Electrophilic substitution in haloarenes

Two effects, two different jobs. A halogen substituent on benzene exerts an inductive ($-I$) effect and a resonance/mesomeric ($+M$) effect, and the two effects act on different things:

• The inductive effect is a through-bond pull of σ-electron density toward the highly electronegative halogen. It is felt everywhere on the ring, roughly equally, so it lowers the electron density — and hence the nucleophilicity — of the whole ring compared with benzene. This is why haloarenes are less reactive than benzene toward electrophiles.
• The resonance effect is a donation of one of the halogen's lone pairs into the ring's π-system. This donation can only be drawn as a resonance structure when the negative character lands on a carbon that is ortho or para to the halogen — never meta. So the resonance effect is positionally selective, and it is what makes halogens ortho/para-directing.

Because the deactivating effect ($-I$) dominates the overall rate while the directing effect ($+M$) governs where on the deactivated ring attack happens, a halogen ends up simultaneously a deactivator and an o,p-director — a combination unique among common substituents. Groups like $-OH$, $-NH_2$, and $-OCH_3$ are also o,p-directors, but for them the resonance donation dominates the inductive withdrawal, so they activate the ring; a halogen's resonance donation is comparatively weak (poor overlap between the halogen's larger $2p/3p/4p$ orbital and the ring carbon's $2p$ orbital), so it cannot outweigh the inductive pull.

The general mechanism. Electrophilic aromatic substitution on a haloarene follows the same two-step arenium-ion (sigma-complex) pathway as on benzene:

where: $\text{ArH}$ is the aromatic substrate (here, chlorobenzene), $E^{+}$ is the electrophile generated in situ ($Cl^{+}$ from $Cl_2/FeCl_3$ for chlorination, $NO_2^{+}$ from $HNO_3/H_2SO_4$ for nitration, or an $SO_3H^{+}$-type species from fuming $H_2SO_4$ for sulphonation), and $[\text{Ar}(H)E]^{+}$ is the resonance-stabilised carbocation intermediate formed before the ring loses a proton and rearomatises. The rate-determining step is the electrophile's attack on the ring; the second step, loss of $H^{+}$, is fast and simply restores aromaticity.

Why ortho/para wins over meta. For chlorobenzene, drawing the arenium-ion intermediate for attack at each position shows why the halogen prefers ortho and para. Attack at the ortho or para carbon gives an intermediate whose positive charge can be delocalised onto the carbon bearing chlorine itself — and only then can the chlorine's lone pair donate in and help stabilise that charge. Attack at the meta carbon gives an intermediate whose delocalised positive charge never reaches the chlorine-bearing carbon, so no such stabilisation is available. The ortho/para intermediates are therefore lower in energy than the meta intermediate, and since the rate-determining step forms this intermediate, ortho/para attack is faster.

The reasoning chain behind "o,p-directing but deactivating" is short and is exactly what an exam answer should reproduce. Start from the rate: the inductive effect withdraws σ-electron density from every ring carbon roughly equally, because it acts through the σ-framework of bonds and falls off only slowly with distance. This lowers the overall electron density of the ring compared with unsubstituted benzene, so the ring is a poorer nucleophile everywhere, and the rate-determining electrophilic attack is slower at every position — hence deactivation is a comparison of haloarene versus benzene, not a comparison between positions on the same ring. Now consider position, which is a separate question of where on this already-deactivated ring the electrophile prefers to attack. Resonance donation of a halogen lone pair into the π-system is only possible when curved arrows can be drawn moving that lone pair onto a ring carbon adjacent to (ortho) or para to the halogen — the geometry of the conjugated π-system simply does not allow a resonance structure that places extra electron density, or later stabilises positive charge, at the meta carbon. So when the arenium-ion intermediate forms after electrophilic attack, only the ortho and para intermediates have an extra resonance contributor with the positive charge on the halogen-bearing carbon, letting the halogen's lone pair partially neutralise it; the meta intermediate has no such contributor and is correspondingly higher in energy. Since a lower-energy intermediate means a lower activation energy by Hammond's-postulate-type reasoning (the intermediate resembles the transition state), ortho/para attack is faster than meta attack within chlorobenzene, even though the whole ring reacts slower than benzene. Both effects must be cited together: $-I$ explains the reduced rate relative to benzene; $+M$ explains the regiochemistry relative to meta.

Problem: Chlorobenzene is treated with a mixture of concentrated $HNO_3$ and $H_2SO_4$. Predict the major product(s), and explain why the para isomer is typically the major single product.

Step 1. Generate the electrophile. Concentrated sulphuric acid protonates nitric acid and helps expel water, forming the nitronium ion: $HNO_3 + 2\,H_2SO_4 \rightarrow NO_2^{+} + H_3O^{+} + 2\,HSO_4^{-}$.

Step 2. Identify the directing behaviour. Chlorine is o,p-directing, so $NO_2^{+}$ attacks preferentially at the carbon ortho to $Cl$ and the carbon para to $Cl$, not at the meta carbon.

Step 3. Justify with the intermediate. For para attack, the arenium-ion intermediate has three resonance contributors, one of which places the positive charge directly on the chlorine-bearing carbon; the chlorine lone pair then donates in to help stabilise that charge. Ortho attack has the same kind of stabilising resonance contributor. Meta attack has no resonance contributor with the charge on the chlorine-bearing carbon, so it is disfavoured.

Step 4. Complete the mechanism. Loss of $H^{+}$ from the carbon that was attacked restores aromaticity and gives the neutral disubstituted product.

Step 5. Compare ortho and para. Both isomers form, but ortho attack places the incoming, comparatively bulky $-NO_2$ group directly next to the existing $-Cl$, creating steric strain in that intermediate and product; para attack places the two groups on opposite sides of the ring, avoiding that clash. Answer: the product is a mixture of 1-chloro-2-nitrobenzene (ortho) and 1-chloro-4-nitrobenzene (para), with the para isomer predominating because it is less sterically hindered than the ortho isomer.

• Assuming every ortho/para-directing group must also activate the ring — halogens are the standard counter-example: o,p-directing yet deactivating, unlike $-OH$, $-NH_2$, and alkyl groups, which are both o,p-directing and activating.
• Explaining the o,p-preference using only the inductive effect — inductive withdrawal alone is felt everywhere and cannot explain why one position is preferred over another; only the resonance ($+M$) donation, which is geometrically restricted to ortho/para, explains the regiochemistry.
• Applying directive-influence rules to disubstituted haloarenes — the ISC syllabus restricts this analysis to monosubstituted compounds (chlorobenzene-type substrates) only.
• Forgetting that deactivation still means chlorobenzene needs more forcing conditions, or gives a slower/lower-yield reaction, than benzene for the same electrophilic substitution, even though the product regiochemistry (o,p) looks like that of an activating group.

Ask the tutor: want the full set of resonance structures for ortho, meta, and para attack on chlorobenzene drawn out step by step, or more practice predicting products for chlorination and sulphonation of haloarenes?