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Benzene's stability arises from the delocalised pi-system - all six electrons contribute to a continuous ring above and below the hexagon. Addition would destroy this arrangement: adding Br2 across a C=C would convert two sp2 carbons to sp3, breaking the pi-system. The resulting cyclohexadiene would be much less stable than benzene plus HBr or Br2.
Substitution, on the other hand, replaces one of the six H atoms with a new group (Nitro, Cl, Br etc.) while keeping the ring intact. The pi-system is momentarily disrupted but reformed in the final product, so the aromatic stability is preserved.
| Reaction type | Example | What happens to benzene ring? |
|---|---|---|
| Addition | C6H6 + Br2 -> C6H6Br2 | Destroys aromatic ring (does not occur) |
| Substitution | C6H6 + Br2 -> C6H5Br + HBr | Preserves aromatic ring (this is what happens) |
This principle governs almost all benzene chemistry: electrophilic substitution is the major reaction type, not electrophilic addition.
Electrophilic substitution has three steps that repeat across all reactions:
The electrophile (E+) is formed from the reagent, often using a catalyst to make a strong enough electrophile. Examples:
The pi-electrons of benzene attack the electrophile. A pair of electrons leaves the delocalised system and forms a new covalent C-E bond. This creates a positively charged intermediate in which the ring has partial delocalisation around the remaining five carbons. This intermediate is sometimes drawn as a Wheland intermediate or sigma-complex:
E
/
C+ <-- loses aromatic sextet; carbocation delocalised over remaining ring
/ \
| |
\ /
The ring is now NOT aromatic - it has lost 2 of its 6 pi electrons to the new C-E bond. This intermediate is high in energy and wants to return to the aromatic form.
The carbon now carrying E also carries an H (from the original C-H of benzene). This H is lost as H+, with both electrons in the C-H bond returning to the ring to restore the aromatic pi-system. The final product is a substituted benzene (E replaces H).
E E
| |
C -- H+ --> C
/ \ / \
| | | |
\ / \ /
(no longer aromatic) (aromatic again)
The overall result: one H on benzene replaced by E, with the ring still aromatic.
graph LR
A[Benzene C6H6<br/>aromatic] --> B[Electrophile E+<br/>formed from catalyst]
B --> C[Attack on ring<br/>sigma complex<br/>NOT aromatic]
C --> D[Loss of H+<br/>to regenerate catalyst<br/>and aromatic ring]
D --> E[Substituted benzene C6H5E]
Nitration is the introduction of a nitro group -NO2 onto the benzene ring. The electrophile is the nitronium ion, NO2+.
Step 1: Generate NO2+
Sulfuric acid is a stronger acid than nitric acid, so it protonates HNO3:
HNO3 + H2SO4 -> H2NO3+ + HSO4-
The protonated nitric acid then loses water to form the nitronium ion:
H2NO3+ -> NO2+ + H2O
Combined: 2 H2SO4 + HNO3 -> NO2+ + H3O+ + 2 HSO4-
The nitronium ion NO2+ is a linear cation (O=N+=O) and is a very strong electrophile because of the positive charge on nitrogen.
Step 2: Attack on Benzene
The benzene ring attacks NO2+:
C6H6 + NO2+ -> C6H5(H)(NO2)+ (sigma complex)
Step 3: Loss of H+
C6H5(H)(NO2)+ -> C6H5NO2 + H+
The H+ is recaptured by HSO4- to regenerate H2SO4 (acid catalyst).
C6H6 + HNO3 -> C6H5NO2 + H2O (with H2SO4 catalyst and 50 degrees C)
Product: nitrobenzene, a pale yellow oil with a characteristic almond smell.
Nitrobenzene is a key industrial intermediate. It can be reduced (using tin and HCl, or H2/Ni, or Sn/H+) to give phenylamine (aniline):
C6H5NO2 + 6 [H] -> C6H5NH2 + 2 H2O
Phenylamine is the starting material for dye manufacture (e.g. azo dyes) and for many pharmaceuticals. Nitration is also the first step in the industrial production of TNT (2,4,6-trinitromethylbenzene, an explosive).
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