Structure-reactivity correlations are reported for general base catalysis of the addition of alcohols to l-(4-(dimethylamino)phenyl)ethyl and 1 -(4-methoxyphenyl)ethyl carbocations in 50:40:10 H20:CF3CH20H:R0H. The addition of trifluoroethanol to the relatively stable ((dimethylamino)phenyl)ethyl cation is catalyzed by substituted acetate ions with ß = 0.33, which is larger than ß = 0.23 for addition to the (methoxy phenyl) ethyl cation. Catalysis is more important for the more stable carbocation, but it decreases faster with increasing alcohol basicity. For the "water catalysis" of alcohol addition to 1-phenylethyl carbocations there is an increased sensitivity to the basicity of ROH with increasing carbocation stability (a "Hammond effect"). This indicates a small involvement of proton transfer in the transition state and is consistent with simple hydrogen bonding of ROH to a base; it is described by a positive interaction coefficient pyy = 5/3nuc¡-da (Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1984,106, 1396. However, for the acetate-catalyzed reaction there is no significant increase in the sensitivity to ROH basicity with increasing carbocation stability. This represents a decrease in the pyy coefficient and a shift toward the negative pyy coefficient that is expected for a fully concerted, coupled mechanism. This change in pyy d3 log k -dpKm(-dpKBOu)(-do) Conclusion. The third-derivative interaction coefficients described here provide a means of characterizing the interrelated changes in transition-state structure and structure-reactivity coefficients that occur as the nature of a reaction mechanism changes. These changes occur as the transition-state structure and structure-reactivity coefficients approach limiting values. They probably also occur for transition states of intermediate structure because the energy surfaces that describe these reactions and their structure-reactivity behavior are not likely to follow simple quadratic equations when changes in transition-state structure are large.

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While the adduct (IV) may be formed by an inter-or intramolecular proton transfer t o (V), its inclusion in the reaction scheme adds very little t o the understanding of the reaction, but complicates the kinetic scheme.

The thermodynamics, pH dependency and solvent effects of the fragmentation reaction of a series of α-oxyiminobenzylphosphonate monomethyl esters [(E)-1a-f ] were examined in water and other hydroxylic solvents by UV and by 31 P NMR spectroscopy at pH 0-3.1. The fragmentation of compounds (E)-1a-f was found to be a first-order reaction in substrate over the acidity range studied, while the dependence on the acidity is more complex, with rate constants k 1 and k 2 . The ρ values corresponding to the first and second order rate constants were Ϫ1.12 and Ϫ0.835, respectively, indicating that the reaction is facilitated by electron-donating substituents, which probably enhance the protonation of the oxime OH group. Activation parameters for k 1 and k 2 reactions were also calculated. The nearzero values of the entropies of activation obtained are consistent with a dissociative transition state with almost no bonding to a nucleophilic solvent. Monitoring the fragmentation reaction of (E)-1a in several binary alcohol-water mixtures at different acidities showed that the reaction rate is enhanced by the alcohol's acidity and not hampered by the steric requirements of the alcohol molecule. This rules out in our opinion, the likelihood for nucleophilic solvent assistance in the rate-determining step. On the other hand, product studies show that both the nucleophilicity and the steric requirements of the alcohol are of importance in determining the product formed in the fragmentation of (E)-1a. The highest selectivity (S) value was found for MeOH, while S values of <1 were observed for 2,2,2trifluoroethanol and the sterically hindered alcohols. The divergence between the effects of the solvent on the rate, on the one hand, and on the products on the other, indicates that the rate limiting step and the product determining step do not share a common transition state and that the reaction coordinate includes at least one reactive intermediate, probably methyl metaphosphate. The results are compatible with a dissociative mechanism (D N *A N or D N ϩ A N ), in which the solvating water molecules pull the departing water molecule into the hydration shell, while the solvated phosphonic group becomes a metaphosphate without nucleophilic assistance. The fragmentation of oxyiminobenzylphosphonates to metaphosphate is perceived as a special case of the "abnormal" Beckmann reaction.