Haloarenes: Reactivity & Reasoning

CHEHaloarenes & Polyhalogen Compounds

Aryl halides resist the nucleophilic substitution that alkyl halides undergo easily — the classic "why is chlorobenzene less reactive than chloroethane?" reasoning question. The explanation rests on resonance, hybridisation, and the resulting C–X bond.

Dow process (industrial phenol)

C_6H_5Cl + NaOH \xrightarrow[300\,\text{atm}]{623\,K} C_6H_5O^-Na^+ \xrightarrow{H^+} C_6H_5OH

harsh conditions are needed because the aryl C–X bond is strong

Reactivity toward nucleophilic substitution

C_6H_5Cl < o\text{-/}p\text{-}O_2N{-}C_6H_4Cl < 2,4\text{-(}O_2N)_2C_6H_3Cl < 2,4,6\text{-(}O_2N)_3C_6H_2Cl

-NO_2

groups at o/p positions stabilise the anionic intermediate

Low reactivity has four causes: (i) resonance gives C–X partial double-bond character (shorter, stronger bond); (ii) the $sp^2$ carbon is more electronegative and holds the bonding electrons tightly; (iii) the phenyl cation is unstable; (iv) the electron-rich ring repels the approaching nucleophile.
The C–X bond is shorter in haloarenes than in haloalkanes, so it is harder to break.
Substitution therefore needs harsh conditions — e.g. chlorobenzene + NaOH at 623 K and 300 atm gives phenol (Dow process).
Electron-withdrawing groups ( $-NO_2$ ) at the ortho/para positions activate the ring toward nucleophilic substitution by stabilising the anionic intermediate.
In electrophilic substitution, halogens are ortho/para-directing but deactivating: $-I$ deactivates the ring while $+R$ steers incoming groups to o/p.
Reactivity toward nucleophilic substitution rises with more o/p nitro groups: chlorobenzene < 1 $-NO_2$ < 2,4-dinitro < 2,4,6-trinitro (picryl chloride).
Activation by $-NO_2$ works only when the group is ortho or para to the halogen, because only then can the negative charge of the intermediate (Meisenheimer-type carbanion) be delocalised onto the nitro oxygens; a meta nitro group does not help.
The chlorine in chlorobenzene is fixed in resonance: lone pairs delocalise into the ring, increasing electron density at o/p positions and explaining both the deactivation and the o/p direction in electrophilic attack.
Chemical test to distinguish chlorobenzene from cyclohexyl chloride: warm with $AgNO_3$ in ethanol — cyclohexyl chloride gives a white $AgCl$ precipitate; chlorobenzene gives none under mild conditions.
Vinyl halides resemble aryl halides — they too are unreactive toward $S_N$ because of resonance and the $sp^2$ C–X bond.
Even though halogens deactivate the ring overall, the extra electron density they push to o/p by resonance is enough to direct, but not to activate, the incoming electrophile.
Dipole moment of chlorobenzene ( $\approx 1.69\ \text{D}$ ) is lower than that of cyclohexyl chloride ( $\approx 2.13\ \text{D}$ ) because the shorter, partial-double-bond aryl C–Cl displaces less charge.

\mathrm{C_6H_5Cl} + \mathrm{NaOH} \xrightarrow[300\ \mathrm{atm}]{623\ \mathrm{K}} \mathrm{C_6H_5ONa} \xrightarrow{\mathrm{H^{+}}} \mathrm{C_6H_5OH}

ConditionsDow process —

623\ \mathrm{K}

and

300\ \mathrm{atm}

Quote the conditions. The whole reason this reaction is famous is that a haloalkane would do the same thing in warm dilute alkali, while chlorobenzene needs a furnace and a pressure vessel.

\mathrm{C_6H_5Cl} \xrightarrow[\text{conc. } \mathrm{H_2SO_4}]{\text{conc. } \mathrm{HNO_3}} \mathrm{o\text{-}} \text{ and } \mathrm{p\text{-}nitrochlorobenzene}

ConditionsNitration — conc.

\mathrm{HNO_3}

with conc.

\mathrm{H_2SO_4}

Halogens are ortho/para-directing yet deactivating: the

-\mathrm{I}

effect slows the ring down overall, while resonance donation still concentrates electron density at the ortho and para positions. The para product dominates for steric reasons.

Where the marks go

Claiming a meta nitro group activates the ring toward nucleophilic substitution; only ortho/para nitro groups stabilise the intermediate.
Saying halogens are activating because they are o/p-directing — they are o/p-directing but deactivating (the $-I$ effect outweighs the $+R$ effect).
Asserting chlorobenzene reacts with NaOH under ordinary conditions; it needs 623 K and 300 atm.
Confusing the reason for low reactivity (a stronger C–X bond) with "chlorine is a poor leaving group" — the issue is bond strength and an unstable phenyl cation, not leaving-group ability alone.
Forgetting that the distinguishing $AgNO_3$ test relies on the alkyl halide reacting while the aryl halide does not under mild conditions.

How the board asks it

Give reasonsresonance and $sp^2$ C–X bond strength1 mk
Account for the following: Aryl halides are much less reactive towards nucleophilic substitution than alkyl halides. Give any three reasons.
Give reasonso/p nitro activation via Meisenheimer intermediate
Give reasons: The presence of a $-NO_2$ group at the ortho or para position greatly increases the reactivity of chlorobenzene towards nucleophilic substitution, but a $-NO_2$ group at the meta position does not.
Give reasonshalogens o/p-directing but deactivating3 mkAsked 2026
Account for the fact that, in electrophilic substitution, the halogen in chlorobenzene is ortho/para-directing yet deactivates the benzene ring.
Distinguishalcoholic $AgNO_3$ chemical test
How will you distinguish between chlorobenzene ( $C_6H_5Cl$ ) and cyclohexyl chloride using a simple chemical test?
Assertion–ReasonC–Cl partial double-bond character and dipole moment1 mkAsked 2025
Assertion: The dipole moment of chlorobenzene ( $\approx 1.69\ \text{D}$ ) is lower than that of cyclohexyl chloride ( $\approx 2.13\ \text{D}$ ). Reason: The aryl C–Cl bond has partial double-bond character and is shorter than the alkyl C–Cl bond. Select the correct option.
Predict the productnucleophilic substitution on a haloarene needs forcing conditions1 mkAsked 2026
When chlorobenzene is heated with aqueous ammonia in the presence of $\mathrm{Cu_2O}$ at high pressure, name the product.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.