Graduate Thesis Or Dissertation
 

Molecular orbital correlation of the relative rates of chlorine atom abstraction from homo- and heteroarylmethyl chlorides by triphenyltin radical

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  • A series of momochloromethyl arenes and heteroarenes have been reacted with triphenyltin hydride at 70° C. Under these radical conditions the only observable reaction is reduction of the starting material to arylmethane or heteroarylmethane. A significant range of relative rates (a factor of eighty) has been found among the compounds studied. The relative reactivities of the homoarylmethylchlorides can be correlated with various calculated energy differences obtained from a standard semi-empirical SCF MO method. Excellent correlations, with coefficients of at least 0.990, were obtained using several different molecular orbital models. The accuracy of these results for a nucleophilic radical is comparable to those previously obtained for hydrogen abstraction by the electrophilic trichloromethyl radical species. The relative reactivities of the heteroarylmethyl chlorides yielded poor correlations when plotted against the same SCF calculated energy differences. Poor results were also obtained when ground state properties such as charge densities were used to correlate the data. The optimum correlation was obtained with the anion intermediate model. This suggests that a charge separated form makes an important contribution to the description of the transition state. The relative reactivities of all compounds were treated using empirically derived four parameter equations. Poor correlation was obtained for that approach which completly neglected charge development in the transition state. However, good correlations were obtained with those models which include anionic character in the transition state. It would appear that the use of the non-alternant heteroarylmethyl systems allows for an evaluation of the importance of charge separation in free radical reactions.
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