Chemical Properties of Benzene
Master the reactivity of the aromatic ring. Decode the general EAS mechanism, the generation of specific electrophiles, and the classic Friedel-Crafts rearrangement traps.
Module Focus: Why Substitution over Addition?
Benzene has three $\pi$ bonds and is highly unsaturated, yet it resists addition reactions under normal conditions. Addition would permanently destroy the delocalized $\pi$ ring, resulting in the loss of its massive Resonance Energy (150 kJ/mol). Instead, benzene strictly undergoes Electrophilic Aromatic Substitution (EAS), where a hydrogen atom is replaced by an electrophile, allowing the highly stable aromatic ring to be completely restored at the end.
1. The General EAS Mechanism
Every classic reaction of benzene (Halogenation, Nitration, Sulfonation, Friedel-Crafts) follows this exact three-step pathway.
In Step 1, the electrophile attaches to one of the carbons, changing its hybridization from $sp^2$ to $sp^3$.
Because one carbon is now $sp^3$ (tetrahedral), the continuous cyclic $p$-orbital overlap is broken. Therefore, the Sigma Complex is NON-AROMATIC. It regains aromaticity only after throwing out the $H^+$ ion in Step 3.
2. The 5 Core EAS Reactions
To master benzene reactions, you must memorize the reagents used and the specific electrophile generated.
| Reaction Name | Reagents | Electrophile ($E^+$) | Product Formed |
|---|---|---|---|
| Halogenation | $Cl_2$ or $Br_2$ + Anhydrous $AlCl_3$ or $FeBr_3$ | $Cl^+$ or $Br^+$ (Chloronium/Bromonium ion) |
Chlorobenzene |
| Nitration | Conc. $HNO_3$ + Conc. $H_2SO_4$ (Nitrating Mixture) |
$NO_2^+$ (Nitronium ion) |
Nitrobenzene |
| Sulfonation | Fuming $H_2SO_4$ (Oleum) | $SO_3$ (NEET Trap: It is Neutral!) |
Benzenesulfonic acid |
| F.C. Alkylation | Alkyl Halide ($R-X$) + Anhydrous $AlCl_3$ | $R^+$ (Carbocation) |
Alkylbenzene |
| F.C. Acylation | Acyl Halide ($R-COCl$) + Anhydrous $AlCl_3$ | $R-CO^+$ (Acylium ion) |
Acetophenone / Ketone |
In the mixture of Conc. $HNO_3$ and Conc. $H_2SO_4$, which one acts as the acid?
$H_2SO_4$ is the stronger acid. Therefore, it forces $HNO_3$ to act as a BASE (it accepts a proton to form $H_2O^+-NO_2$, which then loses water to generate the $NO_2^+$ electrophile).
3. The Friedel-Crafts Alkylation Trap
Because Friedel-Crafts Alkylation generates a true Carbocation ($R^+$) as the electrophile, it is heavily susceptible to carbocation rearrangements (1,2-hydride or 1,2-alkyl shifts) to form a more stable intermediate before attacking the benzene ring.
Reaction of Benzene with n-Propyl Chloride does NOT yield n-Propylbenzene.
4. Addition Reactions (Breaking Aromaticity)
Under extreme conditions, benzene can be forced to undergo addition, completely losing its aromatic character.
Requires high temperature and pressure with a Nickel catalyst to break the resonance energy.
In the presence of excess chlorine and UV light ($h\nu$) at 500K, addition occurs via a free-radical mechanism.
BHC (Benzene hexachloride) / Gammexane
NEET Grand Test: Benzene Reactions
15 High-Yield Questions testing electrophile identification, rearrangement traps, and BHC formation.
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