Haloarenes and polyhalogen compounds

5.1 Preparation of haloarenes

Chlorine or bromine plus a Lewis acid catalyst, usually iron or iron(III) chloride, converts an arene into its aryl chloride or bromide by electrophilic substitution. The ortho and para isomers separate easily because their melting points differ widely. Iodination is reversible, so an oxidising agent such as nitric acid or periodic acid is added to remove the hydrogen iodide as it forms. Fluorination is not done this way because fluorine is too reactive.

The Sandmeyer route starts from a primary aromatic amine held in cold dilute mineral acid. Adding sodium nitrite converts it into a diazonium salt. Mixing that freshly made salt with cuprous chloride or cuprous bromide replaces the diazonium group with chlorine or bromine. Iodine needs no copper salt: shaking the diazonium salt with potassium iodide is enough.

5.2 Why haloarenes are so unreactive

Aryl halides are extremely slow to undergo nucleophilic substitution, and the textbook gives four reasons. First, the lone pairs on the halogen are conjugated with the ring pi electrons, so the carbon chlorine bond picks up partial double bond character and becomes harder to break. Second, the carbon carrying the halogen is sp2 hybridised. An sp2 carbon has more s character, so it is more electronegative and holds the shared pair more tightly; the carbon chlorine bond in chlorobenzene is shorter than in a haloalkane. Third, if the halogen left on its own the phenyl cation formed would have no resonance stabilisation, so the SN1 route is closed. Fourth, the ring is electron rich from the pi cloud, so an approaching electron rich nucleophile is repelled.

Substitution on an aryl ring is not impossible. Chlorobenzene can be converted to phenol by heating with aqueous sodium hydroxide at 623 K and about 300 atmospheres, which is a harsh set of conditions compared with the mild ones that work for a haloalkane. The rate can be brought down to normal temperatures by putting an electron withdrawing nitro group at the ortho or para position. The nitro group pulls electron density out of the ring, the nucleophile attacks more easily, and the negative charge on the intermediate is delocalised onto the nitro group. A meta nitro group does nothing, because the negative charge in the intermediate never lands on the carbon bearing the nitro group, so there is no stabilisation.

5.3 Electrophilic substitution

Haloarenes behave like other aromatic rings towards electrophiles and undergo halogenation, nitration, sulphonation and Friedel-Crafts reactions. The halogen is slightly deactivating but ortho and para directing, so the second substituent enters at the positions on either side of it or directly opposite. The reason is a tug of war between two effects. The inductive effect of the halogen pulls electrons away from the ring, which deactivates it and makes the ring need harsher conditions than benzene. The resonance effect releases the halogen lone pair into the ring and stabilises the intermediate, and that stabilisation is strongest at the ortho and para positions. Reactivity is set by the stronger inductive effect, orientation by the resonance effect.

5.4 Reactions with metals

The Wurtz-Fittig reaction joins an alkyl halide and an aryl halide with sodium in dry ether, giving an alkylbenzene. The Fittig reaction joins two aryl halides under the same conditions and gives a biaryl, such as biphenyl. These reactions show that the carbon halogen bond of an aryl halide can still be broken by a reactive metal even though a nucleophile struggles with it.

5.5 Polyhalogen compounds

Several polyhalogen compounds are useful enough to discuss by name. Dichloromethane is a solvent, paint remover and propellant, and it harms the central nervous system and the eyes and skin on contact. Chloroform is a solvent for fats, alkaloids and iodine, and it is used to make the refrigerant R-22; it was once a general anaesthetic, and it is stored in a filled dark bottle because light and air slowly convert it to the poisonous gas carbonyl chloride, also called phosgene. Iodoform was used as an antiseptic, its action coming from free iodine released in the tissue rather than from the compound itself. Carbon tetrachloride is a feedstock for refrigerants and propellants, and it damages the liver, the kidneys and the nerves; it also rises to the stratosphere and depletes the ozone layer.

Freons are the chlorofluorocarbons of methane and ethane. They are stable, unreactive, non-toxic, non-corrosive and easy to liquefy, which made them popular as refrigerants and aerosol propellants. Freon 12, CCl2F2, is made from carbon tetrachloride by the Swarts reaction. Their stability is exactly the problem: freons reach the stratosphere unchanged and start radical chain reactions that disturb the natural ozone balance.

DDT, p,p'-dichlorodiphenyltrichloroethane, was first prepared in 1873 and its insecticidal action was found by Paul Muller in 1939, work that brought him the Nobel Prize in Medicine in 1948. It helped control the mosquitoes that spread malaria and the lice that carry typhus. Insect resistance appeared, the compound is toxic to fish, and being fat soluble it is stored in fatty tissue instead of being broken down, so it builds up in animals over time. The United States banned it in 1973, though other countries still use it.