The ion-pair mechanism and bimolecular displacement at saturated carbon. VII. Racemization of substituted 1-phenylbromoethanes; ionic strength effects

Abstract

ALLAN R. STEIN. Can. J. Chem. 67,297 (1989). No, or at most very small, salt effects have been detected for second-order racemizations of various 1-phenylbromoethanes, confirming that the "mixed kinetics", previously reported for 4-methyl-and 3,4-dimethylphenylbromoethanes with tetrabutylammonium bromide in acetonitrile and nitromethane, did not reflect a salt effect on a unimolecular reaction. Thus the uni-and bimolecular rate components can be evaluated using the equation:where kl = first-order or unimolecular rate constant, the intercept, and k; = 2k2, the latter being the rate constant for the bimolecular substitution. A "special salt effect" was found for the unimolecular component; it was especially pronounced for 3,4-dimethyl substrate in acetonitrile with Bu4NC104 as the electrolyte. In acetone, where no unimolecular reaction component was detected even with those substrates giving the most stable carbocations, theonly salt effect was a common ion, Bu4N+, effect on the incomplete dissociation of Bu4NBr. From an iterative best fit for plots of observed rate versus calculated bromide ion activity for unsubstituted, 4-methyl, and 3,4-dimethyl substrates, K,,so, -15 * 1 at 40°C. The results are interpreted as additional support for a progression of mechanism with nucleophilic attack possible at any stage of the series of equilibria:substrate S contact ion pair = various solvated ion pairs e dissociated ions.Key words: special salt effects, ionic strength effects, racemization of 1-phenylbromoethanes, mechanism of nucleophilic substitution, ion pair mechanism. ALLAN R. STEIN. Can. J. Chem. 67, 297 (1989). Pour les rackmisations du deuxieme ordre de divers phenyl-1 bromokthanes, on n'a dttecti aucun, ou tout au plus de tres faibles, effet de sels; ces resultats confirment que les (< cinCtiques mixtes >> qui ont t t t observCes anttrieurement lors des reactions des mCthyl-4 et dimtthyl-3,4 phCnylbromotthanes avec le bromure de tttrabutylammonium dans l'acktonitrile et le nitromethane, ne sont pas le reflet d'un effet de sel sur une rtaction unimol6culaire. On peut donc Cvaluer les composantes uni-et bi-molCculaires de la vitesse en utilisant 1'Cquationdans laquelle kl = constante de vitesse du premier ordre ou unimoltculaire, l'ordonde a l'origine, et k; = 2k2 oh k2 est la constante de vitesse de la substitution bimolCculaire. On a observC un << effet de sel spCcial,, sur la composante unimoltculaire; cet effet Ctait particulierement prononcC pour le substrat dimCthyle-3,4, dans l'acetonitrile, avec le Bu2NC104 comme tlectrolyte. Dans l'acCtone, on n'a detect6 aucune composante relite a une reaction unimoltculaire (mCme pas avec les substrats qui donnent les carbocations les plus stables) et le seul effet de sel qui a pu Ctre observe est celui d'un ion commun, B u~N ' , d i a une dissociation incomplete du Bu4NBr. En se basant sur des ittrations des courbes des vitesses observtes versus l'activitt calculCe de I'ion bromure pour les reactions des substrats mtthyle-4, dimkthyl-3,4 ou non-substituts, on a tvalue q...