Radical writing mechanisms involving

Notice the use of fish-hook arrows to show the movement of a single electron. Such arrows are used a great deal when writing mechanisms for reactions involving radical species. 

All three fluorides XeF (n = 2,4,6) are prepared by the action of elemental F2 on Xe under various conditions of temperature and pressure. Almost certainly, the mechanisms involve radical reactions of F atoms. We won t be discussing them here, but feel free to sharpen your skills with fishhook arrows by writing out some of them. 

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... 

Once you have determined which type of mechanism you will write, use a systematic approach to the problem. At this point, we consider mostly the electrophilic reactions covered in recent chapters. Suggestions for drawing the mechanisms of reactions involving strong nucleophiles and free-radical reactions are collected in Appendix 3A. 

The difficulties involved in the direct determination of the momentary concentration of active centres are the most serious shortcoming in studies of termination itself. With radical polymerizations we at least know the most probable method of centre decay, and thus the molecular scheme of the termination reaction. In ionic and coordination polymerizations, the termination mechanism is mostly unknown. Quite generally we can write... 

It is tempting to write a radical coupling to form product 5-33, because it involves fewer steps. However, this mechanism would be energetically inefficient because it does not continue the chain process. Thus, the following is not the major product-forming step ... 

The process might very well involve single-electron transfers from Sg and various radical intermediates. Here, however, we ask you to write out a polar mechanism for the reaction. 

Unusual, but occasionally useful, reactions involve the addition of thiols or CX4 (X = Cl, Br) under radical conditions. Examples are shown in Figure 11.43—try writing a mechanism for each of them. 

---