Construction and Application of Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles

Abstract

A basis-set of reference electrophiles (benzhydrylium ions) and reference nucleophiles (pi-systems) has been constructed on the basis of the equation log k = s(E+N), which covers reactivity ranges of several orders of magnitude. The reference compounds of this basis-set proved to be very useful for the determination of nucleophilicity parameters (N) and electrophilicity parameters (E) of new compounds by kinetic measurements. This was shown by adding various new classes of nucleophiles like enamines, phosphanes and phosphites, and pyridines as well as by adding new electrophiles. Kinetic data for electrophile-nucleophile combinations were used for discussing structure-reactivity relationships, as well as for the analysis of mechanistic aspects. Reversible additions of nucleophiles to benzhydrylium ions allowed the determination of intrinsic barriers for these reactions, which are a key to the understanding of organic reactivity. The catalytic activities of several para-substituted pyridine derivatives in esterification reactions were compared with their nucleophilicities (N).

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