Preparing Benzene and Understanding Its Reactions

Different routes supply the same aromatic ring

Benzene is commercially obtained from aromatic-containing feedstocks such as coal-tar and petroleum fractions. The laboratory routes developed here either build its six-carbon ring or remove a substituent from an existing aromatic ring. Keep this distinction visible: trimerisation constructs the ring from ethyne, while decarboxylation and phenol reduction retain an existing carbon ring.

Arenes are generally non-polar and poorly soluble in water but readily soluble in suitable organic solvents. They can be liquids or solids. Naphthalene’s volatility and moth-repellent effect underlie its traditional use in stored clothing, but these uses do not make exposure harmless. Aromatic compounds often burn with a sooty flame because incomplete combustion is favoured by their relatively high carbon content under ordinary flame conditions.

Five substitution patterns share one mechanism

Electrophilic substitution replaces a ring hydrogen with an electrophile-derived group. Nitration introduces nitro, halogenation introduces halogen, sulphonation introduces a sulphonic acid group, and the Friedel–Crafts reactions attach alkyl or acyl groups. Each requires a reagent system able to create or strongly activate an electrophile. Merely mixing benzene with any source of the desired atom is not enough.

The electron-rich ring can donate pi electron density towards an electrophile. An ordinary nucleophile is also electron-rich, so an unactivated benzene ring is an unfavourable target for its attack. Benzene also has no ordinary leaving group in place of hydrogen. Nucleophilic aromatic substitution therefore requires suitable structural or reaction features; its difficulty in ordinary benzene does not mean that every aromatic derivative is incapable of it.

Chloroethane similarly supplies an ethyl group and gives ethylbenzene. Anhydrous aluminium chloride is a Lewis acid: it accepts an electron pair and activates the electrophile source. The reactive species is often represented in an elementary mechanism as an alkyl cation or a strongly cation-like complex. Rearrangement is possible, so alkylation does not always preserve the position of attachment suggested by the original halide.

An acyl group contains a carbonyl carbon at its attachment point. Benzene reacts with an acyl chloride or an acid anhydride in the presence of aluminium chloride to give an aryl ketone. Acetyl chloride and acetic anhydride both provide routes to acetophenone; their accompanying small products differ.

Generate an electrophile, disturb the ring, then restore it

The common mechanism has three conceptual stages. First, the reagent and catalyst generate or activate an electrophile. Second, the aromatic pi system forms a sigma bond to that electrophile. The attacked carbon becomes approximately sp3 and the ring temporarily loses its complete aromatic delocalisation. The resulting positively charged intermediate is the sigma complex, or arenium ion.

Its remaining positive charge is stabilised through several resonance contributors. This stabilisation does not mean the intermediate is as aromatic as benzene: the sp3 centre interrupts the complete cyclic p system. Finally, a base removes the proton from that centre. The electron pair from its C–H bond restores the ring pi system, giving the substituted aromatic product.

For halogenation and Friedel–Crafts reactions, aluminium chloride activates chlorine, an alkyl halide, or an acyl halide. Simplified schemes write chloronium-like electrophilic chlorine, an alkyl cation, or an acylium ion. These symbols describe the electron-deficient attacking part; they need not imply a free isolated ion existing indefinitely in solution. Tetrachloroaluminate can remove the final proton, regenerating aluminium chloride and forming hydrogen chloride.

In this acid–base step, sulphuric acid supplies a proton and nitric acid accepts it. Water loss produces nitronium, the nitrating electrophile. After ring attack, hydrogen sulphate can remove the ring proton and regenerate the acid. A catalyst participates in the sequence without being consumed in the overall equation.

Addition requires conditions that overcome aromatic stability

Benzene can add hydrogen under vigorous catalytic conditions to form cyclohexane. Under ultraviolet light, three chlorine molecules can add to form hexachlorocyclohexane, traditionally called benzene hexachloride or BHC. This must be distinguished from chlorine substitution with a Lewis acid. The gamma stereoisomer of hexachlorocyclohexane is called lindane or gammexane; the whole stereoisomer mixture should not be treated as that one isomer.

With excess chlorine and suitable substitution conditions, all six ring hydrogens can instead be replaced, giving hexachlorobenzene. Its formula is C6Cl6 and its aromatic ring remains. Comparing the two formulas prevents confusion between adding six chlorine atoms and replacing six hydrogen atoms.

Combustion follows a general atom balance

Complete combustion converts each carbon into one carbon dioxide and every pair of hydrogens into one water. Counting the oxygen atoms in these products gives a formula applicable to any hydrocarbon, not only an alkane. The state of water matters for a quoted enthalpy but does not change the atom balance.