Halogenation . Halogenation constitutes in most cases the chlorination of a certain chemical compound, because the difference in cost between chlorine and the rest of the halogens, particularly on a molar basis, is quite substantial. In some cases, the presence of bromine (Br), iodine (I) and fluorine (F) confers additional properties to a certain chemical compound that make it possible to ensure its manufacture.
Summary
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- 1 Obtaining
- 2 Chlorination process
- 3 Equipment selection
- 4 Low temperature chlorination
- 5 Chlorination reactions
- 6 Use of chlorobenzene
- 7 Obtaining chlorobenzene
- 8 Sources
Obtaining
Chlorination takes place by:
- adding an unsaturated link,
- hydrogen substitution
- replacement of another group such as hydroxyl (–OH) or sulfonic (–SO 3H).
Light catalyzes some chlorination reactions, temperature has a profound influence, and polychlorination almost always occurs to some degree. All halogenation reactions are strongly exothermic .
Chlorination process
In the chlorination process, chlorine and methane (fresh and recycled) are fed in a 0.6 / 1.0 ratio to the reactor , in which the temperature is maintained between 340 – 370 ºC. The reaction product contains chlorinated hydrocarbons together with unreacted methane, hydrogen chloride, chlorine and heavy chlorinated products. Secondary chlorination reactions take place at room temperature in a light-catalyzed reactor, which converts methylene chloride to chloroform, then converts the latter to carbon tetrachloride in another reactor. By varying the ratio of the reagents, the temperatures and the recirculation ratio, it is possible to alter to a certain extent the concentration of the final product in order to satisfy market demands. Ignition during the manufacturing process can be avoided by using narrow channels and high speeds in the reactor. The chlorine conversion is total, and the methane conversion is around 65%.
Equipment Selection
The selection of the equipment necessary to carry out the chlorination reactions is very difficult, since the combination of halogens , oxygen , halogenated acids , water and heat is particularly corrosive. Alloys such as Hastelloy and Duriclor resist very well and are used very often, while glass, glass- coated steel and tantalum can also be used , which are very strong although not always available. Anhydrous conditions allow the use of steels or nickel alloys . In non-aqueous media it is more appropriate to use appliances constructed of iron and lined with plastics and / or lead or tiles.
Chlorination at low temperature
When chlorination is to be carried out at a low temperature, it is often beneficial to circulate cooling water through the interior of a lead coil located inside the chlorinator or to circulate the reaction mixture through an external cooling system rather than employing a outer jacket. When the temperature must be maintained at or below 0ºC, a calcium chloride brine is used , which is cooled by means of a cooling device.
Chlorination reactions
Most chlorination reactions produce hydrogen chloride as a by-product, and a method has been investigated to make this compound useful for later uses: However, this is not an equilibrium reaction, which has a tendency in favor of hydrogen. The reaction can be used and completed using an oxychlorination procedure, in which the chlorine , once formed, is reacted as quickly as possible with a reactive substance, thus leading to the completion of the reaction. The oxychlorination of methane can be given as an example of the above: This chlorination can be carried out using chlorine, although one mole of hydrogen chloride will be formed for each atomof chlorine introduced into methane, and this should be discarded to prevent environmental contamination. In this way, hydrogen chloride is frequently used as a by-product of other processes, while the use of cuprous chloride (CuCl) and cupric chloride (CuCl 2 ), along with some amount of potassium chloride (KCl) as a catalyst of molten salts, will improve the progress of the reaction. The ethane can be chlorinated under conditions very similar to those found in the chlorination of methane , which deliver a mixture of chloroethanes.
Chlorobenzene use
Chlorobenzene is used as a solvent and to make nitrochlorobenzenes.
Obtaining chlorobenzene
This compound is obtained by passing dry chlorine through benzene , using ferric chloride (FeCl 3) as catalyst: The reaction rate favors the production of chlorobenzene over dichlorobenzene in a ratio of 8.5: 1, which is obtained by keeping the temperature below 60 ºC. The generated hydrogen chloride, which is free of chlorine, is washed with benzene, to then be absorbed in water. Distillation separates chlorobenzene, leaving a mixture of dichlorobenzene isomers. In aqueous medium, when you have hydrochloric acid in both the liquid and vapor phases, and particularly under high pressure conditions, tantalum is undoubtedly the most resistant material for the construction of the containers that will handle these fluids. Reactors and catalytic tubes lined with this metal provide satisfactory service for extended periods of time.