Preparation of aromatic compounds

Coal tar and petroleum are the major industrial sources of aromatic compounds generally, exactly as they are for benzene specifically (section 15.4.1). Beyond simply isolating benzene from these natural sources, three synthetic routes can also build it up directly. The first is trimerization of ethyne: passing ethyne through a red-hot iron tube at 873 K causes three ethyne molecules to cyclise together into one molecule of benzene. (Related to this addition chemistry, benzene itself can separately be made to add three molecules of chlorine gas under UV light -- a genuine addition, not substitution -- forming benzene hexachloride, whose gamma-isomer is the insecticide gammexane/lindane.) The second synthetic route is decarboxylation of sodium benzoate: heating anhydrous sodium benzoate with soda-lime (a mixture of NaOH and CaO) drives off carbon dioxide (trapped as sodium carbonate) and leaves ben …