Revise Arenes for Cambridge 9701 (30.1)

Benzene (C₆H₆) is a planar, cyclic molecule where each carbon atom is sp² hybridised. Each carbon forms sigma bonds to two other carbons and one hydrogen atom. The remaining p-orbital on each of the six carbon atoms overlaps sideways, above and below the plane of the ring. This overlap creates a continuous system of delocalised pi electrons, often represented as a circle inside the hexagon.

The enhanced stability of benzene is supported by two key pieces of evidence: thermochemical data and bond length measurements. X-ray diffraction shows all C-C bonds are 0.139 nm, intermediate between a single (0.154 nm) and double (0.134 nm) bond, indicating electron delocalisation. Thermochemically, the enthalpy of hydrogenation of benzene is much less exothermic than predicted for a hypothetical 'cyclohexa-1,3,5-triene' molecule, with the difference being the delocalisation energy.

Due to its stability, the benzene ring does not readily undergo addition reactions which would destroy the delocalised system. Instead, it undergoes substitution reactions where a hydrogen atom is replaced by an electrophile. An electrophile is an electron-pair acceptor, and the high electron density of the pi-system makes benzene susceptible to attack by strong electrophiles.

Nitration is a key reaction that introduces a nitro group (-NO₂) onto the benzene ring to form nitrobenzene. This is an important industrial process as nitrobenzene is a precursor for the synthesis of aniline, which is used to make dyes, pharmaceuticals, and polyurethane foams. The reaction requires a mixture of concentrated nitric and sulfuric acids, often called 'nitrating mixture', and is carefully controlled to prevent further nitration.

Reagents: Conc. HNO₃ + Conc. H₂SO₄ (catalyst) Conditions: Heat to 50-55°C Electrophile: NO₂⁺ (nitronium ion) Overall Equation: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

Benzene can be halogenated (e.g., with Cl₂ or Br₂) in the presence of a Lewis acid catalyst, also known as a halogen carrier. This is necessary because the benzene ring is not nucleophilic enough to polarise the non-polar halogen molecule on its own. This reaction is an electrophilic substitution, unlike the free-radical substitution seen with alkanes which requires UV light.

Reagents: Br₂ + FeBr₃ (or AlBr₃) catalyst Conditions: Room temperature, anhydrous Electrophile: Br⁺ (generated in situ) Overall Equation: C₆H₆ + Br₂ → C₆H₅Br + HBr

The halogen carrier, a Lewis acid, polarises the halogen molecule to generate the electrophile. For example: Br₂ + FeBr₃ → Br⁺ + [FeBr₄]⁻. The Br⁺ then attacks the benzene ring. The [FeBr₄]⁻ intermediate later reacts with the H⁺ eliminated from the ring to regenerate the catalyst: [FeBr₄]⁻ + H⁺ → FeBr₃ + HBr.

Friedel-Crafts reactions are methods of attaching substituents to an aromatic ring. There are two main types: alkylation, which adds an alkyl group, and acylation, which adds an acyl group. Both reactions use a strong Lewis acid catalyst, typically anhydrous aluminium chloride (AlCl₃), to generate the electrophile from a haloalkane or an acyl chloride.

Acylation Example: Formation of Phenylethanone Reagents: Acyl chloride (e.g., CH₃COCl) + AlCl₃ catalyst Conditions: Anhydrous, heat under reflux (approx. 60°C) Electrophile: Acylium ion (e.g., CH₃CO⁺) Overall Equation: C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl

While acylation is a reliable method for adding a carbonyl group, Friedel-Crafts alkylation has significant limitations. The alkyl group product is electron-donating, activating the ring and making it more susceptible to further substitution, leading to polyalkylation. Additionally, the carbocation intermediate can undergo rearrangement to form a more stable carbocation, leading to a mixture of isomeric products.