Electrophilic Aromatic Substitution (EAS)

A representative example — structures drawn live.

Why benzene substitutes instead of adds

Alkenes react with electrophiles by addition, because breaking one π bond costs little and the product keeps its stability. Benzene is different: its six π electrons are delocalized over the ring, giving roughly 36 kcal/mol of aromatic stabilization. Adding across the ring the way an alkene does would permanently destroy that aromaticity, which is far too costly. Instead benzene reacts by substitution — it trades one hydrogen for the electrophile and, crucially, regenerates the aromatic ring at the end. This is why the reaction class is called electrophilic aromatic substitution.

The general two-step mechanism

Every EAS reaction, no matter the electrophile, follows the same two-step core:

1. Attack on the electrophile (slow, rate-determining). Two of benzene's π electrons reach out and form a bond to the electrophile E⁺. This creates a positively charged, non-aromatic intermediate called the arenium ion (also the sigma complex or Wheland intermediate). The carbon bonded to E becomes sp³, and the positive charge is spread over the three remaining ring carbons by resonance.
2. Loss of a proton (fast). A base removes the H⁺ from the sp³ carbon. The electrons from that C–H bond drop back into the ring, restoring the full aromatic π system and releasing the neutral substituted product.

The first step is rate-determining because it temporarily sacrifices aromaticity. The arenium ion's stability — how well the ring can spread out that positive charge — controls how fast the reaction goes, and this is exactly what ring substituents modulate.

Generating the electrophile

Benzene is a weak nucleophile, so a run-of-the-mill electrophile will not react. Each EAS reaction therefore begins with a step that generates a very strong electrophile, usually with the help of a Lewis-acid or Brønsted-acid catalyst. The five reactions you are expected to know differ only in what electrophile is made:

• Halogenation. X₂ (Cl₂ or Br₂) with a Lewis-acid catalyst FeX₃ (or AlX₃). The catalyst polarizes the halogen to deliver X⁺. Product: a chloro- or bromobenzene.
• Nitration. HNO₃ with H₂SO₄. The sulfuric acid protonates and dehydrates nitric acid to make the nitronium ion, NO₂⁺. Product: nitrobenzene (a handy precursor to anilines).
• Sulfonation. SO₃ in H₂SO₄ (fuming sulfuric acid). The electrophile is SO₃ (or protonated SO₃H⁺). Product: a benzenesulfonic acid. Notably, sulfonation is reversible — hot dilute acid removes the –SO₃H group.
• Friedel–Crafts alkylation. R–X with AlCl₃ generates a carbocation R⁺. Product: an alkylbenzene. Watch for carbocation rearrangements and over-alkylation.
• Friedel–Crafts acylation. R–COCl with AlCl₃ generates a resonance-stabilized acylium ion. Product: an aryl ketone. Acylium ions do not rearrange, and the ketone product cannot be over-acylated, so acylation avoids the two big drawbacks of alkylation.

Limitations of Friedel–Crafts reactions

The two Friedel–Crafts reactions carry important restrictions. Alkylation suffers from carbocation rearrangements (a primary halide can rearrange to a more stable secondary or tertiary cation) and from polyalkylation (the product is more reactive than the starting material, so it keeps reacting). Both Friedel–Crafts reactions also fail on strongly deactivated rings — benzene bearing a nitro group, for instance — and fail if a basic amino group is present, because it ties up the AlCl₃ catalyst.

Substituents steer the next reaction

Once benzene already carries a substituent, that group controls both how fast a second EAS occurs and where the new group lands. Electron-donating groups stabilize the arenium ion, speeding the reaction (activators) and directing to the ortho and para positions. Electron-withdrawing groups destabilize the intermediate, slowing the reaction (deactivators) and usually directing to the meta position. The halogens are the well-known exception — deactivating yet ortho/para-directing. Understanding these directing effects is what lets you plan a multi-step synthesis of a specific disubstituted benzene.