Substituent Effects in Electrophilic Aromatic Substitution
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Direct answer
Nitrate toluene and you get roughly 58% ortho, 4% meta and 38% para nitrotoluene; nitrate nitrobenzene and 93% of the product is meta. The substituent already on the ring decides both speed and position. Electron-donating groups (-OH, -NH2, -OR, -NHCOR, alkyl) activate the ring and direct ortho/para; electron-withdrawing groups (-NO2, -CN, -CHO, -COOH, -SO3H, -NR3+) deactivate and direct meta. Halogens break the pattern memorably: they deactivate (strong inductive withdrawal) yet direct ortho/para (weak resonance donation) — the anomaly every paper re-tests. Both position and rate follow from one question: how does the substituent stabilise the arenium-ion intermediate at each possible attack site?
What you must remember
- Activating order: -NH2 > -OH > -OR > -NHCOR > alkyl > H; deactivators run halogen > -CHO ≈ -COR ≈ -COOH ≈ -COOR > -CN > -SO3H > -NO2 > -NR3+.
- Director rule: every ortho/para director is an activator or a halogen; every meta director is a deactivator — no exceptions.
- Toluene nitration data: about 58% ortho, 4% meta, 38% para; nitrobenzene nitration needs fuming acid and heat and gives about 93% meta — the two reference experiments.
- Why aniline fails Friedel-Crafts: the nitrogen lone pair complexes with AlCl3, wrecking catalysis and deactivating the ring — acetanilide, with the lone pair partly tied up, nitrates cleanly instead.
- Steric bias: bulky rings or electrophiles push ortho/para ratios toward para; sulfonation of toluene at high temperature is the classic reversible, para-favouring case.
- Two-substituent logic: the stronger activator governs; if directives conflict, the activating group wins because it accelerates its preferred sites.
- Blocking strategy: sulfonate to occupy the para position, carry out the next substitution, then steam out the SO3H group — a synthesis trick worth one question every few years.
Reasoning with the arenium ion
Attack of an electrophile on anisole: ortho or para attack generates a sigma complex with a fourth resonance form in which oxygen donates its lone pair and bears the positive charge — four ways to spread the charge. Meta attack offers only three. The extra stabilisation sends anisole's nitration mostly ortho/para and fast. Now nitrobenzene: ortho or para attack places a resonance form with positive charge on the carbon bearing NO2, adjacent to the already positive nitrogen — electrostatic punishment. Meta attack never touches that carbon, so meta wins by default, slowly. One framework, both answers.
For chlorobenzene, run both currencies. Chlorine's inductive pull drains the ring overall (deactivation — slower than benzene), but when the cation does form, the lone pair resonance again supplies an extra stabilising form only for ortho/para attack. Weak donation competes with strong withdrawal, so the rate stays low while the directing sense stays ortho/para. The halogen anomaly is not a contradiction; it is two effects measured on different scales.
How the exam frames it
The workhorse question names one or two substituents and asks for the major product — decide activator strength, decide the directing site, apply sterics for para when ortho is crowded. The traps: CF3 and CCl3 direct meta (pure inductive withdrawal, no resonance); phenol brominates in water without any Lewis acid (so strongly activated) but needs the usual conditions once you acetylate it; Friedel-Crafts fails outright on rings bearing meta directors or free amino groups, and nitrobenzene consequently serves as an inert solvent for the reaction. JEE Advanced adds synthesis logic — reaching a meta product from a para-directing starting ring requires changing the substituent class mid-route (for example, reducing a nitro group after using its meta direction), which is exactly the multi-step planning the paper rewards.
Frequently asked questions
Why do halogens direct ortho/para despite deactivating the ring?
Their lone-pair resonance stabilises the ortho/para sigma complex with an extra contributing structure, while their inductive withdrawal still slows the overall rate — deactivating director, ortho/para sense.
Which groups are the strongest activators in EAS?
-NH2, -NHR, -NR2 and -OH, because their lone pairs donate directly into the ring by resonance; acylation of the amine (-NHCOR) tones this down.
Why does nitrobenzene nitration give mostly meta product?
Ortho/para attack generates a resonance form with positive charge on the nitro-bearing carbon, which the positively charged nitrogen forbids; meta attack avoids it, so meta dominates.
Why is aniline unsuitable for Friedel-Crafts alkylation?
The amino lone pair complexes with AlCl3 and the ring deactivates, so acetylate first (acetanilide), carry out the substitution, then hydrolyse back.
How is sulfonation used to control product position?
SO3H occupies a site reversibly, blocking it; after the next substitution, steam removes the group — the standard trick to force otherwise-favoured positions.