Free Energy Relationships for Reactions of Substituted Benzhydrylium Ions: From Enthalpy over Entropy to Diffusion Control
J. Am. Chem. Soc., 2012, 134, 13902-13911 published on 27.07.2012
Second-order rate constants k2 for the reactions of various donor- and acceptor-substituted benzhydrylium ions Ar2CH+ with π-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 k2(20 °C) = sN(N + E). The new correlation analysis also leads to the N and sN parameters of 18 π-nucleophiles, which have only vaguely been characterized previously. The correlations of log k2 versus E are linear well beyond the range where the activation enthalpies ΔH of the reactions are extrapolated to reach the value of ΔH = 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 k2 > 108 M–1 s–1 when the diffusion limit is approached.