Benzene Reactions
Benzene (C₆H₆) is the archetypal aromatic compound, consisting of a planar hexagonal ring of six carbon atoms with delocalized π electrons spread uniformly above and below the ring plane. Unlike alkenes, benzene does not readily undergo addition reactions that would disrupt its stable aromatic system. Instead, it undergoes electrophilic aromatic substitution (EAS), in which an electrophile replaces one of the ring hydrogen atoms while the aromatic π system is restored. This selectivity is the hallmark of aromatic chemistry. The four principal EAS reactions of benzene are nitration (introduction of a –NO₂ group), sulfonation (introduction of a –SO₃H group), halogenation (introduction of –Cl or –Br using a Lewis acid catalyst), and Friedel–Crafts reactions (alkylation and acylation). Each reaction follows the same two-stage mechanistic blueprint: attack of an electrophile on the electron-rich ring to form a carbocation intermediate called an arenium ion (or sigma complex / Wheland intermediate), followed by loss of a proton to regenerate aromaticity. Understanding EAS is fundamental to organic synthesis, because the substituents already present on a benzene ring direct incoming electrophiles to specific positions (ortho, meta, or para) and either activate or deactivate the ring toward further substitution. Mastery of these reactions connects directly to the manufacture of dyes, pharmaceuticals, explosives, detergents, and polymers that underpin modern industry.
- General EAS Mechanism
- Nitration
- Sulfonation
- Halogenation
- Friedel–Crafts Alkylation
- Friedel–Crafts Acylation
General EAS Mechanism
Step 1 – Electrophile generation: A reactive electrophile (E⁺) is generated, often with the help of a Lewis acid catalyst. Step 2 – Electrophilic attack: E⁺ attacks one carbon of the benzene ring, using two electrons from the π system to form a new C–E σ bond. Aromaticity is temporarily lost; the ring becomes a cyclohexadienyl carbocation (arenium ion / Wheland intermediate) with positive charge delocalized over three carbons. Step 3 – Deprotonation: A base (often the counterion or solvent) removes the H⁺ from the sp³ carbon bearing E, restoring the aromatic π system and completing substitution.
Nitration
Reagents: concentrated HNO₃ + concentrated H₂SO₄ (nitrating mixture), 50–55 °C. The strong acid H₂SO₄ protonates HNO₃, which then loses water to generate the nitronium ion: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻. The electrophile NO₂⁺ attacks benzene, forming the arenium ion, then deprotonation gives nitrobenzene (C₆H₅NO₂). Nitrobenzene is a pale yellow liquid used to make aniline for dyes and pharmaceuticals. TNT (2,4,6-trinitrotoluene) is produced by exhaustive nitration of toluene.
Sulfonation
Reagents: fuming H₂SO₄ (oleum, H₂SO₄·SO₃) or concentrated H₂SO₄, heat (~80 °C). The electrophile is SO₃ (or protonated SO₃, i.e., H₂S₂O₇ species). SO₃ attacks benzene to give an arenium ion; deprotonation yields benzenesulfonic acid (C₆H₅SO₃H). Crucially, sulfonation is reversible: heating benzenesulfonic acid with dilute H₂O re-generates benzene. This reversibility is exploited in synthesis as a temporary blocking group strategy. Benzenesulfonic acids are the basis of detergents and ion-exchange resins.
Halogenation
Reagents: Cl₂ or Br₂ with a Lewis acid catalyst (AlCl₃ or FeBr₃), room temperature, anhydrous conditions. The Lewis acid polarizes the halogen–halogen bond: Cl₂ + AlCl₃ → Cl⁺···AlCl₄⁻ (a highly polarized complex). The electrophilic Cl⁺ (or complexed Cl) attacks benzene; deprotonation gives chlorobenzene (C₆H₅Cl) and regenerates AlCl₃. Without the Lewis acid, Cl₂ is not electrophilic enough to attack the ring. Note: Cl₂/UV light gives free-radical chlorination of the side chain of alkylbenzenes, not EAS — conditions dictate the pathway.
