Linear free energy and reactivity-selectivity relationships in reactions of diarylcarbenium ions with .pi.-nucleophiles

Abstract

Second-order rate constants for the reactions of para-substituted diarylcarbenium ions (X-C&)(Y-c&&)CH+ with various alkenes have been determined in CH,C12 solution at -70 to -30 "C.Within each reaction series (constant alkene, variable X and Y), the activation entropy remains constant, while the activation enthalpy increases with increasing electron releasing ability of X and Y. Rate equilibrium relationships AG' = -cxAG; + c (AGY = ionization free energy for the equilibrium Ar2CHCl + BC13 G= Ar,CH+.BCI,-) give a values of 0.64-0.67 for terminal alkenes and 0.75-0.94for nonterminal alkenes, indicating late transition states.All reaction series are connected by well-behaved linear reactivity-reactivity relationships.The relative reactivities of terminal alkenes are independent of X and Y, Le., selectivity is independent of reactivity (constant selectivity relationship).When nonterminal alkenes are included, linear selectivity-reactivity relationships are observed (as a consequence of the linear reactivity-reactivity relationships) which have positive, negative, or zero slopes, i.e., selectivity may increase, decrease, or remain constant, as the reactivity of the carbenium ions increases.In the reaction series with trimethylethylene and tetramethylethylene, isoparametric values have been directly measured: This is the rate constant in a reaction series, characteristic for the isokinetic substituent (here @CH3<6H4)2CH+) at all temperatures and for all compounds of a reaction series (here (X<&)(Y<&f4)CH+)at the isokinetic temperature.The meaning of negative activation enthalpies in these reaction series is discussed.Correlation equations are given, which allow the prediction of rate constants for the reactions of diarylcarbenium ions with alkenes on the basis of pKR+ values or ethanolysis rate constants of the corresponding alkyl chlorides.