How Constant Are Ritchie's “Constant Selectivity Relationships”? A General Reactivity Scale for n-, π-, and σ-Nucleophiles

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Abstract
The kinetics of 82 reactions of benzhydrylium ions (Ar2CH+) with n-nucleophiles has been determined at 20 °C. Evaluation by the equation log k = s(N + E) delivered the reactivity parameters N and s for 15 n-nucleophiles (water, hydroxide, amines, etc.). All nucleophiles except water (s = 0.89) and -SCH2CO2- (s = 0.43) have closely similar slope parameters (0.52 < s < 0.71), indicating that the reactions of most n-nucleophiles approximately follow Ritchie's constant selectivity relationship (s = constant). The different slope parameter for water is recognized as the main reason for the deviations from the Ritchie relationship reported in 1986. Correlation analysis of the rate constants for the reactions of benzhydrylium ions with the n-nucleophiles (except H2O) on the basis of Ritchie's equation log k = N+ + log k0 yields a statistically validated set of N+ parameters for Ritchie-type nucleophiles and log k0 parameters for benzhydrylium ions. The N and s parameters of the n-nucleophiles derived from their reactions with benzhydrylium ions were combined with literature data for the reactions of these nucleophiles with other carbocations to yield electrophilicity parameters E for tritylium, tropylium, and xanthylium ions. While the E parameters for tropylium and xanthylium ions appear to be generally applicable, it is demonstrated that the E parameters of tritylium ions can be used to predict reactivities toward n-nucleophiles as well as hydride transfer rate constants but not rates for the reactions of tritylium ions with π-nucleophiles. It is now possible to merge the large data sets determined by Ritchie and others with our kinetic data and present a nucleophilicity scale comprising n- (e.g., amines), π- (e.g., alkenes and arenes), and σ-nucleophiles (e.g., hydrides).

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