1. The C–X bond and preparation
X on an sp² ring carbon (aryl halide); benzylic halides (C₆H₅CH₂X) are different. C–Cl in chlorobenzene is shorter (169 pm vs 177 pm) and stronger (resonance gives partial double-bond character; sp² carbon) and less polar (1.69 D; +R opposes −I).
Preparation: benzene + Cl₂/Br₂ with FeCl₃/FeBr₃/AlCl₃ in the dark (iodination needs HNO₃ to remove HI); from benzenediazonium chloride (aniline + NaNO₂ + HCl at 273–278 K): Sandmeyer CuCl/HCl or CuBr/HBr; Gattermann Cu powder + HX; KI → iodobenzene.
Formulas
- Sandmeyer
Key terms
- Aryl halide
A compound with a halogen bonded directly to an sp² carbon of an aromatic ring.
- Sandmeyer reaction
Replacement of the diazonium group by Cl or Br using CuCl/HCl or CuBr/HBr.
- Gattermann reaction
Replacement of the diazonium group by Cl or Br using copper powder and HX.
2. Physical properties
Colourless liquids or solids; insoluble in water (no H-bonding), soluble in organic solvents; bromo- and iodo-arenes denser than water. B.p. rises C₆H₅F < C₆H₅Cl < C₆H₅Br < C₆H₅I. Isomeric dihalobenzenes boil at similar temperatures, but the para isomer melts much higher (symmetry, better packing): 1,4-dichlorobenzene 323 K vs 1,2- 256 K and 1,3- 249 K.
| Isomer | m.p. / K |
|---|---|
| 1,2-Dichlorobenzene | 256 |
| 1,3-Dichlorobenzene | 249 |
| 1,4-Dichlorobenzene | 323 |
Key terms
- Crystal packing
How closely molecules fit together in a solid; better packing gives a higher melting point.
3. Electrophilic substitution
Halogen: −I deactivates the ring (slower than benzene); +R directs to ortho/para; para usually major (steric).
Nitration mechanism: ; NO₂⁺ attacks o/p (slow) giving a σ-complex stabilised by the Cl lone pair; loss of H⁺ (fast).
Products from chlorobenzene: Cl₂/FeCl₃ → 1,2- and 1,4-dichlorobenzene; HNO₃/H₂SO₄ → 1-chloro-2- and 1-chloro-4-nitrobenzene; conc. H₂SO₄ → 2- and 4-chlorobenzenesulphonic acid; CH₃Cl/AlCl₃ → 2- and 4-chlorotoluene.
| Effect of Cl | Consequence |
|---|---|
| −I | Deactivating |
| +R | o/p directing |
Formulas
- Nitration of chlorobenzene
Key terms
- σ-complex
The positively charged, non-aromatic intermediate (arenium ion) formed when an electrophile adds to a benzene ring.
4. Nucleophilic substitution and reduction
Difficult because of partial double-bond character, sp² carbon, unstable phenyl cation (no SN1), and repulsion/blocked backside (no SN2). Dow: C₆H₅Cl + NaOH, 623 K, 300 atm → C₆H₅ONa → phenol.
o/p-NO₂ groups help (addition–elimination via a Meisenheimer complex; charge delocalised onto NO₂): 4-nitro 443 K; 2,4-dinitro 368 K; 2,4,6-trinitro warm water. m-NO₂ helps little.
Cl → NH₂: NH₃ + Cu₂O, 475 K, 60 atm. Reduction: H₂/Pd or Ni–Al alloy/NaOH → benzene.
| Substrate | Conditions |
|---|---|
| C₆H₅Cl | 623 K, 300 atm |
| 4-NO₂ | 443 K |
| 2,4-(NO₂)₂ | 368 K |
| 2,4,6-(NO₂)₃ | Warm water |
Formulas
- Dow process
Key terms
- Meisenheimer complex
The negatively charged σ-complex formed when a nucleophile adds to an activated aryl halide.
- Dow process
Industrial hydrolysis of chlorobenzene with NaOH at 623 K and 300 atm.
5. Reactions with metals; DDT
Wurtz–Fittig: ArX + RX + 2Na (dry ether) → Ar–R. Fittig: 2ArX + 2Na → Ar–Ar. Grignard: C₆H₅Br + Mg (dry ether) → C₆H₅MgBr; + H₂O → benzene.
DDT = 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane, from chlorobenzene + chloral (conc. H₂SO₄). Effective insecticide (malaria), but not biodegradable, fat-soluble, biomagnifies in food chains (thin eggshells in birds); insect resistance; banned or restricted.
| Reaction | Product |
|---|---|
| Wurtz–Fittig | Alkylbenzene |
| Fittig | Biphenyl |
| PhMgBr + H₂O | Benzene |
Formulas
- Manufacture of DDT
Key terms
- Biomagnification
Increase in concentration of a persistent substance at each higher level of a food chain.
Exam tips
Common mistakes
- Calling halogen meta-directing (it is o/p-directing but deactivating).
- Saying m-NO₂ activates nucleophilic substitution as much as p-NO₂.
- Confusing Sandmeyer (CuX/HX) with Gattermann (Cu/HX), or Fittig with Wurtz–Fittig.
- Explaining the p-dichlorobenzene melting point by molar mass.
How marks are usually awarded
- Mechanisms (2–3 marks): electrophile formation, σ-complex, loss of H⁺; or addition then elimination with the role of NO₂.
- Reasons for low reactivity (2 marks): resonance, sp² carbon, unstable phenyl cation, repulsion.
- DDT (2 marks): structure/equation and an environmental effect.