Aromatic chemistry · 9 min read

Aromatic substitution: EAS, directing effects and SNAr

Aromatic rings are unusually stable, so they do not undergo addition like simple alkenes. They react by substitution, keeping the ring, and there are two main families: electrophilic substitution on electron-rich rings and nucleophilic substitution on electron-poor ones.

Electrophilic aromatic substitution (EAS)

The same two-step pattern applies to nitration, halogenation, sulfonation, Friedel–Crafts alkylation and acylation. The general mechanism is on the Bromination of benzene (FeBr₃-catalyzed) page.

Attack. A π electron pair of the ring attacks a strong electrophile (E⁺). This is the slow, rate-determining step because it temporarily destroys aromaticity and gives a delocalised carbocation called the arenium ion (or sigma complex or Wheland intermediate).
Loss of H⁺. A weak base removes the proton on the carbon that bears E. The electron pair from the C–H bond returns to the ring and restores aromaticity. This step is fast.

Substitution, not addition, is the outcome because loss of H⁺ regenerates a very stable aromatic ring. Adding a nucleophile to the arenium ion would leave the ring non-aromatic.

Generating the electrophile

Nitration: HNO₃ with H₂SO₄ gives the nitronium ion NO₂⁺. See Nitration of benzene.
Halogenation: Br₂ or Cl₂ with a Lewis acid (FeBr₃, AlCl₃) polarises the halogen so that one atom becomes strongly electrophilic.
Sulfonation: SO₃ in H₂SO₄. This step is reversible, which is why sulfonic acid groups are used as temporary blocking groups. See Sulfonation of benzene.
Friedel–Crafts alkylation: an alkyl halide and AlCl₃ give a carbocation or a polarised complex. Rearrangements and over-alkylation are the usual problems; see Friedel-Crafts alkylation of benzene.
Friedel–Crafts acylation: an acyl chloride and AlCl₃ give an acylium ion, which does not rearrange. The ketone product is deactivated, so it stops after one substitution: see Friedel-Crafts acylation (contrast with alkylation).

Directing effects: where does the second group go?

A substituent already on the ring changes both the rate (activating or deactivating) and the position (ortho/para or meta) of the next substitution. You can predict both by drawing the resonance forms of the arenium ion.

Type	Examples	Effect on rate	Directs to
Strong donors (lone pair on the ring atom)	–NH₂, –NR₂, –OH, –OR	strongly activating	ortho / para
Alkyl and aryl groups	–CH₃, –R, –Ph	weakly activating	ortho / para
Halogens	–F, –Cl, –Br, –I	deactivating	ortho / para
Carbonyl and related π-acceptors	–CHO, –COR, –CO₂R, –CN, –SO₃H	deactivating	meta
Strong acceptors	–NO₂, –NR₃⁺, –CF₃	strongly deactivating	meta

Why donors direct ortho and para

When the electrophile attacks the ortho or para position relative to a donor such as –OMe, one resonance form of the arenium ion places the positive charge on the carbon that bears the donor. The oxygen lone pair can then stabilise it, giving a form in which every atom has an octet. At the meta position that extra form does not exist, so meta attack is slower.

Why acceptors direct meta

For a group such as –NO₂ or –C=O, ortho and para attack put the positive charge on the carbon bearing the acceptor, which is the worst possible place. Meta attack avoids that, so it is "least bad" and the ring reacts slowly but at the meta position.

Why halogens are the odd case

Halogens withdraw electrons inductively (so the ring is deactivated) but donate a lone pair by resonance (so ortho/para attack is still favoured over meta). The two effects pull in different directions, giving a slow reaction with ortho/para selectivity.

Practical points for synthesis

Order of steps matters. To make m-bromonitrobenzene, nitrate first and brominate second. To make p-bromonitrobenzene, brominate first and nitrate second.
Strongly activated rings over-react. Aniline brominates three times in water. Converting –NH₂ to an amide (–NHCOCH₃) moderates the ring, and the amide is removed afterwards.
Strongly deactivated rings do not undergo Friedel–Crafts. Nitrobenzene is so electron-poor that it is used as a solvent for the reaction.
Sterics decide ortho vs para. With bulky groups, para is usually favoured.

Nucleophilic aromatic substitution (SNAr)

If the ring is electron-poor, it can be attacked by a nucleophile instead. The classic requirements are:

a good leaving group (usually a halide) on the ring, and
one or more strong electron-withdrawing groups, such as –NO₂, ortho or para to the leaving group.

The mechanism, shown in SNAr on 1-fluoro-2,4-dinitrobenzene, is again two steps. The nucleophile adds to the carbon bearing the leaving group, giving an anionic Meisenheimer complex whose charge is delocalised onto the nitro group. Then the leaving group departs and the ring becomes aromatic again. The first step is rate-determining, which is why fluoride is the best leaving group in SNAr, the opposite of SN2: the very electronegative fluorine makes the carbon more electrophilic and stabilises the intermediate, and the C–F bond is not broken until the fast second step.

The nitro group must be ortho or para, not meta, because only then can the negative charge be placed on the carbon that carries it (and onto its oxygen atoms). A meta nitro group cannot stabilise the intermediate by resonance.

Related ways to modify an aromatic ring

Diazonium chemistry: an aniline can be turned into a diazonium salt (Diazotization of aniline) and then into halides, nitriles or phenols with the Sandmeyer reaction.
Directed metalation: a directing group lets an organolithium remove the adjacent ortho proton; see directed C–H functionalisation.
Cross-coupling: palladium catalysis joins aryl halides to many partners, as in Suzuki coupling and Buchwald–Hartwig amination. See the cross-coupling guide.

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