What are haloarenes?
In a haloarene (aryl halide) the halogen is joined directly to a carbon of an aromatic ring: C₆H₅Cl (chlorobenzene), C₆H₅Br (bromobenzene). If X is on a side-chain carbon (C₆H₅CH₂Cl) it is a benzylic halide, which behaves like a haloalkane. Dihalobenzenes have three isomers: 1,2 (ortho), 1,3 (meta), 1,4 (para).
Preparation of haloarenes
- Electrophilic substitution: benzene + Cl₂ (or Br₂) with a Lewis acid such as FeCl₃ or anhydrous FeBr₃, in the dark and cold. The Lewis acid makes Cl⁺, which attacks the ring. Iodination needs an oxidiser (HNO₃ or HIO₄) because HI formed would reverse it; fluorination is too violent.
- Sandmeyer reaction: aniline + NaNO₂ + HCl at 273–278 K gives benzenediazonium chloride. Then CuCl/HCl gives chlorobenzene, CuBr/HBr gives bromobenzene. N₂ gas is released.
- Gattermann reaction: the same swap using copper powder with HCl or HBr.
- Iodobenzene: diazonium salt + KI (no copper needed).
Why nucleophilic substitution is hard
- Resonance: the Cl lone pair is shared with the ring, giving C–Cl part double-bond character (169 pm vs 177 pm in CH₃Cl).
- sp² carbon: it holds the bond electrons closer, making the bond shorter and stronger.
- Phenyl cation is unstable, so SN1 fails.
- Repulsion: the electron-rich ring pushes an incoming nucleophile away, and backside (SN2) attack is blocked by the ring.
Dow process: chlorobenzene + NaOH at 623 K and 300 atm → sodium phenoxide → phenol (with acid).
–NO₂ helps: an electron-pulling group at ortho or para pulls electrons away and steadies the negative intermediate. 4-nitro: 443 K; 2,4-dinitro: 368 K; 2,4,6-trinitro: warm water (323 K). A –NO₂ at meta does little.
Electrophilic substitution and the directive effect
Halogen does two things to the ring:
- −I effect (pulls electrons through the bond) → the whole ring is a little less reactive than benzene (deactivating).
- +R effect (lone pair shared by resonance) → extra electron density appears only at ortho and para. So new groups go to ortho and para.
Para is usually the major product because ortho is crowded next to Cl.
- Halogenation: Cl₂/FeCl₃ → o- and p-dichlorobenzene.
- Nitration: conc. HNO₃ + conc. H₂SO₄ → o- and p-chloronitrobenzene.
- Sulphonation: conc. H₂SO₄ → o- and p-chlorobenzenesulphonic acid.
- Friedel–Crafts: CH₃Cl/AlCl₃ → o- and p-chlorotoluene; CH₃COCl/AlCl₃ → o- and p-chloroacetophenone.
Reactions with metals
- Wurtz–Fittig: Ar–X + R–X + 2Na (dry ether) → Ar–R + 2NaX. Example: chlorobenzene + CH₃Cl → toluene.
- Fittig: 2Ar–X + 2Na (dry ether) → Ar–Ar + 2NaX. Chlorobenzene → biphenyl.
- With Mg in dry THF/ether: aryl Grignard reagent ArMgX.
Physical properties and board focus
Haloarenes are colourless liquids or solids, insoluble in water, heavier than water. p-dichlorobenzene melts much higher than its o- and m- isomers because its symmetric shape packs well in the crystal. Board questions: "why is chlorobenzene less reactive than CH₃Cl?", "why is Cl o/p directing though deactivating?", name the reaction (Sandmeyer, Wurtz–Fittig, Fittig, Dow), conversions (benzene → chlorobenzene → phenol).
Key formulas and definitions
- C₆H₆ + Cl₂ → C₆H₅Cl + HCl (FeCl₃, dark)
- C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂ (Sandmeyer)
- C₆H₅Cl + NaOH → C₆H₅ONa (623 K, 300 atm) → C₆H₅OH (with H⁺) (Dow)
- C₆H₅Cl + CH₃Cl + 2Na → C₆H₅CH₃ + 2NaCl (Wurtz–Fittig)
- 2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl (Fittig)
- Directive effect of X: −I (deactivates) + +R (o/p directing)
Worked examples
1. How will you convert benzene into chlorobenzene?
Pass Cl₂ into benzene with anhydrous FeCl₃ in the dark: C₆H₆ + Cl₂ → C₆H₅Cl + HCl. FeCl₃ makes the Cl⁺ electrophile.
2. How will you convert aniline into bromobenzene?
Step 1: aniline + NaNO₂ + HCl at 273–278 K → C₆H₅N₂⁺Cl⁻. Step 2: add CuBr/HBr → C₆H₅Br + N₂ (Sandmeyer).
3. Why is the C–Cl bond in chlorobenzene shorter than in chloromethane?
Cl shares its lone pair with the ring (resonance), giving the bond some double-bond character. Also the carbon is sp² (more s-character) instead of sp³. Both make the bond shorter (≈169 pm vs 177 pm).
4. Arrange for reaction with aqueous NaOH: chlorobenzene, 2,4-dinitrochlorobenzene, 4-nitrochlorobenzene, 2,4,6-trinitrochlorobenzene.
More –NO₂ at o/p = easier: chlorobenzene < 4-nitro < 2,4-dinitro < 2,4,6-trinitro.
5. Chlorobenzene is nitrated. Name the products and say which is major.
o-chloronitrobenzene and p-chloronitrobenzene. Para is major, because the ortho position is crowded by the nearby Cl.
6. Find the percentage of chlorine in chlorobenzene (C = 12, H = 1, Cl = 35.5).
M = 6×12 + 5×1 + 35.5 = 112.5 g/mol. %Cl = 35.5 ÷ 112.5 × 100 = 31.6%.
Common mistakes
- Writing SN2 for chlorobenzene with plain NaOH at room temperature: it does not happen; the ring blocks backside attack.
- Saying Cl activates the ring because it is o/p directing: it deactivates (−I) but directs o/p (+R).
- Mixing up Wurtz–Fittig (aryl + alkyl halide) with Fittig (two aryl halides).
- Putting –NO₂ at meta and expecting faster substitution: only ortho/para –NO₂ helps.