Electrophilic Substitution Reactions
The characteristic reactions of Benzene and its derivatives.
By chemca Team • Updated Jan 2026
Aromatic compounds undergo Electrophilic Aromatic Substitution (EAS) because the $\pi$-electron cloud above and below the benzene ring attracts electrophiles ($E^+$), while the ring structure is retained to preserve aromaticity.
1. General Mechanism
Three Step Process
- Generation of Electrophile ($E^+$): The reagent reacts with a catalyst (usually a Lewis Acid) to produce a strong electrophile.
- Formation of Carbocation (RDS): The electrophile attacks the aromatic ring to form a resonance-stabilized non-aromatic cation called the Arenium Ion or Sigma ($\sigma$) Complex. The carbon attacked becomes $sp^3$ hybridized.
- Removal of Proton: A base removes a proton from the $sp^3$ carbon to restore aromaticity.
$$ C_6H_6 + E^+ \xrightarrow{\text{Slow}} [C_6H_6E]^+ \xrightarrow{\text{Base, Fast}} C_6H_5E + H^+ $$
2. Key EAS Reactions
A. Nitration
Reagent: Conc. $HNO_3$ + Conc. $H_2SO_4$ (Nitrating Mixture).
$$ C_6H_6 + HNO_3 \xrightarrow{H_2SO_4, 323K} \underset{\text{Nitrobenzene}}{C_6H_5NO_2} + H_2O $$
Electrophile: Nitronium Ion ($NO_2^+$) generated by protonation of nitric acid by sulfuric acid.
B. Halogenation
Reagent: $Cl_2$ or $Br_2$ + Lewis Acid ($FeCl_3, AlCl_3$).
$$ C_6H_6 + Cl_2 \xrightarrow{Anhyd. AlCl_3} C_6H_5Cl + HCl $$
Electrophile: Chloronium Ion ($Cl^+$) or Bromonium Ion ($Br^+$).
C. Sulphonation
Reagent: Fuming Sulphuric Acid (Oleum, $H_2S_2O_7$) or Conc. $H_2SO_4$.
$$ C_6H_6 + H_2SO_4 (SO_3) \xrightarrow{\Delta} C_6H_5SO_3H $$
Electrophile: Sulphur Trioxide ($SO_3$), a neutral electrophile.
3. Friedel-Crafts Reactions
Alkylation & Acylation
Catalyst: Anhydrous $AlCl_3$.
A. Alkylation: Reagent is Alkyl Halide ($R-X$).
$$ C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} \underset{\text{Toluene}}{C_6H_5-CH_3} + HCl $$
Electrophile: Carbocation ($R^+$).
Drawback: Carbocations can rearrange (e.g., Propyl $\to$ Isopropyl), and polyalkylation can occur.
Drawback: Carbocations can rearrange (e.g., Propyl $\to$ Isopropyl), and polyalkylation can occur.
B. Acylation: Reagent is Acyl Chloride ($R-COCl$) or Anhydride.
$$ C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} \underset{\text{Acetophenone}}{C_6H_5-COCH_3} + HCl $$
Electrophile: Acylium Ion ($R-C^+=O$). No rearrangement occurs.
4. Effect of Substituents
Activators vs Deactivators
| Group Type | Examples | Effect | Directive Influence |
|---|---|---|---|
| Activating | $-OH, -NH_2, -OR, -CH_3$ | Increases Rate ($+R, +I$) | Ortho / Para |
| Deactivating | $-NO_2, -CN, -COOH, -CHO$ | Decreases Rate ($-R, -I$) | Meta |
| Halogens | $-F, -Cl, -Br, -I$ | Decreases Rate ($-I > +R$) | Ortho / Para |
Knowledge Check
Test your understanding of EAS Reactions
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