Reaction with halogenated methyl radicals

About 5% of the ethyl radical adduct was isolated in the reaction with r-butyl iodide and about 20% of the ethyl radical adduct was formed in the reaction with i-propyl iodide. These ratios compare quite well with those calculated by using the rate constants for halogen transfer in the above reference and the rate constants for addition of a primary radical to methyl vinyl ketone. 

Non-activated methyl groups are never attacked in these reactions toluene can be functionalized but isolation of the resulting benzyl halides from these PTC mixtures is difficult. Most remarkable is that strained hydrocarbons such as cubane (5) or 2,4-didehydroadamantane (6) can also be halogenated with conservation of the cage [27], in marked contrast with the halogenation reactions of these substrates with halogen radicals [40]. Dihalogenations with either the same or a differ-... 

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. 

Consequently, ozone is a H atom acceptor similar to methyl radical or halogen atoms. This is evidence for the biradical nature of ozone and, hence, for a principal feasibility of the reaction proceeding through an intermediate biradical state by analogy with peroxide radical reactions. It may be expected that ozone as biradical may change to the triplet state readily the mechanism of the reaction through triplet states has not been elucidated yet. 

Step (3), attack of methyl radicals on halogen, is exothermic for all four halogens, and for chlorine, bromine, and iodine it has very nearly the same A/f. For these reactions. Fact could be very small, and does indeed seem to be so probably only a fraction of a kcal. Even iodine has been found to react readily with methyl radicals generated in another way, e.g., by the heating of tetramethyllead. In fact, iodine is sometimes employed as a free-radical trap or scavenger in the study of reaction mechanisms. The third step, then, cannot be the cause of the observed relative reactivities. 

Of the two chain-propagating steps, then, step (2) is more difficult than step (3) (see Fig. 2.8). Once formed, methyl radicals react easily with any of the halogens it is how fast methyl radicals are formed that limits the rate of overall reaction. Fluorination is fast because fluorine atoms rapidly abstract hydrogen atoms from methane E cx. is only 1 kcal. lodination does not take place because iodine atoms. And it virtually impossible to abstract hydrogen from methane tLCt is more than 33 kcal. 

Another class of compounds that have been used are the halogens and halogen acids, Cl2 S HCP, HBr and HH° . These react with radicals to give molecular products that are relatively stable (with X2), or very stable (with HX), where X stands for halogen. Clearly, the trapping reactions with methyl radicals will be (37) and (38)... 

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