Building on electrophilic aromatic substitution: once the ring carries a substituent, that group sets both how fast the next electrophile adds and which position it takes — and both answers come from how it stabilizes the arenium ion.
Three substituents, three verdicts. Two independent properties — activating vs. deactivating, and o/p- vs. meta-directing — are set by the same electronic cause.
1. A Group Already on the Ring Sets Both the Rate and the Position
Rate and regiochemistry are two readings of one electronic effect: a group that pushes density in stabilizes the arenium ion (activator), one that pulls density out destabilizes it (deactivator).
2. Electron-Donating Groups Are Ortho/Para Directors and Activators
Groups that donate into the ring — a lone pair adjacent to it (–OH, –OR, –NH₂, –NHCOR) or an alkyl group (–CH₃) — react faster than benzene and steer the electrophile ortho/para; lone-pair donors are strongest, alkyl mild.
Donors: a lone pair (N, O) or an alkyl group next to the ring pushes density in — all ortho/para directors, all faster than benzene.
3. Resonance Explains Why Donors Favor Ortho and Para Attack
Ortho or para attack puts the positive charge on the carbon bearing the donor, where its lone pair adds an extra stabilizing contributor; meta attack never does, so o/p arenium ions form faster and dominate.
Nitration of toluene: the methyl group directs the nitro group ortho and para (para-nitrotoluene shown), never meta.
Bromination of phenol gives the two o/p products predicted by the resonance argument — the –OH lone pair stabilizes exactly these arenium ions.
4. Electron-Withdrawing Groups Are Meta Directors and Deactivators
Electron-poor groups (–NO₂, –C=O, –CN, –SO₃H, –CF₃, –⁺NR₃) react slower than benzene; the flip of section 3, o/p attack stacks positive on positive, so meta wins as the least disfavored path.
Withdrawers: each has an electron-poor atom bonded to the ring. All are deactivators and all are meta directors.
Bromination of nitrobenzene is slow (deactivated ring) and gives the meta product almost exclusively.
5. Halogens Are the Exception: Deactivating but Ortho/Para Directing
Halogens split the two properties: induction withdraws density and slows the rate (deactivating), while lone-pair resonance still stabilizes the o/p arenium ions (o/p-directing).
The halogen paradox: slower than benzene (induction wins the rate) but ortho/para (resonance wins the position).
6. Summary
Donors = activating o/p-directors · withdrawers = deactivating meta-directors · halogens = deactivating but o/p-directing · draw the three arenium ions and count contributors to predict any monosubstituted benzene.
Three products, three rules confirmed — donor sends the group o/p, withdrawer sends it meta.
Worked example
- The methyl group donates electron density → it activates the ring.
- It stabilises the arenium ion best when the electrophile adds ortho or para to it.
- Para is usually the largest single product (less steric crowding than ortho).
Answer. Mainly ortho- and para-nitrotoluene (para major); methyl is an activating o/p-director.
Quiz yourself
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Faster, and at the ortho/para positions. The –OCH₃ oxygen lone pair donates into the ring by resonance, making methoxy a strong activator and an ortho/para director. Para usually dominates for steric reasons.
For ortho or para attack, one resonance form of the arenium ion places a positive charge on the ring carbon that already bears the electron-poor nitro group — positive next to positive, a badly destabilized intermediate. Meta attack avoids that carbon, so meta is the least disfavored pathway and dominates.
Two different effects answer two different questions. Chlorine's electronegativity withdraws electron density by induction, which lowers the rate (deactivating). Separately, chlorine's lone pairs donate by resonance to stabilize the ortho and para arenium ions specifically, which sets the position (o/p-directing). Induction wins the rate; resonance wins the regiochemistry.
The methyl group is a mild activator and ortho/para director, so bromine adds ortho and para to the methyl, giving mainly o-bromotoluene and p-bromotoluene (para favored by sterics). No meta product of significance.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.