Chapter 16: Chemistry of Benzene: Electrophilic Aromatic Substitution
The two-step mechanism of electrophilic aromatic substitution
Benzene has a π system rich in electron density, yet it does not undergo the addition reactions characteristic of alkenes. The difference is thermodynamic. Addition across one double bond of benzene would leave a nonaromatic cyclohexadiene, destroying roughly 150 kJ/mol of aromatic stabilization; substitution of a ring hydrogen preserves the aromatic ring in the product. Strong electrophiles therefore react with benzene by a two-step pathway in which aromaticity is sacrificed only temporarily. In the first step, two π electrons of the ring attack the electrophile (the electron-donor role of…
Generating the electrophile: nitration, sulfonation, and halogenation
Every reaction in the electrophilic aromatic substitution family faces the same requirement: the aromatic ring is a weak nucleophile, so the electrophile must be strong. The reagent mixtures of this section exist to manufacture those strong electrophiles in situ. Nitration employs a mixture of concentrated nitric and sulfuric acids. Sulfuric acid protonates nitric acid on its hydroxyl oxygen; loss of water then leaves the nitronium ion, NO₂⁺, a linear cation with a full positive charge on nitrogen. Nitronium attacks benzene to give an arenium ion, and deprotonation gives nitrobenzene. The…
Friedel–Crafts alkylation and acylation: carbon electrophiles and their limits
The Friedel–Crafts reactions extend electrophilic aromatic substitution to carbon electrophiles, forming the carbon–carbon bonds that build alkyl- and acylbenzenes. In alkylation, aluminum chloride abstracts chloride from an alkyl chloride to form a carbocation–AlCl₄⁻ ion pair. The carbocation attacks the ring, and loss of the ring proton to AlCl₄⁻ gives the alkylbenzene and regenerates the catalyst. Alkylation carries four practical limitations, each traceable to the mechanism. First, the halide must be an alkyl halide: aryl and vinylic halides cannot form the required cations, because those…
How substituents change the reactivity of the ring
Benzene's substitution chemistry becomes predictive the moment one question is asked of every substituent: does it push electron density toward the ring or pull density away? The answer controls the rate of electrophilic substitution because the rate-determining step creates a positively charged arenium ion — anything that stabilizes positive charge in the ring accelerates the reaction, and anything that destabilizes it applies a brake. Substituents interact with the ring through two channels. The inductive effect operates through σ bonds: alkyl groups are weak donors, for the same reason…
Where the next group goes: ortho–para and meta direction
Directing effects are resonance arguments, and every prediction in this section can be reconstructed by drawing three arenium ions and comparing them. Consider anisole. Attack of an electrophile at the para position gives a cation whose three resonance forms place the charge ortho and para to the point of attack — and one of those carbons is the carbon bearing the methoxy group. There the oxygen lone pair engages, giving a fourth resonance form with the positive charge on oxygen and every heavy atom holding a full octet. This oxocarbenium-type form is the most stable contributor available to…
Planning polysubstituted benzenes: directors in combination and order of steps
Predicting substitution on a disubstituted ring begins by marking the positions each group favors. When both substituents direct to the same positions, the prediction is immediate. When they disagree, the more strongly activating group controls, a consequence of the same arenium-ion analysis as before: the intermediate stabilized by the stronger donor is the more stable one. In p-methylanisole, methoxy outranks methyl, so nitration occurs ortho to the methoxy group. One further rule is steric rather than electronic: the carbon between two substituents in a 1,3 relationship is flanked on both…
When nucleophiles substitute the ring: addition–elimination and benzyne
The substitution chemistry of alkyl halides fails completely on aromatic carbon. Backside displacement is impossible because the ring blocks the trajectory and the carbon cannot invert; ionization is impossible because an aryl cation would place an empty sp² orbital in the ring plane with no stabilization available. Aryl halides are therefore inert to the SN1 and SN2 conditions of earlier chapters, and the two pathways that do substitute them are distinctive enough to carry their own names. Nucleophilic aromatic substitution operates on aryl halides that carry strong electron-withdrawing…
Oxidation and reduction around the ring
The aromatic ring is a bystander in most oxidation chemistry: reagents that cleave or dihydroxylate alkenes leave benzene untouched. Alkyl groups attached to the ring are another matter. Hot aqueous KMnO₄ attacks the benzylic C–H bond and, once begun, degrades the entire side chain to a single carboxyl carbon regardless of the chain's original length — butylbenzene gives benzoic acid, losing three carbons in the process, and both methyls of p-xylene are oxidized to give terephthalic acid. The requirement is a benzylic hydrogen: tert-butylbenzene, whose benzylic position is fully substituted,…
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