Alkyl groups free radicals

N. J. Friswell and B. G. Gowenlock Advan. Free Radical Chem. 1, 39 (1965) Inorganic hydrogen and alkyl-containing free radicals Part I, groups II, III and IV 36 (200)... 

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

Like the 5/) -hybridized carbons of carbocations and free radicals, the sp -hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and aie, therefore, better able to stabilize an alkene. 

Recently, an interesting reaction of -dinitrobenzene v/ith trialkylborane has been reported, in which the iritro group is replaced by an alkyl group in good yield fEq 913 " The reacdon is not a simple ionic reacdon, but proceeds via free radical intermediates... 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

The alkyl group R of certain carboxylic esters can be reduced to RH by treatment with lithium in ethylamine. The reaction is successful when R is a tertiary or a sterically hindered secondary alkyl group. A free-radical mechanism is likely. Similar reduction, also by a free-radical mechanism, has been reported with sodium in HMPA-r-BuOH. In the latter case, tertiary R groups give high yields of RH, but primary and secondary R are converted to a mixture of RH and ROH. Both of these methods provide an indirect method of accomplishing 10-81 for tertiary R. 

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