Difficulty: Intermediate | Prerequisites: Resonance structures, basic EAS mechanism
Directing effects determine where a new substituent lands on a benzene ring that already carries a group. This is one of the most heavily tested topics in organic chemistry because it combines resonance theory, electronic effects, and practical synthesis logic. If you can predict products correctly here, you have the tools for multi-step aromatic synthesis. You should be comfortable drawing resonance structures of the arenium ion intermediate before tackling this material.
Substituents already on a benzene ring control where the next group attaches. Electron-donating groups (like –OH, –NH₂, –OR) are activating and direct incoming electrophiles to the ortho and para positions. Electron-withdrawing groups (like –NO₂, –SO₃H, –COR) are deactivating and direct to the meta position. Halogens are the odd ones out: deactivating but still ortho/para-directing.
Electrophilic aromatic substitution (EAS)
A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring. The aromaticity of the ring is preserved in the product.
In simple terms, something electron-hungry attacks the ring, kicks out a hydrogen, and the ring stays aromatic.
Nitration
An EAS reaction that introduces a nitro group (–NO₂) onto the aromatic ring. Uses HNO₃ and H₂SO₄ to generate the nitronium ion (NO₂⁺) as the active electrophile.
Nitronium ion (NO₂⁺)
The electrophile in aromatic nitration. Formed by protonation of HNO₃ by H₂SO₄, followed by loss of water.
Arenium ion (sigma complex, Wheland intermediate)
The carbocation intermediate formed when the electrophile bonds to the ring. The ring temporarily loses aromaticity at this stage. Loss of H⁺ restores aromaticity and gives the product.
Think of it as the ring's momentary sacrifice of stability to let the new group attach.
Activating group
A substituent that donates electron density into the ring (by resonance, induction, or both), making the ring more reactive toward electrophiles than unsubstituted benzene. Examples: –OH, –NH₂, –OR, –alkyl.
Deactivating group
A substituent that withdraws electron density from the ring, making it less reactive toward electrophiles than benzene. Examples: –NO₂, –SO₃H, –COR, –CN, –COOH.
Ortho/para director
A substituent that directs the incoming electrophile to the ortho (positions 2 and 6) and para (position 4) positions relative to itself. All activating groups are ortho/para directors. Halogens are also ortho/para directors despite being deactivating.
Meta director
A substituent that directs the incoming electrophile to the meta position (position 3 and 5). All strong deactivating groups (except halogens) are meta directors.
The existing substituent alters the electron density at specific ring positions
An electron-donating group increases density at ortho and para, stabilising the arenium ion intermediate when the electrophile attacks those positions
An electron-withdrawing group destabilises the arenium ion at ortho and para more than at meta, so meta attack is the least disfavoured path
Draw the resonance structures of the arenium ion for each attack position: the one with the most stable set of resonance contributors wins
Substituent | Activating or Deactivating | Directing Effect |
|---|---|---|
–OH, –OR | Strongly activating | Ortho/para |
–NH₂, –NHR, –NR₂ | Strongly activating | Ortho/para |
–Alkyl (–CH₃, –C(CH₃)₃) | Weakly activating | Ortho/para |
–F, –Cl, –Br, –I | Weakly deactivating | Ortho/para |
–NO₂ | Strongly deactivating | Meta |
–SO₃H | Strongly deactivating | Meta |
–COR, –COOR, –COOH | Moderately deactivating | Meta |
–CN | Moderately deactivating | Meta |
Reagents: HNO₃ / H₂SO₄ (the H₂SO₄ protonates HNO₃ to generate the nitronium ion NO₂⁺)
The NO₂⁺ electrophile attacks the ring at the position dictated by the existing substituent
Activating ortho/para director (e.g. –NH₂ on a ring also bearing –Br): nitration gives a mixture of ortho and para products relative to the strongest activating group
Example from HW4 Q2b: aminobromobenzene under nitration gives products where NO₂ goes ortho and para to –NH₂ (the stronger director)
Deactivating meta director (e.g. –NO₂ already on ring): new NO₂ goes meta to the existing NO₂
Example from HW4 Q2c: chloronitrobenzene under nitration gives products where the new NO₂ goes meta to the existing NO₂
When two substituents compete, the stronger activating group generally wins
If both are ortho/para directors, the new group goes ortho/para to the more activating one
Steric effects also matter: a bulky group at ortho reduces ortho substitution, favouring para
If an activating and a deactivating group are both present, the activating group dominates the directing
When both groups direct to the same position, that position is strongly favoured (reinforcing directors)
Nitration is the standard way to introduce nitrogen functionality onto aromatic rings in industrial synthesis. TNT (2,4,6-trinitrotoluene) is made by successive nitrations of toluene. Pharmaceutical intermediates often require precisely placed nitro groups that are later reduced to amines for further functionalisation.
