Evidence for phenyl cation as an intermediated in reactions of benzenediazonium salts in solution

Displacements on benzenediazonium cation, C,jHsNz+, in solution in the absence of strong bases, reducing agents, or light proceed by rate-determining cleavage to a singlet phenyl cation intermediate, CsH5+, rather than by the bimolecular mechanism proposed in 1969. In aqueous solution at 2 5 O , only unrearranged products are obtained, and negligible incorporation of deuterium from solvent D2O occurs, showing that aryne processes do not occur to a significant extent. The low selectivity between nucleophiles nevertheless requires a highly reactive intermediate. In the hydrolysis of C,jHsNz+, the high entropy of activation (+10.5 cal mol-’ deg-’) and the constancy of k I (within 4%) from H l O to D20 demonstrate that water is not involved as a nucleophile in the rate-determining step. The constancy of k 1 (within 10%) in solutions as diverse as 14-21 M H2S.04 and 100% CH3C02H and CH2Cl2 is in accord with a common rate-determining step over this whole range of solvents. Para-substituent effects can be dissected into nearly equal field (predominantly transition state) and resonance (predominantly reactant) contributions by an analysis using dual substituent constants (previously published), with a correlation coefficient of 0.992, although analysis using the Hammett pu equation, i .e . , with only a single set of substituent constants, yields a correlation coefficient worse than zero. The purpose of this work was to determine whether a weakly basic nucleophile Y (e.g., H20, Br-, CI-, or F-) replacing the N2+ group in a n ordinary substitution reaction (“dediazoniation”) of benzenediazonium cation (I) in solution is in the transition state or not.h The observed kinetics