Haloalkanes and Haloarenes • Topic 3 of 3

Haloarenes & Polyhalogen Compounds

Swap the sp3 carbon of a haloalkane for the sp2 carbon of a benzene ring and the chemistry changes completely. Chlorobenzene looks like an alkyl halide on paper, yet it shrugs off the nucleophiles that attack alkyl halides easily. Understanding why is the heart of this topic.

Why haloarenes resist nucleophilic substitution

Four factors, working together, make the C–X bond in haloarenes short, strong and hard to replace.

  1. Resonance (partial double-bond character): a lone pair on the halogen overlaps with the ring π-system. The C–X bond gains partial double-bond character, becomes shorter and stronger, and the carbon’s electrophilicity drops — so nucleophiles attack reluctantly.
  2. sp2 hybridised carbon: the ring carbon bonded to X is sp2 (more s-character) and holds its electrons more tightly than the sp3 carbon of an alkyl halide, strengthening and shortening the C–X bond.
  3. Instability of the phenyl cation: the aryl cation that an SN1 path would need is highly unstable and is not stabilised by the ring, so that route is shut.
  4. Electron repulsion: the electron-rich ring repels the approaching (usually electron-rich) nucleophile.

Consequently aryl halides react with nucleophiles only under drastic conditions (e.g. chlorobenzene + NaOH at ~623 K and 300 atm gives phenol — the Dow process), and strong electron-withdrawing groups (–NO2) at the ortho/para positions sharply increase reactivity by stabilising the intermediate.

Electrophilic substitution in haloarenes

Halogens are deactivating but ortho/para-directing. By their −I (inductive) effect they pull electron density from the ring, so reactions (halogenation, nitration, sulphonation, Friedel–Crafts) are slower than for benzene. But by their +R (resonance/lone-pair donation) effect they make the ortho and para positions relatively electron-rich, so the incoming electrophile goes mainly there. For example, chlorination/nitration of chlorobenzene gives chiefly the ortho and para products.

Reaction with metals

The Wurtz–Fittig reaction couples an aryl halide with an alkyl halide using sodium in dry ether to give an alkylarene (e.g. chlorobenzene + chloromethane + Na → toluene). The Fittig reaction couples two aryl halides to give a biaryl (e.g. 2C6H5Cl + 2Na → biphenyl).

Important polyhalogen compounds

  • Dichloromethane (CH2Cl2): a widely used solvent and paint remover; harmful to the central nervous system, can damage eyes/skin on contact.
  • Trichloromethane / chloroform (CHCl3): once a common anaesthetic; slowly oxidised by air and light to poisonous phosgene (COCl2), so it is stored in dark bottles filled to the brim with ~1% ethanol as a stabiliser. It depresses the CNS.
  • Triiodomethane / iodoform (CHI3): a pale-yellow solid once used as an antiseptic, but the antiseptic action is due to the liberated iodine, not the compound itself.
  • Tetrachloromethane / carbon tetrachloride (CCl4): used as a solvent and earlier in fire extinguishers (Pyrene); its vapours can cause liver damage and it contributes to ozone depletion.
  • Freons (chlorofluorocarbons, e.g. CCl2F2, Freon-12): stable, non-toxic refrigerants and aerosol propellants; in the stratosphere they release Cl radicals that destroy the ozone layer, so they are being phased out under the Montreal Protocol.
  • DDT (p,p′-dichlorodiphenyltrichloroethane): the first major synthetic insecticide; it is cheap and effective against malaria-carrying mosquitoes, but it is chemically stable and fat-soluble, so it bioaccumulates up the food chain and is now banned or restricted in many countries.
Chlorobenzene resonance: partial double-bond character of C-ClChlorobenzene: lone-pair donation → partial C=Cl double bondringCllone pairsI (single bond)↔Cl+−II (o/p carbanion)↔Cl+−III (other position)C-Cl shorter & stronger → poor leaving group → very low Sₙ reactivityNegative charge sits on o/p carbons → halogens are o/p-directing (but deactivating)
Solved Examples
1
Worked Example
Give two reasons why chlorobenzene is far less reactive than chloromethane toward nucleophilic substitution.
Solution
  1. In chlorobenzene a lone pair on Cl overlaps with the ring π-system (resonance), giving the C–Cl bond partial double-bond character; it becomes shorter and stronger and harder to break.
  2. The ring carbon is sp2 (more s-character) and holds electrons more tightly than the sp3 carbon of chloromethane, further strengthening the bond.
  3. (Additionally, no stable phenyl cation forms and the electron-rich ring repels nucleophiles.) Chloromethane has none of these, so it is much more reactive.

Answer: Resonance gives the C–Cl bond partial double-bond character (short, strong), and the sp2 carbon binds it tightly, so chlorobenzene resists nucleophilic substitution.

