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

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Abstract
Heats of methanolysis and dimethylaminolysis of substituted benzoyl chlorides (4-X-C6H4-COCl, X = H, CH3, OCH3, Cl, NO2) 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: CH3O, −80.29 ± 0.70; CH3, −48.10 ± 1.46; NO2, −47.70 ± 0.87), methyl benzoate esters (4-X-C6H4-COOCH3, X, DHf: CH3O, −124.50 ± 0.39; CH3, −93.99 ± 0.58; Cl, −92.09 ± 0.53; NO2, −92.55 ± 0.31), and N,N-dimethylbenzamides (4-X-C6H4-CON(CH3)2, X, DHf: CH3O, −75.87 ± 1.42; CH3, −46.62 ± 1.99; H, −40.96 ± 1.41; Cl, −49.33 ± 1.09; NO2, −48.05 ± 1.53). Free energies 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−1): CH3O, 0.14; CH3, 0.14; H, 0.12; Cl, 0.15; NO2, 0.13) and N,N-dimethylaminolysis in methanol (X, K (M−1): CH3O, 8.16; CH3, 17.5; H, 26.5; Cl, 22.6; NO2, 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-COOCH3, X, DGmw: CH3O, 3.12; CH3, 3.17; H, 3.01; Cl, 3.43; NO2, 2.89; 4-X-C6H4-CON(CH3)2, X, DGmw: CH3O, 0.96; CH3, 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(CH3)2, X, K(M−1, reactants and products as neutral molecules): CH3O, 767; CH3, 752; H, 2050; Cl, 1020; NO2, 2350). In the course of our calorimetric measurements we derived an improved value for the heat of solution of HCl in methanol.

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