Aromatic compounds, with benzene as the prime example, possess a unique stability derived from their delocalized π electron system. This stability profoundly influences their reactivity. Unlike alkenes, they resist addition reactions that would disrupt their aromaticity. Instead, their characteristic reactions are electrophilic substitution reactions, where an electrophile replaces a hydrogen atom on the aromatic ring, preserving the stable aromatic system.

General Mechanism (Two Steps): Step 1: Attack on the Aromatic Ring by an Electrophile (Rate-Determining Step)

• The highly delocalized π electrons of the benzene ring act as a nucleophile and are attracted to a strong electrophile (E+).
• A pair of π electrons breaks from the delocalized system to form a new σ bond with the electrophile. This breaks the aromaticity of the ring and generates a resonance-stabilized, non-aromatic carbocation intermediate. This intermediate is often called a sigma complex or arenium ion.

Step 2: Loss of a Proton and Regeneration of Aromaticity (Fast Step)

• The sigma complex is unstable because it has lost its aromaticity. A base (typically the conjugate base of the acid used, or a base generated by the catalyst, e.g., HSO4− from nitration, or FeBr4− from halogenation) abstracts a proton (H+) from the carbon that is bonded to the electrophile.
• The electrons from the C-H bond then move back into the ring, reforming the delocalized π electron system and restoring the aromaticity. This step is very fast because it is highly energetically favourable (regains stability).

Common Electrophilic Substitution Reactions and Their Electrophiles:

• Nitration: Introduction of a nitro group (-NO2 ).
• Reagents: Concentrated nitric acid (HNO3 ) and concentrated sulfuric acid (H2 SO4 ) (the latter acts as a catalyst to generate the electrophile).
• Electrophile: Nitronium ion (NO2+ ).
• Example: Benzene + HNO3 Conc. H2 SO4 ,50 ∘C → Nitrobenzene + H2 O
• Halogenation: Introduction of a halogen atom (-X).
• Reagents: Halogen (Cl2 or Br2 ) and a Lewis acid catalyst (e.g., iron(III) halide like FeCl3 or FeBr3 ).
• Electrophile: Polarized halogen, effectively X+ (e.g., Br+).
• Example: Benzene + Br2 → Bromobenzene + HBr
• Friedel-Crafts Alkylation: Introduction of an alkyl group (-R).
• Reagents: Alkyl halide (R-X) and a Lewis acid catalyst (e.g., AlCl3 ).
• Electrophile: Carbocation (R+).
• Example: Benzene + CH3 Cl + AlCl3 → Methylbenzene (Toluene) + HCl
• Friedel-Crafts Acylation: Introduction of an acyl group (-COR).
• Reagents: Acyl halide (RCO-X) or acid anhydride ((RCO)2 O) and a Lewis acid catalyst (e.g., AlCl3 ).
• Electrophile: Acylium ion (RCO+).
• Example: Benzene + CH3 COCl + AlCl3 → Phenylethanone (Acetophenone) + HCl

Significance: Electrophilic substitution is the single most important reaction type for functionalizing aromatic rings. It allows for the synthesis of a vast range of substituted aromatic compounds that are vital as starting materials in the pharmaceutical industry, for dyes, polymers, and many other fine chemicals. Its importance stems from the fact that it allows for chemical transformations while maintaining the unique and highly stable aromatic character of the ring.