Halogenation Electrophilic Aromatic Substitution — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Halogenation (Electrophilic Aromatic Substitution) Halogenation by Electrophilic Aromatic Substitution (EAS) is a reaction where a hydrogen atom on an aromatic ring is replaced by a halogen atom through the attack of an electrophilic halogen species, typically catalyzed by a Lewis acid. Typically, the pure reactants are a colorless liquid (benzene) and a colored gas/liquid (e.g., yellowish-green Cl2, reddish-brown Br2). During the reaction, the color of the halogen might fade. Evolution of a colorless acidic gas (HCl or HBr) can be observed, which turns moist blue litmus paper red. The product (aryl halide) is usually a colorless liquid. Generally exothermic due to the formation of a strong C-X bond and the restoration of aromatic stability after the initial electrophilic attack. Fluorination is extremely exothermic. Generation of the electrophile: A Lewis acid (e.g., FeCl3, FeBr3) reacts with the halogen molecule (X2) to form a highly electrophilic species (e.g., Cl+FeCl4- or Br+FeBr4-). For iodination, an oxidizing agent is needed to generate I+. For fluorination, specific conditions or complexes may be required. Electrophilic attack: The electron-rich aromatic ring acts as a nucleophile and attacks the electrophilic halogen. This forms a resonance-stabilized carbocationic intermediate called the sigma complex (or arenium ion), temporarily breaking the aromaticity. Deprotonation and rearomatization: A base (e.g., the Lewis acid anion, FeX4-, or solvent) abstracts a proton from the carbon bearing the halogen in the sigma complex. This restores the aromaticity of the ring and regenerates the Lewis acid catalyst. Forgetting the Lewis acid catalyst: Direct halogenation of benzene (Cl2, Br2) without a catalyst typically does not occur via EAS; UV light/heat would promote free radical substitution on alkyl side chains. Misunderstanding iodination: I2 is not electrophilic enough for direct EAS; it requires an oxidizing agent (e.g., HNO3, HIO3) to generate I+. Underestimating fluorination: F2 reacts violently with benzene; direct fluorination via EAS is challenging and often requires special, carefully controlled conditions or indirect methods. Ignoring polyhalogenation: With excess halogen and catalyst, especially with activating groups present, polysubstitution can occur. Confusing EAS with Free Radical Halogenation: Free radical halogenation (e.g., with UV light) occurs on alkyl side chains, not the aromatic ring itself.