Haloarenes Polyhalogen

6.9 Reactions of Haloarenes

6.9.1 Why Aryl Halides are Much Less Reactive Toward Nu⁻

Compare the rates: boiling C₆H₅Cl with aqueous NaOH at 1 atm does essentially nothing, whereas CH₃CH₂Cl is hydrolysed in hours under the same conditions. Three structural reasons explain this sluggishness:

1. Resonance stabilisation. A lone pair on X delocalises into the ring, giving the C–X bond partial double-bond character. A double bond is harder to break than a single bond.
2. sp² vs sp³ carbon. The ring carbon bonded to X is sp²-hybridised, so its orbital is 33 % s-character (vs 25 % in sp³). Electrons in higher-s-character orbitals are held more tightly by the nucleus, making the C–X bond shorter and stronger.
3. Destabilised transition state. Adding Nu⁻ to the ring disrupts aromaticity; the energy cost of losing the 6π cloud is enormous.

6.9.2 Nucleophilic Substitution (only under harsh conditions)

Aryl halides can be forced to react if we either apply brute-force conditions or decorate the ring with electron-withdrawing groups at the ortho or para position.

When a nitro group sits ortho or para to Cl, the chloride is hydrolysed easily; two nitro groups at o,p make it easier still; three of them (picryl chloride) reacts with cold aqueous NaOH:

Mechanism — Addition–Elimination via a Meisenheimer Complex

6.9.3 Electrophilic Substitution on Haloarenes

In spite of its electron-withdrawing inductive effect, a halogen is a weak ortho-/para-director because its lone pair can donate into the ring by resonance. The ring is slightly deactivated compared to benzene, but only at the meta position — the ortho and para positions remain reactive.

6.9.4 Reactions with Metals

Wurtz Reaction (haloalkane only)

Fittig Reaction (aryl halide only)

Wurtz–Fittig Reaction (alkyl + aryl halide mixed)

Grignard Reagent (R–Mg–X or Ar–Mg–X)

The Grignard reagent is one of the most versatile synthons in organic chemistry: it carries a highly polar C–Mg bond (carbon is δ⁻, which is the opposite of what it is in R–X) and acts as a carbanion equivalent. It will add to carbonyls, open epoxides, couple with other halides, and must be kept scrupulously dry (water destroys it: R–MgX + H₂O → R–H + Mg(OH)X).

6.10 Polyhalogen Compounds

Compounds carrying two or more halogen atoms often have properties very different from their mono-halide cousins — useful ones, and, as we will see, some deeply troubling ones.

Dichloromethane (CH₂Cl₂)

Common laboratory solvent, paint stripper and aerosol propellant. Long-term inhalation causes CNS damage, impaired hearing and vision, and at high levels is metabolised to CO which starves tissues of oxygen.

Trichloromethane (Chloroform, CHCl₃)

Obstetrician James Young Simpson introduced chloroform as a surgical anaesthetic in 1847. It is now replaced by safer agents (halothane, sevoflurane) because of cardiac arrhythmia, liver damage and carcinogenicity. Chloroform slowly oxidises in air and light to the extremely poisonous gas phosgene (COCl₂):

For this reason, chloroform is stored in dark amber bottles filled to the brim; 1 % ethanol is added to convert any phosgene formed into harmless diethyl carbonate.

Triiodomethane (Iodoform, CHI₃)

Yellow crystalline solid with a distinctive hospital smell; was used as an antiseptic. Ethanol, acetaldehyde, acetone and any methyl ketone give a positive iodoform test — a yellow precipitate of CHI₃ on treatment with I₂/NaOH.

Tetrachloromethane (Carbon tetrachloride, CCl₄)

Formerly used as a fire-extinguisher ("Pyrene") and a dry-cleaning solvent. Banned under the Montreal Protocol (1987) because it destroys ozone and is hepatotoxic. Acute exposure causes liver and kidney damage; chronic exposure causes cancer.

Freons (CFCs)

Chlorofluorocarbons, chiefly CCl₂F₂ (Freon-12) and CCl₃F (Freon-11), are stable, non-flammable, non-toxic — ideal for refrigerators and aerosol cans. Precisely because they are so unreactive they survive long enough to drift up to the stratosphere, where UV cleaves a C–Cl bond and liberates Cl atoms:

DDT — p,p′-Dichlorodiphenyltrichloroethane

First synthesised in 1874, DDT was identified as an insecticide by Paul Müller in 1939 (Nobel Prize, 1948). It helped eliminate malaria from wide swathes of Europe and North America. But DDT is lipid-soluble and chemically inert — it persists in fat tissue and biomagnifies up the food chain, culminating in eggshell thinning in predatory birds, famously documented by Rachel Carson's Silent Spring (1962). DDT is now banned for agricultural use in most countries; a restricted indoor-spraying role is permitted against malaria vectors where no safer alternative exists.

Environmental Themes

• Persistent organic pollutants (POPs): CFCs, DDT, dioxins, PCBs — Stockholm Convention (2001) targets for phase-out.
• Bioaccumulation: lipid-soluble halocompounds concentrate up food chains; apex predators (eagles, orcas, humans) carry the highest body burdens.
• Stratospheric ozone depletion: CFCs → Cl atoms → catalytic destruction of O₃; addressed by the Montreal Protocol and its amendments.

Worked Examples