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Minegishi, Shinya (2004): Reactivity Selectivity Relationships in Reactions of Carbocations with Nucleophiles. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



Benzhydrylium ions, which have previously been characterized as reference electrophiles in the Mayr group, have been employed for the construction of general nucleophilicity and electrophilicity scales. The kinetics of the reactions of benzhydrylium ions with 15 n-nucleophiles in water and DMSO were measured to yield the N- and s-parameters. Most of the nucleophiles have closely similar slope parameters indicating that the reactions of most n-nucleophiles approximately follow Ritchie's constant selectivity relationship. The different slope parameter for water is recognized as the main reason for deviations from the Ritchie relationship reported in 1986. The rates of the reactions of benzhydrylium ions with solvent mixtures of variable composition have been determined. From plots of the first-order rate constants (log k) versus E of benzydrylium ions, the solvent nucleophilicity parameters are derived. This allow the systematic design of Friedel-Crafts reactions with solvolytically generated carbocations. Rate constants for the reactions of laser flash photolytically generated benzhydrylium ions with chloride and bromide anions have been determined in various solvents and compared with literature data. The rate constants for the ionization of benzhydryl halides and for the reactions of benzhydrylium ions with halide anions and with solvent are combined to give complete free energy profiles for solvolysis reactions. The first SN1 reaction, where ionization and trapping of the carbocation by the solvent could directly be observed, is reported.