Methane, bromination mechanism

Methane, bromination mechanism, 60 Methide shift, 94 Methylene in synthesis, 67 Methyl salicylate, 440 Michael addition, 385, 459 Microscopic reversibility, 98 Migratory aptitude, 293 Molecular orbital, 14 Molecularity, 40 Molecules, geometry of, 18 polar, 27... 

Chlorination (and bromination) of alkanes such as methane, CH., has a radical-chain mechanism, as follows INITIATION STEP... 

It will be worthwhile to examine the mechanism of chlorination of methane in some detail. The same mechanism holds for bromination as well as chlorination, and for other alkanes as well as methane it even holds for many compounds which, while not alkanes, contain alkane-like portions in their molecules. Closely... 

Suggest a reasonable mechanism for this reaction. Hint Bromine vapor is deep red methane is colorless.)... 

If propagation reactions compete favorably with termination reactions, the formation of two chlorine radicals could result in the reaction of many molecules of methane. It should be noted that this effect is important in the loss of stratospheric ozone resulting from the production of chlorine or bromine radicals from freons. Free radicals are very important in biological processes involving oxygen and their production is involved in some toxicological mechanism. In organic chemistry, free radicals are a major factor in polymerization processes. 

For example, the reaction of bromine with methane in the presence of light is an example of a chain reaction. The mechanism by which the reaction proceeds is as follows. 

The overall A//° is not very different from those of typical alkane C—H bonds methane, A//° = —6 kcal mol-1 1° C—H, A//° = -10 2° C—H, Atf° = -13.5 3° C—H, A//° = -15.5. However, the rate-determining first propagation step in the reaction of benzene is much more endothermic than any of the alkane reactions, due to the exceptional strength of the C—H bonds in benzene. The result is that bromination of benzene by this mechanism is exceedingly difficult (very slow) and does not compete x kinetically with bromination reactions of typical alkanes. 

In the previous section, we explored the proposed mechanism for the chlorination of methane. We will now explore whether this reaction can be accomplished with other halogens as well. Is it possible to achieve a radical fluorination, bromination, or iodination To answer this question, we must explore some thermodynamic aspects of haiogenation. 

If the mechanisms for fluorination, bromination, and iodination of methane are the same as for its chlorination, we can explain the wide variation in reactivity of the halogens by a careful examination of A//° and act for each step. 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

Fluorine and bromine, but not iodine, also react with methane by similar radical mechanisms to furnish the corresponding halomethanes. The dissociation energies of X2 (X = F, Br, I) are lower than that of CI2, thus ensuring ready initiation of the radical chain (Table 3-4). 

In Summary Fluorine, chlorine, and bromine react with methane to give halomethanes. All three reactions follow the radical chain mechanism described for chlorination. In these processes, the first propagation step is always the slower of the two. It becomes more exo-... 

Section 3-4 presented the mechanism for the chlorination of methane in full, step-by-step detail. Section 3-5 discussed the reactions of methane with the other three halogens, but it did not actually display either the full equations for any of these reactions, nor did it illustrate any of the individual steps of their mechanisms. Why not The reactions of methane with the four halogens all proceed through mechanisms that are qualitatively identical to one another. To write a mechanism for the reaction of methane with fluorine, bromine, or iodine you need only (1) copy the mechanism for reaction with chlorine and (2) replace every Cl in those equations with the symbol for the new halogen. The energy values will be different because the bond strengths are different, but the overall appearance of the mechanisms will be the same. Solved Exercise 3-4 already illustrated this strategy for the chlorination of ethane. Let us try some examples ... 

Methane reacts with bromine to give bromomethane and hydrogen bromide. The mechanism forthe reaction is called free-radical substitution and involves homolytic fission of chemical bonds. The reaction proceeds via initiation, propagation and termination steps, a By what mechanism does bromine react with methane b Write a balanced symbol equation forthis reaction, c Bonds break in this reaction. What type of bond breaking is involved d What essential conditions are required forthis reaction Why e Forthis reaction, write down an equation for an initiation step a propagation step... 

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