Chlorination, of methane

Chlorination of methane provides approximately one third of the annual U.S. production of chloromethane. The reaction of methanol with hydrogen chloride is the major synthetic method for the preparation of chloromethane. 

Chlorination of methane is a reaction of industrial importance and is carried out in the gas phase to give a mixture of chloromethane (CH3CI), dichloromethane (CH2CI2), trichloromethane (CHCI3), and tetrachloromethane (CCI4). 

Dichloromethane, trichloromethane, and tetrachloromethane are widely known by their common names methylene chloride, chloroform, and carbon tetrachloride, respectively. 

One of the chief uses of chloromethane is as a starting material from which silicone polymers are made. Dichloromethane is widely used as a paint stripper. Trichloromethane was once used as an inhalation anesthetic, but its toxicity caused it to be replaced by safer materials many years ago. Tetrachloromethane is the starting material for the preparation of several chlorofluorocarbons (CFCs), at one time widely used as refrigerant gases. Most of the world s industrialized nations have agreed to phase out all uses of CFCs because these compounds have been implicated in atmospheric processes that degrade the Earth s ozone layer. 

The chlorination of methane is carried out at rather high temperatures (400 40°C), even though each substitution in the series is exothermic. The high temperature provides 

Dichioromethane, trichioromethane, and tetrachioromethane are wideiy known by their common names methyiene chioride, chioroform, and carbon tetrachioride, respectiveiy. 

We will now use the skills developed in the previous section to explore the mechanisms of radical reactions. As a first example, consider the reaction between methane and chlorine to form methyl chloride  

Evidence suggests that this reaction proceeds via a radical mechanism (Mechanism 11.1). 

This now provides us with a new, refined definition for propagation steps. Specifically, the sum of the propagation steps gives the net chemical reaction. AU other steps must be either initiation or termination, not propagation. We will revisit this refined definition later in the chapter. 

There is one more critical aspect of the propagation steps shown above. Notice that the first propagation step consumes a chlorine radical, while the second propagation step regenerates a chlorine radical. In this way, one chlorine radical can ultimately cause thousands of molecules of methane to be converted into chloromethane (assuming enough Cl, is present). Therefore, the reaction is called a chain reaction. 

The initial product, methyl chloride (CH3CI), is even more reactive toward radical halogenation than methane. As methyl chloride is formed, it reacts with chlorine to produce methylene chloride. The process continues until carbon tetrachloride is produced. In order to produce methyl chloride as the major product (monohalogenation), it is necessary to use an excess of methane and a small amount of Cl,. Unless otherwise indicated, the conditions of a halogenation reaction are generally controlled so as to produce monohalogenation. 

This reaction may continue heat or light is needed for each step  

This sequence raises several questions about the chlorination of methane. Why is heat or light needed for the reaction to go Why do we get a mixture of products Is there any way to modify the reaction to get just one pure product Are the observed products formed because they are the most stable products possible Or are they favored because they are formed faster than any other products  

The answers to these questions involve three aspects of the reaction the mechanism, the thermodynamics, and the kinetics. 

The mechanism is the complete, step-by-step description of exactly which bonds break and which bonds form in what order to give the observed products. 

Thermodynamics is the study of the energy changes that accompany chemical and physical transformations. It allows us to compare the stability of reactants and products and predict which compounds are favored by the equilibrium. 

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CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

Methylene chloride CHjCl, b.p. 41°, is obtained as a by product in the com mercial preparation of chloroform by the reduction of carbon tetrachloride with moist iron and also as one of the products in the chlorination of methane it is a useful extraction solvent completely immiscible with water. 

Like carbocations most free radicals are exceedingly reactive species—too reac tive to be isolated but capable of being formed as transient intermediates m chemical reactions Methyl radical as we shall see m the following section is an intermediate m the chlorination of methane... 

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

Like the chlorination of methane chlorination of ethane is carried out on an industrial scale as a high temperature gas phase reaction... 

Consider the chlorination of methane to chloromethane The heats of formation of the reac tants and products appear beneath the equation These heats of formation for the chemical com pounds are taken from published tabulations the heat of formation of chlorine as it is for all elements IS zero... 

Thus the chlorination of methane is calculated to be an exothermic reaction on the basis of heat of forma tion data... 

Compare chlorination of methane with lodina tion The relevant bond dissociation energies are given in the equation... 

As in the chlorination of methane it is often difficult to limit the reaction to monochlo rmation and derivatives having more than one chlorine atom are also formed... 

Chlorination of methane and halogenation of alkanes generally proceed by way of free radical intermediates Alkyl radicals are neutral and have an unpaired electron on carbon... 

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

Thermal chlorination of methane was first put on an industrial scale by Hoechst in Germany in 1923. At that time, high pressure methanol synthesis from hydrogen and carbon monoxide provided a new source of methanol for production of methyl chloride by reaction with hydrogen chloride. Prior to 1914 attempts were made to estabHsh an industrial process for methanol by hydrolysis of methyl chloride obtained by chlorinating methane. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

A representative technical grade of methyl chloride contains not more than the following indicated quantities in ppm of impurities water, 100 acid, such as HCl, 10 methyl ether, 20 methanol, 50 acetone, 50 residue, 100. No free chlorine should be detectable. Traces of higher chlorides are generally present in methyl chloride produced by chlorination of methane. The boiling point should be between —24 and —23° C, and 5—95% should distill within a range of about 0.2°C. It should be clear, colorless, and free from visible impurities. 

Exothermic processes, with cooling through heat transfer surfaces or cold shots. In use are sheU-and-tube reactors with smaU-diameter tubes, or towers with internal recirculation of gases, or multiple stages with intercoohng. Chlorination of methane and other hydrocarbons results in a mixture of products whose relative amounts... 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

As mentioned in Chapter 2, methane is a one-carhon paraffinic hydrocarbon that is not very reactive under normal conditions. Only a few chemicals can he produced directly from methane under relatively severe conditions. Chlorination of methane is only possible by thermal or photochemical initiation. Methane can be partially oxidized with a limited amount of oxygen or in presence of steam to a synthesis gas mixture. Many chemicals can be produced from methane via the more reactive synthesis gas mixture. Synthesis gas is the precursor for two major chemicals, ammonia and methanol. Both compounds are the hosts for many important petrochemical products. Figure 5-1 shows the important chemicals based on methane, synthesis gas, methanol, and ammonia. ... 

As an example of an industrially useful radical reaction, look at the chlorination of methane to yield chloromethane. This substitution reaction is the first step in the preparation of the solvents dichloromethane (CHoCl ) and chloroform (CHCI3). 

Figure 10.1 Mechanism of the radical chlorination of methane. Initiation step Three kinds of steps are required initiation, propagation, and termination. The propagation steps are a repeating cycle, with Cl- a reactant in step 1 and a product in...

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