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Concept 07 / 07Hydrocarbons

Electrophilic Aromatic Substitution: Nitration, Halogenation, Sulfonation and Friedel-Crafts Reactions

Mechanism of electrophilic substitution in benzene via the arenium ion (sigma complex), generation of the electrophile in nitration, halogenation and sulfonation, Friedel-Crafts alkylation and acylation with their limitations, and why benzene undergoes substitution rather than addition.

Figure 1
General mechanism. Benzene's delocalised π\pi cloud attacks electrophiles, but the aromatic sextet is restored at the end, so the net result is substitution. Step 1, generation of the electrophile E+\mathrm{E^{+}} (usually with a Lewis or Bronsted acid catalyst). Step 2, attack of E+\mathrm{E^{+}} on the ring to form a carbocation, the arenium ion or σ\sigma-complex, in which one carbon is sp3sp^{3} and the positive charge is delocalised over three ring carbons. Aromaticity is lost, so this step is slow and rate-determining (see figure). Step 3, loss of H+\mathrm{H^{+}} from the sp3sp^{3} carbon (removed by a base such as HSO4−\mathrm{HSO_{4}^{-}} or AlCl4−\mathrm{AlCl_{4}^{-}}), restoring the aromatic ring. If the arenium ion instead added a nucleophile, the product would lose the resonance energy; losing H+\mathrm{H^{+}} is far more favourable. Nitration. Concentrated HNO3\mathrm{HNO_{3}} and concentrated H2SO4\mathrm{H_{2}SO_{4}} (nitrating mixture) at about 330 K330\,\mathrm{K}. Sulfuric acid, the stronger acid, protonates nitric acid to give the nitronium ion: HNO3+2H2SO4⇌NO2++H3O++2HSO4−\mathrm{HNO_{3} + 2H_{2}SO_{4} \rightleftharpoons NO_{2}^{+} + H_{3}O^{+} + 2HSO_{4}^{-}}. C6H6+NO2+→C6H5NO2+H+\mathrm{C_{6}H_{6} + NO_{2}^{+} \rightarrow C_{6}H_{5}NO_{2} + H^{+}} (nitrobenzene). Halogenation. Cl2\mathrm{Cl_{2}} or Br2\mathrm{Br_{2}} with a Lewis acid (FeCl3\mathrm{FeCl_{3}}, FeBr3\mathrm{FeBr_{3}}, AlCl3\mathrm{AlCl_{3}}) in the dark: Cl2+FeCl3→Cl++FeCl4−\mathrm{Cl_{2} + FeCl_{3} \rightarrow Cl^{+} + FeCl_{4}^{-}}, giving chlorobenzene. (In UV light and without a catalyst, addition gives BHC instead.) Iodination is reversible and requires an oxidising agent to remove HI. Sulfonation. Fuming sulfuric acid (oleum); the electrophile is SO3\mathrm{SO_{3}} (or HSO3+\mathrm{HSO_{3}^{+}}): C6H6+H2SO4⇌C6H5SO3H+H2O\mathrm{C_{6}H_{6} + H_{2}SO_{4} \rightleftharpoons C_{6}H_{5}SO_{3}H + H_{2}O}. Sulfonation is reversible: heating benzenesulfonic acid with dilute acid (steam) removes the −SO3H\mathrm{-SO_{3}H} group, so it can serve as a temporary blocking group. Friedel-Crafts alkylation. C6H6+CH3Cl→C6H5CH3+HCl\mathrm{C_{6}H_{6} + CH_{3}Cl \rightarrow C_{6}H_{5}CH_{3} + HCl} (anhydrous AlCl3\mathrm{AlCl_{3}}); the electrophile is the carbocation (or a polarised R−Cl⋯AlCl3\mathrm{R-Cl \cdots AlCl_{3}} complex). Limitations: (a) polyalkylation, since the alkylbenzene product is more reactive than benzene; (b) carbocation rearrangement: n-propyl chloride gives mainly isopropylbenzene (cumene); (c) fails with strongly deactivated rings (nitrobenzene, which can even be used as a solvent) and with rings bearing −NH2\mathrm{-NH_{2}} (which complexes AlCl3\mathrm{AlCl_{3}}); (d) aryl and vinyl halides cannot be used as alkylating agents. Friedel-Crafts acylation. C6H6+CH3COCl→C6H5COCH3+HCl\mathrm{C_{6}H_{6} + CH_{3}COCl \rightarrow C_{6}H_{5}COCH_{3} + HCl} (anhydrous AlCl3\mathrm{AlCl_{3}}; acetic anhydride also works); the electrophile is the acylium ion CH3C≡O+\mathrm{CH_{3}C \equiv O^{+}}, which is resonance-stabilised and does not rearrange. The ketone product deactivates the ring, so there is no polyacylation. Clemmensen reduction of the ketone then gives the straight-chain alkylbenzene, avoiding rearrangement. Key Takeaways: - Mechanism: electrophile generation, slow formation of the arenium ion, fast loss of H+\mathrm{H^{+}}. - Electrophiles: NO2+\mathrm{NO_{2}^{+}} (nitration), Cl+\mathrm{Cl^{+}} (halogenation), SO3\mathrm{SO_{3}} (sulfonation), R+\mathrm{R^{+}} (alkylation), RCO+\mathrm{RCO^{+}} (acylation). - Sulfonation is reversible. - Alkylation: polyalkylation and rearrangement; acylation: neither. - Pitfall: Friedel-Crafts reactions fail on nitrobenzene and aniline. Brain-Teaser 1: What is the product when benzene reacts with 1-chloropropane and anhydrous AlCl3\mathrm{AlCl_{3}}? How can n-propylbenzene be made? Solution: The primary cation rearranges by a hydride shift to the more stable isopropyl cation, so the main product is isopropylbenzene (cumene). n-Propylbenzene is made by Friedel-Crafts acylation with propanoyl chloride (giving propiophenone, no rearrangement) followed by Clemmensen reduction (Zn(Hg)\mathrm{Zn(Hg)}, HCl). Brain-Teaser 2: Why is concentrated H2SO4\mathrm{H_{2}SO_{4}} needed in nitration, when nitric acid alone contains the nitrogen? Solution: Nitric acid itself is a poor electrophile. Sulfuric acid, a stronger acid, protonates HNO3\mathrm{HNO_{3}}, which then loses water to give the powerful electrophile NO2+\mathrm{NO_{2}^{+}}; H2SO4\mathrm{H_{2}SO_{4}} also absorbs the water formed. Brain-Teaser 3: In the nitration of C6D6\mathrm{C_{6}D_{6}} (hexadeuterobenzene), the rate is the same as for C6H6\mathrm{C_{6}H_{6}}. What does this show about the mechanism? Solution: There is no kinetic isotope effect, so the C-H (C-D) bond is not broken in the rate-determining step. This confirms that formation of the arenium ion is slow and loss of H+\mathrm{H^{+}} (or D+\mathrm{D^{+}}) is a separate, fast step.

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