Bond making, homolytic

In radical reactions, bonds break homolytically with one electron going one way and one the other. The radicals that are formed have an odd number of electrons, one of which must be unpaired. This makes them very reactive and they are not usually isolated. Even strong bonds can break into ions provided they are polarized, but to make radicals we need weak symmetrical bonds such as 0-0, Br-Br or I-I. Dibenzoyl peroxide, the Ph(C02)2 catalyst in this reaction, readily undergoes homolysis like this—the one-electron movements are represented by fish-hook arrows having one barb and odd electrons on atoms are represented by dots. 

Finally, it is noted that chromophores other than CO have been incorporated into polymer backbones to make the polymers photodegradable. An example is the class of polymers with metal-metal bonds along the polymer backbones. " Photochemical degradation occurs with these polymers because metal-metal bonds cleave homolytically when irradiated with visible light (Eq. 7 scheme 7) The remainder of this review focuses on these materials. 

CMRP makes use of the stable Co-C bond that forms in less active CCT monomers. The Co-C bond undergoes homolytic cleavage with release of a persistent cobalt radical and transient alkyl radical. " Hence, the cobalt complex end-caps the polymer chain providing a level of control over the polymerization. This research area is quite large in its own right and is not discussed in this chapter however, a more detailed description can be found in a review. ... 

Site receptivity. As discussed in Section 8.2, hydrogen transfer to a radical site at a heteroatom or multiple bond forms a new bond, making the site coordina-tionally less unsaturated. The strength of this new bond RY H is thus a measure of the site receptivity. This favors H transfer that increases the coordination number at electronegative atoms the homolytic bond dissociation energies of Table A.3 show RC=N- —H > RF —H > R(HO)C—0+—H > R(RO)C—0+-H > R(H)0+-H > RjC—H = RCl -H = RHjN —H>R20 —H>R2HN —H>R(H)S —H, 6.6 eV- 3.9 eV. Thus transfers to all of these sites must be considered for most slerically allowed H atoms, since D(alkyl-H) = 4.0 — 4,3 eV,... 

Since the aquohydroxo complex is the active species the decrease of the reaction rate at pH >13 has been explained with the decrease of the concentration of this reacting form. The first step of the reaction is the homolytic cleavage of Co-OH bond, followed by deprotonation of the resulting hydroxo radical with formation of O-", which eventually dimerizes to give hydrogen peroxide, as revealed by iodometry. Deprotonation of the hydroxo radical is necessary to make less effective the backward reaction of recombination with the Co2 + complex. 

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