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

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).