acid. As you remember, this aromaticity comes from delocalization of the p-orbital carbons on the sp2 hybridized carbons. This creates a “doughnut”-shaped singular orbital above and below the carbon atoms that can have electrons anywhere in it. This planar molecule has a bond angle between carbon atoms of about 120 degrees. The bond length is 1.39 Angstroms, which is between that of a single and a double bond.

If there is a great deal of temperature or a catalyst, substitution reactions can occur rather than addition reactions that are seen in alkenes. This is directly related to the fact that this is a stable resonance molecule. The addition of hydrogen to make cyclohexane leads to a release of heat amounting to 28.6 kilocalories per mole. This is about 35 kilocalories per mole more stable than expected.

As an aromatic structure, it must meet four criteria to be considered aromatic:

• It must be cyclic

• It must be planar

• It must have fully conjugated double bonds

• It must have 2n + 2 pi electrons

According to the 2n + 2 rule (Huckel’s rule), there are six pi-electrons in the pi bond, leading to six delocalized electrons that “travel” throughout the perpendicular pi bonds of the benzene ring structure. This is because each double bond (with a pi bond) always contributes 2 pi electrons for a total of six electrons in benzene.

Huckel’s rule also applies to ions as long as there are 4n + 2 pi electrons, it doesn’t matter if there is a charge on the ion. This means that cyclopentadienyl anion is aromatic. This is a pentene ring with two double bonds and a carbon atom that is negatively charged without a hydrogen ion. It has an unbound pair of electrons that, along with the four donated electrons in the pi bonds of the other carbon atoms, has 6 pi electrons, which satisfies the 4n + 2 rule. Figure 54 shows the cyclopentadienyl anion structure:

Figure 54.

As we have discussed, there are many aromatic cyclic compounds that do not just contain carbon atoms. There is, as you remember, furan, which contains an oxygen atom. It is aromatic because one of the lone pairs of electrons is sp2 hybridized. The other pair is in the p orbital. Because it has a pair in the sp2 hybridized orbital, it is considered resonance. It has two electrons in the sp2 hybridized orbital and four carbon atoms, leading to 6 total electrons contributing to resonance structure that looks very similar to figure 54 but with an oxygen atom instead of a carbon atom at the top. This also means that it is a planar molecule with a trigonal planar shape of the oxygen atomic orbitals.

There are aromatic heterocycles, which are unsaturated cyclic compounds that are considered aromatic. There are several compounds that do not have the typical benzene compound associated with them. These include the molecules listed in figure 55:

Figure 55.

In imidazole, for example, there are six pi-electrons: 3 from the carbon atom and three from the nitrogen atoms, making a total of six, which fits the 2n + 2 pi-electron rule. With this molecule, one nitrogen has an H electron added to it with a lone pair adding to the sexted, while one nitrogen is free of a hydrogen so only one electron contributes to the aromaticity. As a general rule, if a compound has the possibility of being aromatic, it will be because of the stability and energy-saving features of aromatic compounds.

Benzene rings will join together and fuse to give larger polycyclic aromatic compounds that are stable. There is some localization of the pi-electrons, leading to an unevenness of the carbon-carbon bond. There are six benzene rings in the substance called coronene, which is a planar molecule. Figure 56 shows some of these polycyclic aromatic compounds: