Real and Ideal Relationships between Selectivity, Reactivity and Equilibrium

Abstract

Reactivity and selectivity are discussed in terms of quadratic rate—equilibrium relationships for reaction series sharing a common intrinsic barrier, notably Marcus' equation. R.D. Levine's emphasis on such reactions as forming a “Brønsted Series” is suggested as a basis for defining “ideal” reactivity behavior. Ideal relationships between selectivity and reactivity and selectivity and equilibrium are derived based on reaction schemes drawing a formal distinction between reagent and substrate. In a simple Brønsted relationship, the reagent varies and the substrate is held constant. In an extended relationship, in which both substrate and reagent vary, the Brønsted series becomes a selectivity “network” or matrix characterized by direct and cross quadratic coefficients reflecting curvature of a free energy relationship and the dependence of selectivity upon rate or equilibrium respectively. In the ideal case, the quadratic coefficients are equal but, in practice, they generally differ. Inspection of the relevant reaction schemes shows that in evaluating the coefficients, there is greater cancellation of experimental and chemical scatter arising from non‐systematic variations in reagent structure for the cross than the direct coefficient; this phenomenon accounts in part for the wider observation of reactivity—selectivity than non‐linear free energy relationships. New measurements are reported for reactions of carboxylic acids with enolate and imine substrates. The results are combined with literature data and plotted as reactivity—selectivity and extended Brønsted relationships. The correlations are satisfactorily considered in the context of departures of real from ideal behaviour.

Determination of the pKaof Ethyl Acetate: Brønsted Correlation for Deprotonation of a Simple Oxygen Ester in Aqueous Solution

“…Therefore, approximately 40% of the substituent effect on the equilibrium constant for formation of the enolate ( K eq , Scheme ) is expressed in the rate constant for its formation ( k HO , Scheme ). The linearity of this empirical rate−equilibrium correlation, which spanned a range of 11 p K units and included both thermodynamically favorable and unfavorable reactions, stands in sharp contrast with the reports of changes in Brønsted exponents for variation of the base catalyst with changing p K a K of a wide range of carbon acids. ,−

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Reaction of carbon monoxide with alkyliron porphyrins generated from alkyl halides and electrochemically produced iron(I) and iron(0)porphyrins

“…The acyl complexes thus obtained have the same characteristics as those formed by the reaction of Fe("0")2" porphyrins with carboxylic anhydrides. 5 In this preliminary study,3® CO insertion in the primary carbon-iron bond was investigated by cyclic voltammetry and thinlayer spectroelectrochemistry of solutions containing a mixture T h i s c o n t e n t i s carried out in the case of the -ethyl complex.…”

Absolute Rate Calculations. Proton Transfers in Solution

“…Linear relations between the rate constants for protonation (k p ) or deprotonation (k -p ) and the acidity constant of the catalyst, have been observed when the substrate (a carbon base or a carbon acid) is held constant and the catalyst is changed within a family of structurally related acids or bases (eg., carboxylic acids or carboxylate bases). Figure 3 illustrates Brönsted relationships obtained in the carboxylic acid catalysis of azulene, 85 acetylacetone, 87 and acetone, 85 and the carboxylate-base-catalyzed enolization of acetylacetone and acetone. 86 We selected these systems to illustrate the Brönsted relationship because they cover a broad range of reaction energies and involve some of the substrates already presented.…”