Radical halogenations of methane

Other Radical Halogenations of Methane Similarities and differences. 

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

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... 

Typical radical reactions are substitution and addition reactions as shown below (Scheme b). A typical substitution reaction is the halogenation of methane with chlorine gas under photolytic conditions, and generally available chlorohydrocarbons are prepared by this method. The chlorination reaction proceeds through a chain pathway via the initiation step, propagation step, and termination step as shown below (Scheme 1.1). 

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. 

Alkenes can undergo free radical substitution reactions with halogens. Which of the following best represents a chain propagation step during the free radical chlorination of methane ... 

Concerning the electrophilic halogenation of methane it should also be pointed out that the singlet-triplet energy difference of positive halogens (as illustrated for the hypothetical ions) favor the latter Electrophilic halogenations thus may be more complex and can involve radical ions even under conditions where conventional radical chain halogenation is basically absent. 

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. 

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... 

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. 

In the light of the more complete study of ring-halogenated triphenylchloro-methanes in this paper, the free radical hypothesis was back - if it ever was excluded in the previous paper - in the final discussion of the constitution of triphenylmethyl , now with two tautomeric triphenylmethyl radical structures in equilibrium with each other and the Jacobson dimer 1 (Scheme 2). Note that the radical was symbolized by an open valence (a thick line is used here for clarity). The strong results obtained with 3 (Scheme 1) were explained by removal of the quinoid bromine atom from 4 giving a radical 6 which tautomerized to the triphenylmethyl analogue 7. By analogy with the... 

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

We now apply what we know about rates to the reaction of methane with halogens. The rate-limiting step for chlorination is the endothermic reaction of the chlorine atom with methane to form a methyl radical and a molecule of HC1. 

The mechanisms of methane-halogen reactions are chain radical processes. Carbon tetrachloride Is also prepared from carbon disulfide and chlorine. Sulfur is formed in this reaction, but it is recovered and used for the regeneration of CS3. Carbon disulfide is obtained from the elements, but is also formed from methane and sulfur (see Section 1,1.5.4). 

Halogenation of Alkanes - Reaction Mechanism Section 4.4D The ethyl radicals formed from the decomposition of tetraethyllead can react with methane to form methyl radicals or with chlorine to form chlorine radicals. Both of these are part of the propagation steps. 

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. 

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