Acetone, dissociation constants indicators

F. M. Cray and G. M. Westrip have determined the influence of acetone upon the transformation interval of a number of indicators. They worked with acetone containing 10% by volume of water. They prepared various buffer mixtures, the pH s of which were measured potentiometrically by means of the quinhydrone electrode. The following table shows how large an effect acetone has upon the magnitude of the dissociation constant. 

Recently an indication of path A has been shown by investigating solvolysis of the 7-substituted norbornadiene complexes of Cr, Mo, and W 90). It was found that in an 80% acetone-water solution at 25°C,(t7-C7H7C1)Mo (CO)4 gives free 7-norbornadienol instead of the expected (t7-C7H70H)Mo (CO)4- Moreover, the solvolysis rate constant was of the same order of magnitude as that for free 7-chloronorbornadiene. All these data support fast dissociation of the type... 

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. 

A dissociative mechanism is proposed for the aquation of // a -[Cr(en)2Cl2] ion from studies in aqueous-organic solvent mixtures (0—50% MeOH, ethylene glycol, glycerol, acetone, and dioxan). Plots of In A with e ( = dielectric constant of solvent) or the Grunwald-Winstein T-values are linear, and comparison with the analogous Co systems indicates far less charge separation in the transition state. 

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