Directing Groups

In every EAS reaction, the electrophile (E⁺) attacks one carbon of the benzene ring, breaking one π bond and forming a carbocation intermediate called the arenium ion (or Wheland intermediate). This intermediate has three resonance structures. The position where the positive charge lands determines which resonance structures are drawn and whether they are stabilised or destabilised by the substituent already on the ring.

Groups such as −OH, −NH₂, −OR, and −NHR donate electrons by resonance directly into the ring. When the electrophile attacks the ortho or para position relative to these groups, one resonance structure of the arenium ion places the positive charge on the carbon bearing the substituent. The lone pair on the oxygen or nitrogen can then delocalise directly into that position, providing an extra resonance structure that stabilises the intermediate dramatically. This makes ortho/para attack faster and these products predominate. Because these groups increase the electron density of the ring above that of benzene, they are also activating — the ring reacts faster than benzene toward electrophiles. Examples: −OH (hydroxyl), −OCH₃ (methoxy), −NH₂ (amino), −NHCOCH₃ (acetamido).

Halogens (−F, −Cl, −Br, −I) are a special case: they are deactivating (inductively withdraw electrons through the σ bond due to high electronegativity) yet ortho/para directing (donate a lone pair by resonance when needed). The inductive effect dominates overall reactivity, making halogenated rings react more slowly than benzene. However, resonance donation still stabilises the intermediate preferentially at ortho/para positions over meta. This means halogenated rings give mostly ortho/para products, but the reaction requires harsher conditions than unsubstituted benzene.

Groups such as −NO₂, −CN, −COOH, −SO₃H, −CHO, −COR, and −CF₃ withdraw electrons from the ring by resonance and/or inductive effects. When the electrophile attacks ortho or para to these groups, one resonance structure places the positive charge on the carbon directly attached to the electron-withdrawing group — a highly destabilised arrangement since two electron-poor centres are adjacent. Meta attack avoids this unfavourable resonance structure entirely. The intermediate from meta attack is therefore the least destabilised (relatively most stable), so meta products predominate. Note: these groups are deactivating — the ring reacts much more slowly than benzene.

Activating power follows a rough order: −NR₂ > −NHR > −NH₂ > −OH > −OR > −NHCOR > phenyl > −F > −Cl > −Br > −I (halogens are the boundary, weakly deactivating) > −NO₂ > −CN > −COOH > −SO₃H. When two substituents are present, their directing effects are combined. If both groups direct to the same position, selectivity is high. If they conflict, the stronger activating group usually wins. Steric effects also reduce ortho product relative to para, since ortho attack is more hindered.

• \ce{C6H5-OH} — Phenol (—): The starting aromatic compound; the −OH group is a strong ortho/para activating director.
• \ce{Br2} — Bromine (—): The electrophilic reagent; acts as E⁺ after polarisation or activation by Lewis acid.
• \ce{FeBr3} — Iron(III) bromide (—): Lewis acid catalyst that polarises Br₂ to generate the electrophilic Br⁺ species (not needed for highly activated rings).
• \ce{HBr} — Hydrogen bromide (—): Byproduct formed when the proton is lost from the arenium intermediate to restore aromaticity.

Problem. Nitrobenzene undergoes sulfonation with fuming sulfuric acid (oleum). Predict the major product and explain your reasoning using directing group theory.

Answer. The major product is 3-nitrobenzenesulfonic acid. The −NO₂ group is a deactivating meta director; attack at the meta position gives the most stable arenium intermediate, and the reaction requires harsh conditions.

• Electrophilic Aromatic Substitution Mechanism — Directing groups only make sense once you understand the two-step EAS mechanism: electrophilic attack to form the arenium ion, followed by proton loss to restore aromaticity.
• Resonance and Aromaticity — The directing effect is entirely explained by resonance stabilisation of the arenium intermediate — you must be comfortable drawing resonance structures to predict regiochemistry.
• Inductive and Mesomeric Effects — Halogens are ortho/para directors yet deactivating because inductive withdrawal competes with resonance donation — understanding both effects is essential for the halogen special case.
• Friedel-Crafts Alkylation and Acylation — These key EAS reactions are strongly influenced by directing groups — for example, the acyl group (−COR) produced in acylation is a meta director, which is exploited in multi-step synthesis.
• Nitration of Aromatic Compounds — Nitration is the classic EAS reaction used to illustrate directing effects — the −NO₂ group introduced is the textbook example of a deactivating meta director for subsequent substitutions.