Halogen compounds

A halogenoarene 卤代芳烃 (also called an aryl halide 芳基卤化物) is formed when an arene 芳烃 reacts with $\text{Cl}_2$ or $\text{Br}_2$, using $\text{AlCl}_3$ or $\text{AlBr}_3$ as a catalyst 催化剂. This is the electrophilic substitution 亲电取代 from the arenes topic — the halogen replaces a hydrogen on the benzene ring 苯环.

• benzene gives chlorobenzene.
• methylbenzene 甲苯 gives 1-chloro-2-methylbenzene and 1-chloro-4-methylbenzene (the methyl group 甲基 directs 定位 the chlorine to positions 2 and 4).

Compare chloroethane (a halogenoalkane 卤代烷) with chlorobenzene (a halogenoarene).

A halogenoalkane reacts easily by nucleophilic substitution 亲核取代: its C–Cl bond is polar 极性的, so a nucleophile can attack the slightly positive carbon and push the halogen out.

A halogenoarene is very unreactive 不活泼的 towards nucleophilic substitution. There are two reasons:

• a lone pair 孤对电子 on the chlorine overlaps sideways 侧面重叠 with the delocalised 离域 ring of electrons. This gives the C–Cl bond partial double-bond character 部分双键性质, making it shorter and stronger, so it is much harder to break.
• the electron-rich 富电子的 ring repels 排斥 an approaching nucleophile 亲核试剂.

So chlorobenzene does not react with nucleophiles such as $\text{OH}^-$ under normal conditions, while chloroethane does.

Seeing the difference in the lab

Warm each compound with aqueous silver nitrate in ethanol. Any halide ions released by hydrolysis 水解 form a silver halide precipitate:

So the ease of hydrolysis is acyl chloride > halogenoalkane > halogenoarene.

Worked example. (a) Explain why chlorobenzene gives no precipitate with warm aqueous silver nitrate, while chloroethane does. (b) Predict what happens with (chloromethyl)benzene, $\text{C}_6\text{H}_5\text{CH}_2\text{Cl}$. (a) In chloroethane the C–Cl bond breaks by nucleophilic substitution, releasing $\text{Cl}^-$, which gives white AgCl. In chlorobenzene, a lone pair on Cl overlaps with the delocalised ring, so the C–Cl bond is stronger, and the ring's electrons repel the nucleophile. No $\text{Cl}^-$ is released, so there is no precipitate. (b) Here the Cl is on the side chain, joined to an $\text{sp}^3$ carbon, not to the ring. It behaves like a halogenoalkane, so a white precipitate forms on warming.

• "The C–Cl bond in chlorobenzene is not polar." It is still polar; the bond is unreactive because it is stronger (partial double-bond character) and the ring repels nucleophiles.
• Giving only one reason. Mark schemes usually want both: the stronger C–Cl bond and the repulsion by the ring's electrons.
• Treating every Cl in an aromatic compound as unreactive. Only a halogen bonded directly to the ring is.

• A halogenoarene is less reactive than a halogenoalkane: the lone pair delocalises into the ring, giving the $\text{C-X}$ bond partial double-bond character.
• Distinguish a halogen on the ring (needs a catalyst, unreactive to substitution) from one on a side chain 侧链 (reacts like a halogenoalkane).