The rate of abstraction of iodine by phenyl radicals from a variety of 2-substituted iodobenzenes relative to the rate of abstraction of chlorine from carbon tetrachloride is reported. Qualitatively, it is found that electron-donating substituents retard and electron-withdrawing groups enhance the rate of homolytic C-I bond cleavage. Attempts are made to correlate the observed rates with various ortho-substituent parameters as well as the extended Hammett relationship (eq 6). It is concluded that steric interactions are important in determining the rate of iodine abstraction from 2-substituted iodobenzenes but that there is no need to invoke anchimeric assistance by a neighboring bromo or iodo group. The rates of iodine abstraction from 2-iodobiphenyl and 2-iodo-2'methylthiobiphenyl are comparable, indicating no participation by the sulfide group in the abstraction process. Iodoferrocene is shown to be slightly more reactive to the phenyl radical than iodobenzene indicating a possible weak interaction of the iron atom in the abstraction process. (X) A . R . Lvons and M. C. R . Symons, ibid., 93, 7330 (1971); Chem.
The rates of abstraction of bromine by phenyl radicals from the cyclic Cg to Cs monobromides and trans-1,2dibromides and meso-and dl-2,3-dibromobutanes relative to the rate of chlorine abstraction from carbon tetrachloride are reported. The rates of bromine abstraction from the cycloalkyl monobromides vary with ring size in a manner
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