Substituent effects unimolecular nucleophilic

Nevertheless, several kinetic studies have shown that certain nucleophilic substitution reactions of aryl diazonium ions are first order and independent of the concentration of the nucleophilic species. Solvent effects, isotope effects, and substituent effects are also in agreement with a rate-determining unimolecular decomposition of the aryl diazonium ion. In other reactions, an adduct of the nucleophile and diazonium ion is a distinct intermediate. Substitution results when nitrogen is eliminated from the adduct. Finally, substitution can occur via radical... 

Rate constants for three distinct competing processes were separately determined solvolysis k, unimolecular k, and bimolecular k2. The Hammett equation with cross-terms was applied to the effects of substituents X in the nucleophile and Z in the leaving group on the analysed rate constants, but in most cases the pxz term was negligible. The Hammett equation with cross-terms has also been applied to the reactions of Z-substituted benzyl X-benzenesulfonates with Y-substituted thiobenzamides in acetone at 45 The findings pz < 0 and pyz > Pxz indicate that this reaction proceeds by a dissociative 5ivr2 mechanism. 

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