Electrophilic Aromatic Substitution General — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Electrophilic Aromatic Substitution (General) Electrophilic Aromatic Substitution (EAS) is an organic reaction where an atom, typically hydrogen, attached to an aromatic ring is substituted by an electrophile. This reaction is a cornerstone for synthesizing a vast array of substituted aromatic compounds. Since 'Electrophilic Aromatic Substitution (General)' encompasses many specific reactions, universal visual observations are limited. However, common observations for specific reactions can include: heat evolution (exothermic reactions), formation of a colored product, precipitation of a solid byproduct, or a change in the clarity/viscosity of the reaction mixture as new products form. Most electrophilic aromatic substitution reactions are exothermic, releasing heat. The stability gained from restoring aromaticity in the final step makes these reactions thermodynamically favorable. 1. Generation of the Electrophile (E+): The electrophile is typically generated in situ from a reagent and a Lewis acid catalyst. 2. Attack of the Aromatic Ring: The pi-electrons of the aromatic ring act as a nucleophile, attacking the electrophile. This forms a sigma complex (arenium ion), a resonance-stabilized carbocation, which temporarily breaks the aromaticity of the ring. 3. Deprotonation and Restoration of Aromaticity: A base (often the conjugate base of the Lewis acid or another species in the reaction mixture) abstracts a proton from the carbon bearing the electrophile in the sigma complex. This restores the aromaticity of the ring, yielding the substituted product. Carbocation Rearrangements in Friedel-Crafts Alkylation: Primary carbocations can rearrange to more stable secondary or tertiary carbocations, leading to unexpected isomeric products. Limitations of Friedel-Crafts Alkylation/Acylation: Highly deactivated aromatic rings (e.g., nitrobenzene) and rings with strongly electron-withdrawing groups (e.g., carboxylic acids, ketones) do not undergo Friedel-Crafts reactions. Directing Effects: Misidentifying whether a substituent is an ortho/para director (activator or weak deactivator) or a meta director (deactivator) is a common error. Polysubstitution in Friedel-Crafts Alkylation: The alkylated product is often more activated than the starting material, leading to further alkylation and a mixture of products. Friedel-Crafts acylation avoids this as the acyl group is deactivating. Reversibility of Sulfonation: Benzenesulfonic acid formation is reversible; heating with dilute acid can lead to desulfonation.