Free Energy Relationships for Reactions of Substituted Benzhydryliunn Ions: From Enthalpy over Entropy to Diffusion Control

Second-order rate constants k(2) for the reactions of various donor- and acceptor substituted benzhydrylium ions Ar2CH+ with pi-nucleophiles in CH2Cl2 were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar2CH-PAr3+X- in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E. for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log k(2)(20 degrees C) = s(N)(N + E). The new correlation analysis also leads to the N and s(N) parameters of 18 pi-nucleophiles, which have only vaguely been characterized previously. The correlations of log k(2) versus E are linear well beyond the range where the activation enthalpies Delta H-double dagger of the reactions are extrapolated to reach the value of Delta H-double dagger = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect A flattening of the correlation lines only occurs for k(2) > 10(8) M-1 s(-1) when the diffusion limit is approached.