Effect of the acyl substituent on the equilibrium constant for hydration of esters

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. Can. J . Chem. 65, 1951Chem. 65, (1987.Heats of hydrolysis have been measured for three acylimidazole acetals. From these results free energies of formation in aqueous solution have been calculated for the acetals and the corresponding tetrahedral intermediates. Having free energies of formation for the tetrahedral intermediates allows a detailed analysis of the energetics of both the unobservable nucleophilic reaction of imidazole with ethyl acetate and also the observable reaction of imidazole with p-nitrophenyl acetate. pK, values of 1-(3-aminopropy1)-imidazole and 1-(2-aminoethy1)-imidazole have been determined to allow calculation of the a* value for the imidazolyl substituent. The dependence of imidazole pK, values on the electron-withdrawing properties of the 1-substituent was also determined. There is a linear free energy relation between the free energy change for replacement of OH in a carbonyl hydrate by imidazolyl and the sum of the a* values for the other substituents. The implications of these results for the question of concerted versus stepwise mechanisms for the reactions of imidazole with aryl acetates have been examined. An equilibrium constant has been calculated for the addition of imidazole to p-nitrophenyl acetate. A simple extension of Marcus theory allows the free energy surface for a three-dimensional reaction coordinate diagram to be calculated using the energy levels of the tetrahedral intermediate determined in this work, the energy level of the acylium ion derived from literature data, and intrinsic barriers for the edge reactions. It is shown that the reaction of imidazole with p-nitrophenyl acetate probably follows a concerted path. General conclusions from this theory of concerted reactions are discussed.

Section: Discussionmentioning

confidence: 99%

Hydration of acylimidazoles: tetrahedral intermediates in acylimidazole hydrolysis and nucleophilic attack by imidazole on esters. The question of concerted mechanisms for acyl transfers

Guthrie¹,

Pike²

1987

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“…There has been criticism directed at the use of Marcus theory for electrophile-nucleophile combination reactions (56), but there is a strong empirical basis for using the equation because so many reactions are described by it (21,57,58).…”

Section: Discussionmentioning

confidence: 99%

Concertedness and E2 elimination reactions: prediction of transition state position and reaction rates using two-dimensional reaction surfaces based on quadratic and quartic approximations

Guthrie¹

1990

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This paper is dedicated to Professor Ross Stewart on the occasion of his 65th birthday J. PETER GUTHRIE. Can. J. Chem. 68, 1643 (1990). An analytical expression for the two-dimensional reaction surface appropriate for discussing a concerted reaction has been derived for the quartic approximation to a reaction coordinate, and shown to be consistent with Marcus theory. By analysis of literature data for SN1 solvolyses and carbanion formation we have been able to reproduce the qualitative behavior of eliminations involving tBuBr, iPrBr, and EtBr in EtOH/EtO-, and to predict the rates of these reactions within the uncertainties of the input data. IntroductionWe have recently reported an equation for the energy surface for the situation where two reaction progress variables are required to describe a reaction ( 1 ) . The equation, which was a generalization of the inverted parabola model for the reaction coordinate of a simple reaction, was shown to be consistent with Marcus theory (2-4) for the edge reactions. It provides a basis for predicting when a reaction will be concerted and when it will be stepwise in terms of the thermodynamics and intrinsic barriers of the edge reactions. A logical consequence of the simple theory used is that the intrinsic barriers are the same for each pair of parallel edge reactions.There has been considerable interest in the use of various energy surfaces to analyze concerted reactions ( 1 , 5-18). We wish now to report that, using information from the literature, it is possible to predict which of a set of simple P-elimination reactions are concerted, and to make semiquantitative predictions of the rate constants, with no adjustable parameters.We ( 8 ) and others (19,20) have discussed the advantages of a quartic equation as an approximation to a reaction coordinate diagram, since it has the correct shape at the extrema. We have now devised an analogous equation with two reaction coordinate dimensions, which provides the simplest generalized surface in the quartic approximation with the correct behavior for the edge reactions, and which satisfies Marcus theory for all sections through it.

“…In all cases the heat of formation of the substituted benzoic acid was known; methyl benzoate was the only ester for which a heat of formation had been reported (3). Heats of formation of the substituted methyl benzoates were determined by measuring the heat of reaction with tetraethylammonium hydroxide in aqueous DMSO.…”

Section: Heats Of Formationmentioning

confidence: 99%

“…We have been interested for some time in the energetics of acyl transfer reactions (1) and in the energy levels of tetrahedral intermediates in ester (2,3) and amide (4) hydrolyses. As part of a project aimed at extending this study to a series of substituted benzamides, we determined heats of formation of several N,N-dimethylbenzamides and methyl benzoates.…”

Section: Introductionmentioning

confidence: 99%

Equilibrium constants and heats of formation of methyl esters and N,N-dimethyl amides of substituted benzoic acids

Guthrie¹,

Pike²,

Lee³

1992

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.Heats of methanolysis and dimethylaminolysis of substituted benzoyl chlorides (4-X-C6H4-COC1, X = H, CH,, 0CH3, C1, NO,) have been measured, as have the heats of hydrolysis of the esters, permitting the calculation of the heats of formation of the benzoyl chlorides (4-X-C6H4-COCl, X, DHf: CH30, -80. 29 of transfer from methanol to water and from gas to water were determined for the esters and amides. Free energies of formation in aqueous solution were calculated for the acids, esters and amides, making use of thermodynamic estimation procedures where necessary. Equilibrium constants were measured for ester formation in water (X, K (M-I): CH30, 0.14; CH,, 0.14; H, 0.12; C1, 0.15; NO,, 0.13) and N,N-dimethylaminolysis in methanol (X, K (M-I): CH30, 8.16; CH,, 17.5; H, 26.5; C1, 22.6; NO,, 41.0). Partition constants for esters and amides were measured for methanol/dodecane and dodecane/water, permitting calculation of the free energy of transfer from methanol to water (4-X-C6H4-COOCH,, X, DG,,: CH30, 3.12; CH,, 3.17; H, 3.01; C1, 3.43; NO,, 2.89; 4-X-C6H4-CON(CH,),, X, DG,,: CH30, 0.96; CH,, 1.48; H, 0.92; Cl, 1.77; NO2, 0.99). These data permit calculation of the equilibrium constants for dimethylaminolysis of substituted methyl benzoates in water, and for amide formation in water (4-X-C6H4-CON(CH,),, X, K(M-', reactants and products as neutral molecules): CH30, 767; CH,, 752; H, 2050; C1, 1020; NO,, 2350). In the course of our calorimetric measurements we derived an improved value for the heat of solution of HC1 in methanol.