Study Guides › Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS)

The question this page answers: how do the C=C bonds in aromatic rings react with electrophiles — and why do substituents already on the ring decide where the new group ends up?

Deeper reading: Clayden, Organic Chemistry 2e, ch. 21 (pp. 473–496) and ch. 24 (pp. 562–567) — see our chapter-by-chapter practice map for Clayden.

Why benzene doesn't react like an alkene

Why won't benzene add Br2 the way an alkene does?

The C=C π bonds in aromatic rings are far less reactive than ordinary alkenes because they are stabilized by aromatic delocalization.

Tetramethylethylene reacts with Br2 to give the dibromide, while benzene shows no reaction with Br2 alone
An ordinary alkene adds Br2 readily; benzene, under the same conditions, does nothing.

Losing aromaticity is expensive, so an aromatic π system will not attack a run-of-the-mill electrophile. To get a reaction, you need a much stronger electrophile — and that requirement is what defines this whole family of chemistry.

The five classic EAS reactions

What are the five classic EAS reactions?

Each named EAS reaction is just a different recipe for generating a strong electrophile.

Table of the five EAS reaction types with general schemes and electrophiles: nitration (HNO3/H2SO4, NO2+), sulfonation (SO3/H2SO4, SO3H+), halogenation (X2/Lewis acid), Friedel-Crafts alkylation (RX/Lewis acid, R+), and Friedel-Crafts acylation (acyl chloride/Lewis acid, acylium ion)
The five common EAS reactions and the strong electrophile each one generates in situ.

One mechanism to rule them all

What mechanism do all EAS reactions share?

Every EAS reaction has the same two steps: (1) addition of the electrophile to a C=C bond to form an arenium ion intermediate, then (2) a β-elimination that re-aromatizes the ring.

Generic EAS mechanism: benzene attacks E+ in an addition step to form the arenium ion, then a base removes the proton in an elimination step that restores aromaticity
The generic EAS mechanism. The formation of E+ itself varies by reaction, but everything after it is the same.

Notice what makes this a substitution overall: the ring temporarily gives up aromaticity in the addition step, then gets it back by kicking out a proton rather than adding a nucleophile. The arenium ion (also called a sigma complex or Wheland intermediate) is the key species — anything that stabilizes it speeds up the reaction.

Directing effects: where does the new group go?

Where does the new group end up — and why?

Groups already on the ring do two things: they direct the electrophile to particular positions, and they activate or deactivate the ring by donating or withdrawing electron density.

Benzene ring with substituent G labeling the ipso, ortho, meta, and para positions
Position nomenclature relative to a substituent G. Since EAS replaces an H, ipso attack is not relevant here.

Directing groups come in exactly two flavors: ortho/para directors and meta directors. Here is nitration of anisole showing that –OMe is an ortho/para director:

Nitration of anisole with HNO3/H2SO4 gives 44% ortho, less than 1% meta, and 55% para nitroanisole
–OMe sends the incoming nitro group ortho and para; the meta product barely forms.

And nitration of nitrobenzene showing that –NO2 is a meta director:

Nitration of nitrobenzene gives 6% ortho, 93% meta, and less than 1% para dinitrobenzene
–NO2 directs the second nitration almost entirely meta.

The rate side of the story: electron-donating groups (EDGs) increase electron density in the π system and make the ring more reactive; electron-withdrawing groups (EWGs) do the opposite.

Relative nitration rates: methyl benzoate 0.004, benzene 1.0, toluene 24.5
Relative nitration rates: an ester-substituted ring reacts ~250× slower than benzene, while toluene reacts ~25× faster.

Taken together, the rules compress to three lines:

Multiple substituents: balancing the effects

What if two directing groups disagree?

With more than one group on the ring, sum the individual effects. When groups disagree, the reaction (1) avoids sterically crowded positions and (2) follows the group whose lone pair best stabilizes the arenium intermediate.

When the directors cooperate, the outcome is clean:

Nitration of meta-dichlorobenzene occurs ortho/para to the chlorines but avoids the position between the two Cl groups
Cooperative directing: both chlorines agree, and the crowded position between them is avoided.

When they disagree, the stronger director wins:

Nitration of 2-chlorotoluene: the chlorine lone pairs do the directing, and para is chosen over ortho
Non-cooperative directing: Cl (lone-pair donor) outranks the methyl group, and para beats ortho on sterics.

Friedel-Crafts reactions: powerful but tricky

Why does Friedel-Crafts alkylation misbehave — and what's the fix?

Friedel-Crafts reactions are prized because they make C–C bonds — but alkylation has two famous failure modes, and acylation is the workaround.

Problem one: alkylations go through carbocation-like intermediates, so they rearrange:

Friedel-Crafts alkylation of benzene with 3-methyl-1-butanol and BF3 gives the rearranged tert-amylbenzene in 81% yield
The primary alcohol ionizes and hydride-shifts to the more stable tertiary cation before the ring attacks.

Problem two: each alkyl group you install activates the ring, so the product outcompetes the starting material and you over-alkylate:

Friedel-Crafts alkylation of benzene with excess MeCl and AlCl3 gives hexamethylbenzene in 100% yield
With excess MeCl, methylation doesn't stop — it runs all the way to hexamethylbenzene.

The fix for both: acylate instead, then reduce. The acylium ion doesn't rearrange, and the ketone product is deactivated toward further EAS:

Friedel-Crafts acylation of benzene with propanoyl chloride and AlCl3 gives propiophenone, which is reduced to propylbenzene by Zn/Hg/HCl (Clemmensen) or NH2NH2/NaOH (Wolff-Kishner)
Acylation then carbonyl reduction (Clemmensen or Wolff–Kishner) delivers the clean mono-alkylated product.

More Practice for this Topic

Reading about directing effects is one thing — predicting real products is another. Try it:

Spotted an error, or want a topic covered next? Let us know.