9.4 Aromatic Hydrocarbons

In 1825 Michael Faraday isolated a sweet-smelling liquid from the oily residue of the gas used to light London's streets. Its formula, $\mathrm{C_{6}H_{6}}$, is missing eight hydrogen atoms compared with hexane — four rings or double bonds' worth of unsaturation — so it "should" react like a super-alkene. It does not. Benzene leaves bromine water and cold permanganate untouched; with bromine and a catalyst it substitutes a hydrogen instead of adding across a double bond, keeping its ring intact. The compounds built on this stubborn ring are called aromatic, after the fragrant oils and resins (oil of wintergreen, vanillin, cinnamon) from which many were first obtained.

This section explains why benzene behaves so differently. It covers the naming and isomerism of benzene derivatives; the Kekulé structures, resonance and delocalised π electrons of benzene; the general conditions for aromaticity (the Hückel $4n+2$ rule); how benzene is prepared; its characteristic reactions — electrophilic substitution (nitration, halogenation, sulphonation and the Friedel–Crafts reactions) with mechanism — and its few addition reactions and combustion; how a group already on the ring decides where and how fast the next one goes; and why benzene and its larger relatives are toxic and carcinogenic.

Naming, Isomerism and the Structure of Benzene

Because the six positions of a benzene ring are equivalent, there is only one monosubstituted benzene $\mathrm{C_{6}H_{5}X}$, but three disubstituted ones $\mathrm{C_{6}H_{4}X_{2}}$ (o, m, p) — whether the two groups are the same or different — and three trisubstituted ones $\mathrm{C_{6}H_{3}X_{3}}$ (1,2,3-, 1,2,4- and 1,3,5-). Arenes also show chain/position isomerism through their side chains: ethylbenzene $\mathrm{C_{6}H_{5}C_{2}H_{5}}$ is an isomer of the three xylenes, $\mathrm{C_{8}H_{10}}$.

The structure of benzene. The molecular formula, the existence of only one monosubstituted benzene, and the formation of cyclohexane ($\mathrm{C_{6}H_{12}}$) by adding three molecules of $\mathrm{H_{2}}$ led August Kekulé in 1865 to propose a six-membered ring of carbon atoms with alternating single and double bonds and one H on each carbon. Two facts did not fit a single fixed Kekulé structure. It predicts two different 1,2-disubstituted benzenes (with a C=C or a C–C between the substituents), but only one is known; Kekulé suggested that the double bonds keep shifting between two arrangements. And benzene does not show the addition reactions of alkenes. Modern measurements settle the matter: all six C–C bonds are equal, $139\ \mathrm{pm}$ — between C–C ($154$) and C=C ($134\ \mathrm{pm}$) — and the molecule is a flat regular hexagon.

Preparing Benzene

Industrially, benzene comes from coal tar and, today mostly, from petroleum by catalytic reforming (aromatisation of hexane, Section 9.1). Three laboratory methods are:

• Cyclic polymerisation of ethyne: $\mathrm{3HC{\equiv}CH}$ through a red-hot iron tube at $873\ \mathrm{K}$ → $\mathrm{C_{6}H_{6}}$ (Section 9.3).
• Decarboxylation of an aromatic acid: sodium benzoate heated with soda lime loses the $\mathrm{{-}COONa}$ group, just like the aliphatic salts of Section 9.1: $$\begin{aligned}&\mathrm{C_{6}H_{5}COONa(s)+NaOH(s)}\\&\quad\xrightarrow[\Delta]{\text{CaO}}\mathrm{C_{6}H_{6}(l)+Na_{2}CO_{3}(s)}\end{aligned}$$
• Reduction of phenol: phenol vapour passed over heated zinc dust loses its oxygen: $$\mathrm{C_{6}H_{5}OH+Zn\xrightarrow{\Delta}C_{6}H_{6}+ZnO}$$

Electrophilic Substitution, Addition and Combustion

Physical properties. Arenes are non-polar, colourless liquids or solids with a characteristic smell, immiscible with water but miscible with organic solvents. Benzene boils at $80\ ^{\circ}\mathrm{C}$ and burns with a very sooty flame (high C : H ratio).

The cloud of six delocalised π electrons makes benzene a source of electrons, so it is attacked by electrophiles (Chapter 8). But where an alkene would then add a nucleophile, benzene instead loses $\mathrm{H^{+}}$ and regains its aromatic ring.

Addition reactions happen only under forcing conditions, because they destroy the aromatic ring. With hydrogen over nickel at high temperature and pressure benzene gives cyclohexane, $\mathrm{C_{6}H_{6}+3H_{2}\rightarrow C_{6}H_{12}}$. With chlorine in ultraviolet light at about $500\ \mathrm{K}$ — no catalyst — it adds three molecules of $\mathrm{Cl_{2}}$, giving benzene hexachloride (BHC), $\mathrm{C_{6}H_{6}Cl_{6}}$ (1,2,3,4,5,6-hexachlorocyclohexane), once used as an insecticide. Combustion: $\mathrm{2C_{6}H_{6}(l)+15O_{2}(g)\rightarrow 12CO_{2}(g)+6H_{2}O(l)}$; for any hydrocarbon, $\mathrm{C_{x}H_{y}+(x+\tfrac{y}{4})O_{2}\rightarrow xCO_{2}+\tfrac{y}{2}H_{2}O}$.

Directive Influence of Substituents; Toxicity

When a benzene ring already carries a group, a second electrophile does not attack the five remaining positions at random. Nitrating methylbenzene gives mainly 2- and 4-nitromethylbenzene (o- and p-nitrotoluene), and faster than benzene; nitrating nitrobenzene gives mainly 1,3-dinitrobenzene, and much more slowly. The first group both directs the newcomer and activates or deactivates the ring.

Carcinogenicity and toxicity. Benzene and many aromatic hydrocarbons with several fused rings (polynuclear hydrocarbons) are toxic and carcinogenic. Long exposure to benzene vapour damages the bone marrow and can cause leukaemia, which is why its content in petrol is strictly limited and it is no longer used as a general laboratory solvent (methylbenzene is used instead). Polynuclear hydrocarbons with four or more fused rings, such as benzo[a]pyrene, are formed when organic matter — tobacco, coal, wood, petroleum and grilled food — burns incompletely; in the body they are converted into reactive species that bind to DNA and can trigger cancer. They are major carcinogens in cigarette smoke and urban air pollution.

Exercises