Measuring Effective Charge in Equilibria

Substituent effects on equilibrium constants from kinetics derive from 

Equation (9b), derived from Equation (9a), which gives p q = Pi - P i 

Since the range of the equilibrium constants can be very large it is not usually a simple procedure to measure both forward and reverse rate constants under the same conditions it must also be borne in mind that when the equilibrium constant is close to unity the expression = Zr, + i for the measured rate constants ( ohs) becomes important. 

The value of Peq for the reaction of substituted phenyl acetates with imidazole (Equation 10) was first measured from kinetic data. In this case the standard equilibrium is the dissociation of substituted phenols (Equation 11). 

It is not a simple procedure to measure both and k under the same conditions as required by application of Equation (9a) and the measurement of may be effected by use of excess imidazole to force the equilibrium of Equation (10) well to the right. Under these circumstances ki = (the rate constant actually measured). The acetyl imidazole is in its neutral form (Acim) at the pH of the measurements (between 5 and 8) knowledge of the dissociation constant of AcImH and that 

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