Electrophilic Aromatic Substitution (EAS)

Quick answer In EAS, benzene's π cloud attacks a strong electrophile E⁺ to form a resonance-stabilized arenium ion, which then loses H⁺ to restore aromaticity — so one ring H is swapped for E. Five classic reactions all run through this same pathway.

⚙ Mechanism · Electrophilic Aromatic Substitution2 steps

Step 1 — the ring attacks the electrophile.

The aromatic π system attacks the electrophile, giving a resonance-stabilised arenium ion (σ complex). Aromaticity is temporarily broken and one carbon becomes sp³, bearing both H and E. This is the slow, rate-determining step.

Step 2 — lose a proton to re-aromatise.

A base removes the proton from the sp³ carbon; those electrons restore the aromatic sextet. Because the ring is regenerated, the overall result is substitution, not addition.

Benzene (starting material)

Bromobenzene (halogenation)

Nitrobenzene (nitration)

Acetophenone (acylation)

Same ring every time; only the incoming group changes.

1. EAS Replaces One Ring Hydrogen With an Electrophile

A strong electrophile takes the place of one ring hydrogen, and the aromatic ring is left intact.

2. Benzene Substitutes Instead of Adds to Preserve Aromaticity

Adding across the ring like an alkene would destroy benzene's ~36 kcal/mol of aromatic stabilization, so it trades one H for E and regenerates the aromatic ring instead.

The model EAS reaction: one H exchanged for Br, ring intact.

3. The Arenium (Sigma-Complex) Intermediate Is the Key Step

Every EAS reaction runs through the same three steps:

Generate the electrophile. A catalyst builds a very strong E⁺.
The ring attacks — slow, rate-determining. π electrons bond to E⁺, giving the arenium ion: one sp³ carbon, + charge spread over three carbons.
Lose H⁺ — fast. A base removes the proton and aromaticity is restored, releasing the product.

Benzene's aromatic π system reaches out toward the electrophile E⁺.

The arenium ion (sigma complex): one carbon is now sp³ and the + charge is spread over three ring carbons.

Loss of H⁺ from the sp³ carbon restores aromaticity and gives the product.

The arenium ion: a cyclohexadienyl cation with charge shared over three carbons.

4. Every EAS Reaction Starts by Generating a Strong Electrophile

Benzene is a weak nucleophile, so the five reactions differ only in how they build a strong electrophile:

Halogenation. Br₂/FeBr₃ (or Cl₂/AlCl₃) delivers Br⁺ → halobenzene.
Nitration. HNO₃/H₂SO₄ builds the nitronium ion, NO₂⁺ → nitrobenzene.
Sulfonation. SO₃ in fuming H₂SO₄ → benzenesulfonic acid; uniquely reversible.
Friedel–Crafts alkylation. R–Cl/AlCl₃ makes R⁺ → alkylbenzene; beware rearrangements and polyalkylation.
Friedel–Crafts acylation. R–COCl/AlCl₃ makes a resonance-stabilized acylium ion → aryl ketone; no rearrangement or over-reaction.

5. The Five Core Reactions All Run Through the Arenium Ion

Once E⁺ exists, steps 2 and 3 are identical for all five — only the electrophile changes.

Nitration: the nitronium ion NO₂⁺ is the electrophile, giving nitrobenzene.

Sulfonation gives benzenesulfonic acid — and it is reversible.

Friedel–Crafts acylation installs a C=O–bearing group through a non-rearranging acylium ion.

All five products: one benzene ring, one new group each.

Bromobenzene

Nitrobenzene

Benzenesulfonic acid

Ethylbenzene (alkylation)

Acetophenone (acylation)

The five core products. Note: Friedel–Crafts fails on strongly deactivated rings and on basic amines that tie up AlCl₃.

6. Substituents Steer the Rate and Position of the Next Substitution

Electron-donors activate the ring and direct ortho/para; electron-withdrawers deactivate and direct meta; halogens are the oddball — deactivating yet still ortho/para.

Toluene — activator, o/p-director

Nitrobenzene — deactivator, meta-director

Chlorobenzene — deactivator but o/p-director

The group already on the ring decides the next reaction.

7. Summary

Swap one aromatic C–H for E, ring stays aromatic · always three steps: make E⁺ · form the arenium ion (slow, rate-determining) · lose H⁺ to rearomatize · the five reactions just differ in how E⁺ is made · existing groups set the next reaction's rate and regiochemistry.

Worked example

Problem. What is the product of benzene + HNO₃ / H₂SO₄?

The mixed acids generate the electrophile nitronium, NO₂⁺.
Benzene attacks it to form the arenium (σ) complex — the slow step, breaking aromaticity.
Loss of the ring proton re-aromatises the ring.

Answer. Nitrobenzene. Only one substitution — the now-deactivated ring resists a second nitration.

Quiz yourself

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Adding across the ring like an alkene would destroy benzene's ~36 kcal/mol of aromatic stabilization. Substitution lets the ring lose only one H and then re-form the aromatic π system, so it keeps that stabilization.

It is the positively charged, non-aromatic intermediate (sigma complex / Wheland intermediate) formed when the ring's π electrons attack the electrophile. That attack is the slow, rate-determining step, because it temporarily breaks aromaticity.

HNO₃ with H₂SO₄. The sulfuric acid protonates and dehydrates nitric acid to generate the nitronium ion, NO₂⁺, which is the electrophile that attacks benzene.

The acylium ion does not rearrange (unlike the carbocation in alkylation), and the ketone product is deactivated, so it will not over-react. Alkylation suffers from both carbocation rearrangements and polyalkylation.

Draw this on the whiteboard

Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.

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