Methane and chlorine

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

An industrial example of series reactions is the substitution process involving methane and chlorine ... 

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

When a mixture of methane and chlorine is irradiated, reaction commences immediately. When irradiation is stopped, the reaction gradually slows down but does not stop immediately. Explain. 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

In CfE Higher Chemistry, you came across free radicals when we considered the mechanism of the substitution reaction between methane and chlorine In the presence of ultraviolet light. You will recall that the initiation step In the mechanism Is the homolytic fission of chlorine molecules to generate chlorine free radicals. 

In the manufacture of methyl chloride a mixture of methane and chlorine is mixed with recycled gas and fed into a reactor. The CH4 Cl2 ratio is 4-5. The reactor temperature is maintained by regulating the gas feed rate. In methane chlorination plants, 95-96% chlorine yields and 90-92% methane yields are typical.176 All four chloromethanes are formed with a typical composition of 35 wt% methyl chloride, 45 wt% methylene dichloride, 20 wt% chloroform, and a small amount of carbon tetrachloride.178 The reaction, however, may be regulated so that either mono- or tetrachlorination predominates.177 179 180... 

Chlorination can also be catalytic 8, 9, or oxide-bound allylrhodium hydride catalyzes the reaction between methane and chlorine to yield chlorinated methanes predominantly methyl chloride. 

The equation for the reaction is simple, the ingredients are cheap, and the product is useful. However, if we want to decide in advance whether such a reaction is actually feasible, we have to know more. Particularly, we have to know whether the reaction proceeds in the direction it is written and, if so, whether conditions can be found under which it proceeds at a convenient rate. Obviously, if one were to mix methane and chlorine and find that, at most, only 1% conversion to the desired product occurred and that the 1% conversion could be achieved only after a day or so of strong heating, this reaction would be both too unfavorable and too slow for an industrial process. 

Presumably, methane could react with chlorine to give chloromethane and hydrogen chloride, or chloromethane could react with hydrogen chloride to give methane and chlorine. If conditions were found for which both reactions proceeded at a finite rate, equilibrium finally would be established when the rates of the reactions in each direction became equal ... 

To reach an understanding of why methane and chlorine do not react in the dark, we must consider the details of how the reaction occurs—that is, the reaction mechanism. The simplest mechanism would be for a chlorine molecule to collide with a methane molecule in such a way as to have chloromethane and hydrogen chloride formed directly as the result of a concerted breaking of the Cl-Cl and C-H bonds and making of the C-Cl and H-Cl bonds (see Figure 4-5). The failure to react indicates that there must be an energy barrier too high for this mechanism to operate. Why should this be so ... 

I. e net result of CH4 + Cl- — CH3- + HC1 and CH3- + Cl2 —> CH3CI + Cl- is formation of chloromethane arid hydrogen chloride from methane and chlorine. Notice that the chlorine atom consumed in the first step is replaced by another one in the second step. This kind of sequence of reactions is called a chain reaction because, in principle, one atom can induce the reaction of an infinite number of molecules through operation of a chain or cycle of reactions. In our example, chlorine atoms formed by the action of light on... 

Another useful reaction worth noting is that between the alkanes and the halogens. For example, methane and chlorine react in the presence of sunlight (or ultraviolet light). The ultraviolet light splits the chlorine molecules into atoms. When this type of reaction takes place, these atoms are called free radicals and they are very reactive. 

Quantum yields for photochemically induced low-temperature reactions are also sensitive to intermolecular effects (i.e., the structure of the solid). For example, the quantum yield for reaction (9.15) in glassy mixtures of methane and chlorine at 20 K varies from 0.1 to 0.001, depending on the method of sample preparation [Benderskii et al., 1983]. The lowest values are obtained for samples prepared by thermal or ultrasonic annealing prior to UV photolysis. 

Hydrocarbons. Methane and chlorine explode in the presence of mercury oxide. Mixtures of chlorine and ethylene explode in sunlight or in the presence of mercury or... 

In the reaction of a 1 1 mixture of methane and chlorine one does not obtain the monochlorination product selectively, but a 46 23 21 9 1 mixture of unreacted methane, mono-, di-, tri-, and tetrachloromethane. Thus, all conceivable multiple chlorination products are also produced. Multiple chlorinations, like monochlorinations, occur as radical chain substitutions. They are based on completely analogous propagation steps (Figure 1.19). 

Radical chlorination is a difficult reaction to control. As the reaction proceeds and the initial product, chloromethane, accumulates, it can also undergo hydrogen abstraction by a chlorine atom, resulting in the formation of dichloromethane. Chloroform is formed from dichloromethane and carbon tetrachloride from chloroform in a similar manner. The reaction of a 1 1 ratio of methane and chlorine at 440°C (at this high temperature. homolytic fission of the chlorine-chlorine bond occurs without light) results in the product mixture shown in the following equation ... 

We can put known amounts of methane and chlorine into a bomb calorimeter and use a hot wire to initiate the reaction. The temperature rise in the calorimeter is used to calculate the precise value of the heat of reaction, AH°. This measurement shows that 105 kJ (25 kcal) of heat is evolved (exothermic) for each mole of methane converted to chloromethane. Thus, AH° for the reaction is negative, and the heat of reaction is given as... 

Kinetics is the study of reaction rates. How fast a reaction goes is just as important as the position of its equilibrium. Just because thermodynamics favors a reaction (negative AG°) does not necessarily mean the reaction will actually occur. For example, a mixture of gasoline and oxygen does not react without a spark or a catalyst. Similarly, a mixture of methane and chlorine does not react if it is kept cold and dark. 

Laboratory halogenations show that our predictions are right. In fact, fluorine reacts explosively with methane, and chlorine reacts at a moderate rate. A mixture of bromine and methane must be heated to react, and iodine does not react at all. 

Preparation of Alkyl Halides.—We have spoken of the formation of the alkyl halides by the direct action of the halogen upon the saturated hydrocarbon. In the case of chlorine this action takes place at ordinary temperatures as in the reaction between methane and chlorine in the sunlight. Bromine, however, does not act directly at ordinary temperatures but by heating in a sealed tube. Iodine does not act directly with the hydrocarbons. In any case the result is a mixture of several substitution products, and the method is not, therefore, of practical value. Where direct action does not occur the presence of iodine chloride or antimony chloride, which act as carriers, is necessary. The two reactions of most importance in the preparation of these compounds are those involving either alcohols or unsaturated hydrocarbons. These will be taken up when these compounds are studied. 

For simplicity we will look at the reaction of methane and chlorine. 

It is not easy to make chloromethane alone from the reaction between methane and chlorine. Explain why. 

The impurities in silicium tetrabromide were identified The packing irreversibly adsorbs SiBr4 methane and chlorine were not separated, but they could be determined simultaneously by using FID for CH4 and an ECD for chlorine. 

Under the influence of ultraviolet light or at a temperature of 250-400 a mixture of the two gases, methane and chlorine, reacts vigorously to yield hydrogen chloride and a compound of formula CH3CI. We say that methane has undergone chlorination, and we call the product, CH Cl, chloromethane or methyl chloride (CH3 = methyl). 

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