Students often think halogens are meta directors because they are deactivating. They are not meta directors. Halogens are ortho/para directors despite being deactivating. The reason is that their lone pairs donate by resonance into the ortho/para positions even though they withdraw by induction overall.
Students forget that "activating" and "ortho/para directing" are separate concepts. They happen to overlap for most groups, but halogens break the pattern. Learn the exception.
Students assume the major product is always ortho. Steric effects often make the para product predominant when the substituent is bulky (e.g. –C(CH₃)₃). A mixture of ortho and para is typical, but para is frequently the major product.
Students treat two substituents as equal when they compete. They are rarely equal. The stronger activating group dominates. Rank your directors by strength before predicting.
⚠️ Predicting the regiochemistry of EAS on substituted benzenes is among the most common exam question types in CHM 26200. You need to be fast at classifying substituents.
⚠️ Expect questions with two substituents that either reinforce or oppose each other. The trick is to identify which group is the stronger director and predict accordingly.
⚠️ You must be able to draw the arenium ion intermediate showing why the product forms where it does. Partial credit often depends on showing the resonance structures.
⚠️ Know the nitration mechanism cold: H₂SO₄ protonates HNO₃, water leaves, NO₂⁺ forms, NO₂⁺ attacks the ring, H⁺ is lost to restore aromaticity.
True or False: –Cl is a meta director.
False. Chlorine is an ortho/para director (despite being deactivating).
Fill in the blank: The electrophile in aromatic nitration is ______.
The nitronium ion, NO₂⁺.
True or False: An –NH₂ group makes the ring less reactive than unsubstituted benzene.
False. –NH₂ is a strong activating group.
Fill in the blank: When –NO₂ and –OH are both on a ring, the incoming electrophile is directed primarily by ______.
–OH (the stronger activating group wins).
True or False: The para product is always the major product over ortho.
False. A mixture is typical, but steric effects often favour para.
Q: Predict the major product(s) when bromobenzene bearing an acyl (–COR) group is treated with HNO₃/H₂SO₄.
A: The acyl group is a meta director. Bromine is an ortho/para director but weaker. The nitro group will go primarily to the position that is meta to the acyl group and also ortho/para to the bromine, if such a position exists. Where directors conflict, the stronger deactivating group's meta-directing effect typically dominates for the major product.
Q: 2-bromoaniline is treated with HNO₃/H₂SO₄. Where does the –NO₂ go?
A: –NH₂ is a strong activating ortho/para director. –Br is a weak deactivating ortho/para director. The –NH₂ group dominates. The NO₂ goes to the positions that are ortho and para to –NH₂. The major products have –NO₂ at position 4 (para to –NH₂) and position 6 (ortho to –NH₂, which also happens to be meta to –Br).
Q: Why is chlorine an ortho/para director if it is electronegative and deactivating?
A: Chlorine withdraws electron density by induction (its electronegativity pulls electrons through the sigma bond), which deactivates the ring overall. However, its lone pairs donate into the ring by resonance, and this resonance donation preferentially stabilises the arenium ion when the electrophile attacks at ortho or para. The resonance effect controls the regiochemistry (where), while the inductive effect controls the rate (slower than benzene).
Q: 4-chloronitrobenzene is nitrated. Predict the product.
A: –NO₂ is meta-directing, –Cl is ortho/para-directing. Both direct to position 3 (meta to –NO₂ and ortho to –Cl). The NO₂ goes to the 3-position, giving 4-chloro-1,3-dinitrobenzene as the major product.
Q: Explain why –NO₂ is a meta director using resonance structures.
A: When the electrophile attacks ortho or para to –NO₂, one of the resonance structures of the arenium ion places a positive charge directly on the carbon bearing the –NO₂ group. Since –NO₂ is already electron-withdrawing, this creates an unstable arrangement with adjacent positive character. At the meta position, no resonance structure places the positive charge next to the –NO₂ carbon, so that intermediate is more stable. The meta product forms preferentially.
Directing effects feed directly into multi-step aromatic synthesis: you cannot plan a synthesis route without knowing which group goes on first and how it steers the next reaction. This connects tightly to Friedel-Crafts alkylation and acylation, where the same directing logic determines product regiochemistry.
The resonance arguments used here (drawing arenium ion intermediates, evaluating stability) are the same tools you use for understanding nucleophilic aromatic substitution, diazonium chemistry, and aromatic side-chain reactions later in the course.
Electrophilic aromatic substitution, EAS, nitration, HNO₃/H₂SO₄, nitronium ion, NO₂⁺, directing effects, ortho/para director, meta director, activating group, deactivating group, arenium ion, sigma complex, Wheland intermediate, resonance structures, halogen directing effects, competing directors, regioselectivity, aromatic substitution, CHM 26200, organic chemistry, Purdue