Friedel–Crafts Alkylation
Reagents: alkyl halide (R–X) + AlCl₃ catalyst. AlCl₃ abstracts X⁻ to generate a carbocation R⁺ (or tightly ion-paired complex), which attacks benzene. After deprotonation, an alkylbenzene is formed. Limitations: (1) Polyalkylation occurs because the alkyl group activates the ring toward further substitution. (2) Rearrangement: primary carbocations can rearrange to more stable secondary or tertiary carbocations before attacking. For example, 1-chloropropane + AlCl₃ can give isopropylbenzene (cumene) instead of n-propylbenzene. (3) Does not work with rings deactivated by strong electron-withdrawing groups (e.g., nitrobenzene).
Friedel–Crafts Acylation
Reagents: acyl chloride (R–CO–Cl) or acid anhydride + AlCl₃. AlCl₃ generates the acylium ion R–C≡O⁺, a resonance-stabilized electrophile that does not rearrange (key advantage over alkylation). Attack on benzene followed by deprotonation gives an aryl ketone (e.g., acetophenone from benzene + CH₃COCl). Because the carbonyl group deactivates the ring (meta-director), polyacylation does not occur, making acylation superior for controlled synthesis. The ketone can subsequently be reduced to an alkyl group (Clemmensen or Wolff–Kishner reduction) to achieve indirect alkylation without rearrangement.
The Key Equation
- \ce{C6H6} — Benzene (—): The aromatic substrate; six π electrons in a delocalized ring.
- \ce{HNO3} — Nitric acid (—): Source of the nitronium ion NO₂⁺ when protonated by H₂SO₄.
- \ce{H2SO4} — Sulfuric acid (catalyst/proton donor) (—): Strong acid that protonates HNO₃ to generate electrophilic NO₂⁺.
- \ce{C6H5NO2} — Nitrobenzene (—): Product; a pale-yellow liquid with a characteristic almond-like odour, precursor to aniline.
Worked Example
Problem. A student reacts benzene with ethanoyl chloride (CH₃COCl) and excess AlCl₃. (a) Name this type of reaction and identify the electrophile formed. (b) Write the overall equation for the reaction. (c) Explain why this method is preferred over reacting benzene with chloroethane to introduce an ethyl group.
Answer. Friedel–Crafts acylation; electrophile is CH₃CO⁺ (acylium ion); product is acetophenone (C₆H₅COCH₃). Acylation is preferred because the acylium ion does not rearrange and polyacylation does not occur.
Origins · 1 minCharles Friedel and how this was discovered
Charles Friedel, collaborating with American chemist James Mason Crafts, discovered Friedel–Crafts alkylation and acylation in 1877 while working at the École Normale Supérieure in Paris. Their discovery that aluminum chloride could catalyze the attachment of carbon-containing groups to benzene rings opened an entirely new dimension of aromatic synthesis, enabling the industrial production of fragrances, pharmaceuticals, dyes, and synthetic rubbers. Friedel was also a distinguished mineralogist who collected over 10,000 mineral specimens; he described several new mineral species and had the mineral 'friedeite' named in his honour. He initially stumbled upon the Friedel–Crafts reaction by accident while studying organochlorosilane compounds.
Related Concepts
- Structure and Stability of Benzene — Understanding benzene's delocalized π system and aromatic stability is prerequisite to explaining why substitution rather than addition occurs.
- Electrophilic Addition in Alkenes — Contrasting EAS with electrophilic addition highlights why benzene sacrifices addition (which would destroy aromaticity) in favour of substitution.
- Directing Effects of Substituents on Benzene — Substituents on benzene direct incoming electrophiles ortho/para or meta; this is the logical next topic after learning the four EAS reactions.
- Nucleophilic Addition to Carbonyl Groups — Friedel–Crafts acylation produces aryl ketones whose carbonyl group can undergo further reactions, linking EAS to carbonyl chemistry.
- Lewis Acids and Bases — AlCl₃ and FeBr₃ catalysts in halogenation and Friedel–Crafts reactions function as Lewis acids; understanding this concept is essential for explaining electrophile generation.