Halogenation is almost always chlorination, for the difference in cost between chlorine and the other halogens, particularly on a molar basis, is quite substantial. In some cases, the presence of bromine (Br), iodine (I), or fluorine (F) confers additional properties to warrant manufacture. Chlorination proceeds (1) by addition to an unsaturated bond, (2) by substitution for hydrogen, or (3) by replacement of another group such as hydroxyl (–OH) or sulfonic (–SO3H). Light catalyzes some chlorination reactions, temperature has a profound effect, and polychlorination almost always occurs to some degree. All halogenation reactions are strongly exothermic.
In the chlorination process (Fig.1), chlorine and methane (fresh and recycled) are charged in the ratio 0.6/1.0 to a reactor in which the temperature is maintained at 340 to 370oC. The reaction product contains chlorinated hydrocarbons with unreacted methane, hydrogen chloride, chlorine, and heavier chlorinated products. Secondary chlorination reactions take place at ambient temperature in a light-catalyzed reactor that converts methylene chloride to chloroform, and in a reactor that converts chloroform to carbon tetrachloride. By changing reagent ratios, temperatures, and recycling ratio, it is possible to vary the product mix somewhat to satisfy market demands. Ignition is avoided by using narrow channels and high velocities in the reactor. The chlorine conversion is total, and the methane conversion around 65 percent.
Equipment for the commercial chlorination reactions is more difficult to select, since the combination of halogen, oxygen, halogen acid, water, and heat is particularly corrosive. Alloys such as Hastelloy and Durichlor resist well and are often used, and glass, glass-enameled steel, and tantalum are totally resistant but not always available. Anhydrous conditions permit operation with steel or nickel alloys. With nonaqueous media, apparatus constructed of iron and lined with plastics and/or lead and glazed tile is the most suitable, though chemical stoneware, fused quartz, glass, or glass-lined equipment can be used for either the whole plant or specific apparatus.
When chlorination has to be carried out at a low temperature, it is often beneficial to circulate cooling water through a lead coil within the chlorinator or circulate the charge through an outside cooling system rather than to make use of an external jacket. When the temperature is to be maintained at 0o C or below, a calcium chloride brine, cooled by a refrigerating machine, is employed.
Most chlorination reactions produce hydrogen chloride as a by-product, and a method was searched for to make this useful for further use:
4HCl + O2 ? 2H2O + 2C12
However, this is not a true equilibrium reaction, with a tendency to favor hydrogen chloride. The reaction can be used and driven to completion by use of the oxychlorination procedure that reacts the chlorine with a reactive substance as soon as it is formed, thus driving the reaction to completion as, for example, in the oxychlorination of methane:
CH4 + HCl + O2 ? CH3Cl + CH2Cl2 + CHCl3 + CCl4 + H2O
This chlorination can be accomplished with chlorine but a mole of hydrogen chloride is produced for every chlorine atom introduced into the methane, and this must be disposed of to prevent environmental pollution. Thus, the use of by-product hydrogen chloride from other processes is frequently available and the use of cuprous chloride (CuCl) and cupric chloride (CuCl2), along with some potassium chloride (KCl) as a molten salt catalyst, enhances the reaction progress.
Ethane can be chlorinated under conditions very similar to those for methane to yield mixed chlorinated ethanes.
Chlorobenzene is used as a solvent and for the manufacture of nitrochlorobenzenes. It is manufactured by passing dry chlorine through benzene, using ferric chloride (FeCl3) as a catalyst:
C6H6 + C12 ? C6H5Cl + HCl
The reaction rates favor production of chlorobenzene over dichlorobenzene by 8.5:1, provided that the temperature is maintained below 60o C. The hydrogen chloride generated is washed free of chlorine with benzene, then absorbed in water. Distillation separates the chlorobenzene, leaving mixed isomers of dichlorobenzene.
In aqueous media, when hydrochloric acid is present in either the liquid or vapor phase and particularly when under pressure, tantalum is undoubtedly the most resistant material of construction. Reactors and catalytic tubes lined with this metal give satisfactory service for prolonged periods.