Reactions of Haloarenes

1. Nucleophilic Substitution Reactions

Aryl halides (haloarenes) are far less reactive than alkyl halides toward nucleophilic substitution — chlorobenzene, for instance, will not hydrolyse to phenol under the mild conditions that easily convert an alkyl chloride to an alcohol. Four structural factors, taken together, explain this sluggishness.

(i) Resonance effect — conjugation of the halogen lone pair with the ring. One of the lone pairs on the halogen atom overlaps with the π-system of the benzene ring. This conjugation generates additional resonance contributors in which the ring carbons ortho and para to the halogen carry a formal negative charge while the halogen itself carries a formal positive charge, with the $\text{C–X}$ bond gaining partial double-bond character:

Because the $\text{C–X}$ bond in a haloarene is effectively part-double-bond, it is shorter and stronger than a normal single bond, so it is harder to break heterolytically than the $\text{C–X}$ bond of an alkyl halide.

(ii) Difference in hybridisation of the carbon atom in the C–X bond. In a haloalkane, the carbon attached to $\text{X}$ is $sp^3$-hybridised; in a haloarene it is $sp^2$-hybridised. An $sp^2$ carbon has more $s$-character and is therefore more electronegative, so it holds the bonding electron pair of the $\text{C–X}$ bond more tightly. Consequently the $\text{C–Cl}$ bond length is only about 169 pm in chlorobenzene against about 177 pm in an alkyl chloride — a shorter bond is harder to cleave, reinforcing the low reactivity of haloarenes.

(iii) Instability of the phenyl cation. If a haloarene tried to ionise on its own (the way many alkyl halides do in an $S_N1$ pathway), it would have to form a phenyl cation. This cation cannot be stabilised by resonance with the ring π-system (the empty orbital lies in the plane of the ring, not conjugated with it), so it is far too high in energy to form. This effectively rules out an $S_N1$-type mechanism for haloarenes.

(iv) Electronic repulsion at the ring. The benzene ring is itself electron-rich (due to the π-cloud), so an incoming electron-rich nucleophile is repelled as it approaches, making the approach of the nucleophile inherently less favourable than at the electron-poorer carbon of an alkyl halide.

Replacement by the Hydroxyl Group

Despite this general unreactivity, chlorobenzene can be forced to give phenol if sufficiently harsh conditions are used — heating with aqueous sodium hydroxide at about 623 K under roughly 300 atmospheres of pressure, followed by acidification:

The reaction becomes dramatically easier — needing far lower temperature — when a strongly electron-withdrawing group such as $-\text{NO}_2$ is present ortho or para to the halogen:

With three $-\text{NO}_2$ groups at the 2,4,6-positions, even warming the chloride with water alone is enough to displace $\text{Cl}$ and give the corresponding trinitrophenol. So the effect is cumulative: each additional ortho/para $-\text{NO}_2$ makes displacement of the halogen easier and lets the reaction run under progressively milder conditions. A $-\text{NO}_2$ group placed meta to the halogen, however, has essentially no effect on reactivity at all.

Mechanism. This activated substitution follows a two-step addition–elimination pathway (never a simple one-step displacement):

In the slow step, the hydroxide ion adds to the ring carbon bearing chlorine, momentarily destroying the aromatic sextet and generating a resonance-stabilised carbanion intermediate (this delocalised, non-aromatic intermediate is the same kind of species commonly called a Meisenheimer-type complex). When $-\text{NO}_2$ sits ortho or para to the site of attack, the negative charge of this intermediate can be pushed out onto the oxygens of the nitro group by resonance, which stabilises the intermediate strongly and speeds up the slow step. In the fast second step, the ring rearomatises as chloride ion departs.

2. Electrophilic Substitution Reactions

Haloarenes still undergo the standard electrophilic aromatic substitution reactions typical of benzene — halogenation, nitration, sulphonation and the two Friedel–Crafts reactions (alkylation and acylation). In every case the halogen substituent directs the incoming electrophile to the ortho and para positions relative to itself, even though the ring as a whole reacts more sluggishly than benzene does.

Why the halogen is ortho/para-directing. The same lone-pair conjugation described above for nucleophilic substitution is responsible here too. Resonance delocalisation of the halogen's lone pair into the ring places extra electron density preferentially at the ortho and para carbons (structures II, III and IV below all carry a formal positive charge on the halogen and enhanced electron density at these ring positions), while comparatively little extra density reaches the meta positions:

So an electrophile attacking ortho or para to the halogen forms an arenium-ion (carbocation) intermediate that the halogen's lone pair can stabilise directly by resonance.

Why the ring is nevertheless deactivated overall. The halogen is strongly electronegative, so it also withdraws electron density from the ring inductively (the $-I$ effect), through the sigma framework, at every position — including the position of attack. This inductive withdrawal destabilises the arenium-ion intermediate regardless of where the electrophile attacks.

(i) Halogenation. Chlorobenzene reacts with $\text{Cl}_2$ in the presence of anhydrous $\text{FeCl}_3$ to give a mixture of dichlorobenzenes, dominated by the para isomer:

(ii) Nitration. With $\text{HNO}_3$ in concentrated $\text{H}_2\text{SO}_4$, chlorobenzene gives mainly the para-nitro product:

(iii) Sulphonation. Heating chlorobenzene with concentrated $\text{H}_2\text{SO}_4$ gives predominantly the para-sulphonic acid:

(iv) Friedel–Crafts reactions. With methyl chloride and anhydrous $\text{AlCl}_3$ (alkylation): …