Metal ion alcoholysis and titration in alcohol

The titration of metal ions in alcohol solvents28 follows the same sort of rules as titrations of metal ions in water29 but poses additional problems due to the lower polarity that increases ion pairing and oligomerization of the metal ions. We have performed several such titrations with the analysis of the potentiometric data depending on the level of information one requires. More complete and time-consuming analyses are reserved for the most effective catalytic metals, namely La3 +, and for the transition metal ion Zn2+ and Cu2+ along with some simple complexes of the latter two which we describe a little later. For the other metal ions described in our titration papers,7,8 we only present the data in terms of the 

For other cases, such as La3+ where more detail is required about the nature of the species present in solution, titration data can be computer fit to more complicated multi-equilibrium models containing Mx 1 v( OR)v forms whose stoichiometry is suggested by information gained from independent spectroscopic or kinetic techniques. One must be mindful of the pitfalls of simply fitting the potentiometric data to complex multi-component models for which there is no independent evidence for the various species. Without some evidence for the species put into the fit, the procedure simply becomes an uncritical mathematical exercise of adding and removing various real and proposed components until the goodness of fit is satisfactory. 

La3 + -catalyzed ethanolysis of paraoxon (1) as a function of jpH. Reprinted with permission from ref. 8. 

Having established the speciation, we now have a very powerful tool for analyzing the kinetic data for the pH dependence of the La3 + catalysis of the alcoholysis of various substrates. Included in the Figs 1 and 2 plots are the second-order rate constants for La3 + -catalysis of the ethanolysis of paraoxon (1) and the methanolysis of /xnitrophenyl acetate (PNPA, 2) as a function of pH in ethanol and methanol, respectively. The kinetic data mainly follow the rise/fall behavior of the Lal+( OR)2 species with some involvement of the other species, La2 + ( OR)i, La2 1 ( OR)3 and Lal+(-OR)4. 

To determine the activities for the various Lal+( OR) we analyze the k2bs data as a linear combination of individual rate constants (Equation 8), where ki4, kf2... are the second-order rate constants for each La2+( OR) promoting ethanolysis and methanolysis of 1 and 2 respectively. 

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