Solvent effects radical reactions

Most researchers distil solvents for radical reactions in the same manner as they might for use with a reactive organometallic. This practice is recommended to ensure that solvents are sufficiently pure. That the solvents are simultaneously dried during purification is of little consequence. Water is a much poorer hydrogen atom donor than any common solvent (conversely, the hydroxyl radical is a powerful hydrogen atom abstractor). Thus, the presence of trace quantities of water will have no adverse effect on most radical reactions. As a corollary, water can be a useful solvent or cosolvent provided that the reagents or substrates are not susceptible to hydrolysis or protonolysis. 

In the previous chapters, Bu3SnH has been used as a typical and useful radical reagent in a benzene solvent. Generally, radical reactions with Bu3SnH initiated by AIBN, proceed effectively in benzene, which bears a conjugated Tr-system. Probably, the formed radicals are somewhat stabilized through the SOMO-LUMO or SOMO-HOMO interaction between the radicals and benzene. 

From substituent and solvent effects on reactions such as Eq. 20 it was concluded [84] that these reactions are of the SnI type, i.e. that alkoxyalkene (enol ether) type radical cations are intermediates. The lifetimes of these radical cations were estimated [84] to be of the order of nanoseconds, much shorter than those [78, 79, 81] of the corresponding l,l-radical cations. This shows the importance of the additional (second) alkoxy group in stabilizing the positive charge on the carbon skeleton. On the basis of these mechanistic model studies, very detailed suggestions could be made [84] regarding the deoxyribose-derived radical reactions that lead to chain breaks in DNA (see below). 

There is also experimental evidence for the influence of radical-solvent complexes in small radical addition reactions. For instance, Busfield and co-workers used radical-solvent to explain solvent effects in reactions involving small radicals, such as t-butoxyl radicals towards various electron donor-electron acceptor monomer pairs. The observed solvent effects were interpreted in terms of complex formation between the t-butoxyl radical and the electron-acceptor monomer, possibly via a sharing of the lone pair on the t-butoxyl oxy-... 

Significant, though smaller, solvent effects have also been reported for alkoxy radical reactions (Section 3.4.2.1).133 137... 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

Solvent effects on the reactions of small radicals have been discussed in general terms in Chapter 2 (see 2.3.6.2 2.4.5). Small, yet easily discernible, solvent effects have been reported for many reactions involving neutral radicals. These effects on the rates of radical reactions often appear insignificant when... 

Studies on the reactions of small model radicals with monomers provide indirect support but do not prove the bootstrap effect.111 Krstina et ahL i showed that the reactivities of MMA and MAN model radicals towards MMA, S and VAc were independent of solvent. However, small but significant solvent effects on reactivity ratios are reported for MMA/VAc111 and MMA S 7 copolymerizations. For the model systems, where there is no polymer coil to solvate, there should be no bootstrap effect and reactivities are determined by the global monomer ratio [Ma0]/[Mb0].1j1... 

Most of the theoretical arguments support the statement of Evans and Polanyi (20), who first argued that AG represents the situation at 0°K better than does AH. The main reason is seen in solvent effects. Dewar expressed this meaning in the most radical manner, saying that determination of AH and AS in solution is simply a waste of time (21). Laidler offered similar ideas and stressed that a theory for AG can be more easily developed than for AH (13, 225). The approach of Hammett is still more general and not restricted to solvent effects (226). According to Hammett, a reaction that is more complex than it appears to the observer and consists of two parallel independent processes will affect the value of AH more than will AG. 

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5),... 

Ketone and the formed a-ketoperoxyl radical are polar molecules. Hence the polar effect influences the reactivity of the ketones and the peroxyl radicals. Polar solvents also influence the reactions of peroxyl radicals with ketones as well as other free radical reactions. 

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