2
Worked Example
Why are halogens called deactivating yet ortho/para-directing in electrophilic aromatic substitution?
Solution
  1. By the −I (inductive) effect the electronegative halogen withdraws electron density from the ring, lowering overall reactivity — hence deactivating (slower than benzene).
  2. By the +R (resonance) effect the halogen donates a lone pair into the ring, and the resulting negative charge sits on the ortho and para carbons.
  3. So those positions are relatively electron-rich and the electrophile attacks there — hence ortho/para-directing.

Answer: The −I effect deactivates the ring overall, while the +R effect makes the o/p positions electron-rich, so substitution is slow but goes mainly ortho/para.

3
Worked Example
Write the product(s) of nitration of chlorobenzene with conc. HNO3/H2SO4.
Solution
  1. Chlorine is ortho/para-directing, so the nitro group enters mainly at the ortho and para positions.
  2. The products are o-chloronitrobenzene (1-chloro-2-nitrobenzene) and p-chloronitrobenzene (1-chloro-4-nitrobenzene).
  3. The para isomer usually predominates because the ortho position is sterically hindered by the chlorine.

Answer: Mainly o- and p-chloronitrobenzene, with the para isomer as the major product.

4
Worked Example
Why is chloroform stored in dark coloured bottles filled to the brim, often with about 1% ethanol?
Solution
  1. In air and light chloroform is slowly oxidised: 2CHCl3 + O2 → 2COCl2 + 2HCl.
  2. The product phosgene (COCl2) is extremely poisonous, so air and light must be excluded.
  3. Dark bottles cut out light, filling to the brim leaves little air, and the small amount of ethanol converts any phosgene formed into harmless diethyl carbonate.

Answer: To prevent its photochemical oxidation by air to poisonous phosgene; the dark bottle and full filling exclude light and air, and ethanol destroys any phosgene formed.

5
Worked Example
Explain why freons (CFCs) are environmentally harmful despite being non-toxic and chemically stable.
Solution
  1. Their very stability lets them drift unchanged up to the stratosphere.
  2. There UV light cleaves a C–Cl bond, releasing chlorine free radicals.
  3. Each Cl radical destroys many ozone molecules in a chain (Cl + O3 → ClO + O2, then ClO regenerates Cl), thinning the protective ozone layer.

Answer: CFCs are so stable they reach the stratosphere, where UV frees Cl radicals that catalytically destroy ozone, damaging the ozone layer (hence the Montreal Protocol phase-out).

6
Worked Example
Give one major benefit and one major drawback of DDT as an insecticide.
Solution
  1. Benefit: it is a cheap, potent insecticide that has saved many lives by controlling malaria-carrying mosquitoes and crop pests.
  2. Drawback: it is chemically very stable and fat-soluble, so it does not break down quickly and bioaccumulates in fatty tissue up the food chain.
  3. This persistence harms birds and other wildlife (and insects develop resistance), which is why DDT is now banned or restricted in many countries.

Answer: Benefit — cheap, effective control of malaria mosquitoes and pests; drawback — it persists and bioaccumulates in the food chain, harming wildlife, so it is largely banned.

Key Points

  • Haloarenes resist nucleophilic substitution because resonance gives the C–X bond partial double-bond character (short, strong), the carbon is sp2, the phenyl cation is unstable, and the ring repels nucleophiles.
  • Replacement needs drastic conditions (chlorobenzene + NaOH, 623 K/300 atm → phenol); –NO2 groups ortho/para to X greatly increase reactivity.
  • Halogens are deactivating (−I) but ortho/para-directing (+R) in electrophilic substitution; chlorobenzene nitrates mainly o/p (para major).
  • With Na/dry ether: Wurtz–Fittig (aryl + alkyl halide → alkylarene) and Fittig (two aryl halides → biaryl).
  • Polyhalogens: CH2Cl2 (solvent), CHCl3 (→ toxic phosgene, store in dark/ethanol), CHI3 (antiseptic via I2), CCl4 (solvent, ozone/liver harm), freons (ozone depletion), DDT (persistent, bioaccumulating insecticide).
Quick Check — 4 MCQs
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Q1.The low reactivity of chlorobenzene toward nucleophilic substitution is best explained by:
Explanation: Resonance (lone-pair donation into the ring) gives the C–Cl bond partial double-bond character, making it shorter and stronger; together with the sp2 carbon this resists nucleophilic attack.
Q2.In electrophilic substitution of chlorobenzene the −Cl group is:
Explanation: The −I effect deactivates the ring (slower than benzene), while the +R effect makes the ortho and para positions electron-rich, so substitution is o/p-directing.
Q3.Chloroform is stored in dark bottles filled to the brim mainly to prevent its conversion to:
Explanation: Air and light slowly oxidise CHCl3 to poisonous phosgene; excluding light and air (and adding ~1% ethanol) prevents this.
Q4.Which compound is mainly responsible for depletion of the stratospheric ozone layer?
Explanation: CFCs (freons) reach the stratosphere and release Cl radicals under UV light, which catalytically destroy ozone; DDT is a persistent insecticide and iodoform an antiseptic.
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