Radical halogenation, methane

Hsu, K. J., and W. B. DeMore, Rate Constants and Temperature Dependences for the Reactions of Hydroxyl Radical with Several Halogenated Methanes, Ethanes, and Propanes by Relative Rate Measurements, J. Phys. Chem., 99, 1235-1244 (1995). 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

The effect of wavelength upon the relative proportion of the products is very marked. When the 2537 A. line from a low-pressure mercury arc is used, the yield of all three ethanes is drastically reduced, while the yield of both halogenated methanes is enhanced. The 1,3-dichlorohexafluoropropane can no longer be detected. This effect is unlikely to be due to alternative primary processes, such as mercury photosensitization, since it is also observed under flash photolysis conditions. As before, it is considered that the additional energy given to the radicals at shorter wavelengths, increases the rate of abstraction and decreases the rate of combination because of the third-body restriction. Some evidence for this... 

One important prerequisite to the application of this reaction in hydrocarbon synthesis is the selective monochlorination of methane. Usual radical chlorination of methane is not selective, and high CH4 CI2 ratios are needed to minimize formation of higher chlorinated methanes (see Section 10.2.5). In contrast with radical halogenation, electrophilic halogenation of methane was shown to be a highly selective process.412... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere, bromotrichloromethane, which has a measured vapor pressure of 39mmHg at 25°C, is expected to exist solely as a vapor in the ambient atmosphere. Based on bromotrichloromethane s structural similarity to bromotrifluoromethane, it is expected to slowly degrade in the atmosphere by reaction with photochemically produced hydroxyl radicals the half-life for bromotrichloromethane s reaction in air is estimated to be greater than 44 years. Photolysis may occur based on bromotrichloromethane s structural similarity to other halogenated methane compounds but not at an environmentally relevant rate. 

Radical chemistiy has advanced tremendously since the discovery of triphenylmethyl radical in 1900 by Goomberg.l The early synthetic work started with Kharasch addition reaction (Scheme 1)2,3 in which halogenated methanes were directly added to olefinic bonds in the presence of free radical initiators or light. However, it was not until early 1980s that the full potential of... 

Because a high yield of one particular compound is usually needed, and the separation of product mixtures is often difficult, radical halogenations are rarely used with substrates other than simple hydrocarbons. This is not to say that this reaction type is unimportant the chlorination of methane is a major industrial process. Chloromethane is not the only product obtainable if the ratio of the reactants and the reaction conditions are varied, dichloromethane (CH2CI2), trichloromethane (chloroform, CHCI3) and tetrachloromethane (carbon tetrachloride, CCI4) can all be produced. 

In general, the radical halogenation of alkanes is indiscriminate, with substitution occurring at every carbon-hydrogen bond. This limits the usefulness of this type of reaction to small hydrocarbon molecules. However, some such reactions (for example, the chlorination of methane) are of great industrial importance. 

Other Radical Halogenations of Methane Similarities and differences. 

The reaction of methane with chlorine (in the gas phase) provides a good example for studying the mechanism of radical halogenation. 

An ab initio study of the kinetics of the reactions of halomethanes with the hydroxyl radical. 2. A comparison between theoretical and experimental values of the kinetic parameters for 12 partially halogenated methanes " ... 

Mechanism and electron pushing for the free radical halogenation of methane. 

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. 

What happens in the radical halogenation of other alkanes Will the different types of R-H bonds—namely, primary, secondary, and tertiary— react in the same way as those in methane As we saw in Exercise 3-4, the monochlorination of ethane gives chloroethane as the product. 

We know that bromine is less reactive than chlorine in the rate-determining step of halogenation of methane. We also recall that bromine is more selective than chlorine. Both the rate of reaction and the selectivity of free radical halogenation are related to the first propagation step, so let s look at the relationship between reactivity and selectivity in terms of the structure of the transition states for chlorination and bromination. 

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