Ester hydrolysis activation parameters

Ester Hydrolysis.—Activation parameters for glycine ester hydrolysis, both of the free ester and of esters complexed as [Co(en)a(NH2CH2COaR)] +, are listed in Table 1. For the ethyl ester, the observed greater rate of hydrolysis of the coordinated ester than of the free ester is a consequence of a much lower... 

Table 7 Activation Parameters for the Resin Catalyzed Hydrolysis of Some Esters in Water-Solvent Mixtures...

Now a very useful feature of the excess acidity method comes into play likely nucleophiles or bases can be tested by subtracting their log activities or concentrations from the left-hand side of equations (59) and (60), and the species reacting with SH+ is uniquely identified when linearity of the result against X is achieved.145,161 For instance, subtraction of twice the water activity is required to attain linearity in ester hydrolysis processes such as equation (42), as shown in Fig. 11 for methyl benzoate41 and ethyl benzoate.210 The water activities given in Table 3 were used. The parameters of the lines in Fig. 11, obtained by curve-fitting, are methyl benzoate, slope 0.921 + 0.010, intercept... 

These relations both demonstrate the inhibition of formation of the tetrahedral state which can be clearly attributed to steric crowding. Such a correlation as (5) confirms the attack at the neighbouring carbonyl group and this intramolecular catalysis for all this series. The activation parameters for the alkaline hydrolysis of these esters were also measured and are shown in Table 1. The enthalpies of activation of the 2-formyl, 2-acetyl, 2-propionyl, 2-isobutyryl and 2-pivaloyl esters are exceptionally small. These are... 

Table 1 Rate coefficients k2, enhanced rate ratios re, ring size and activation parameters for the alkaline hydrolysis of esters in 70% (v/v) aqueous dioxan at 20°C. 

The rate-limiting step is almost certainly attack of OH on the original ester (164). This is borne out by the activation parameters for the base-induced hydrolysis of MeC02Et AH° = 112kJmor ... 

Similarly, vesicular reactivity is dependent on bilayer fluidity and Arrhenius (or Eyring) plots for the decarboxylation of 6-NBIC show a break around Tm. " For the Kemp elimination in different bilayers, it was found that the bilayer for which kinetic data had been gathered below its was least effective as a catalyst. Ester hydrolysis has also been found to be faster above r. For the decarboxylation of 6-NBIC, the increase in catalytic efficiency was attributed to different aggregate surface dynamics based on the observation that vesicles above showed intermediate activation parameters between vesicles below and micelles. One could, of course, discuss causality here considering the fact that many of the bilayer... 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

ACTIVATION PARAMETERS FOR ACID-CATALYZED ESTER HYDROLYSIS AND... 

The data for acid-catalyzed ester formation in cyclohexanol are doubly interesting. The activation parameters are closely similar to those for the acid-catalyzed hydrolysis of the corresponding ethyl esters. The enthalpy of activation is considerably higher than for esterification in methanol this is probably a result of steric inhibition of solvation, as well as non-bonded compression in the transition state, as suggested by the entropies of activation, which are also significantly higher than with methanol, especially for compounds without ortho substituents which presumably have more transition state solvation to lose. 

Apart from ethyl acetate, the least reactive ester studied is N,0-diacetyl serinamide, which is hydrolyzed in a pH-independent reaction between pH 7 and 8 with a rate coefficient193 of 2.66 x 10-5 sec-1. Salmi and Suonpaa194 and Palomaa et al. 9S, have measured the rates of neutral hydrolysis of a number of chloroacetate esters, and this work has been extended more recently by Euranto and Cleve196-198, who have measured the activation parameters for the hydrolysis of several compounds. Motfat and Hunt199 have obtained the same data for the hydrolysis of a variety of alkyl and aryl trifluoroacetates, and the data for substituted phenyl acetates191 have been plotted in Fig. 14. Most of the available data are collected in Table 27. 

RATE COEFFICIENTS AND ACTIVATION PARAMETERS FOR THE NEUTRAL HYDROLYSIS OF ESTERS IN WATER AT 25°C... 

Activation parameters have been determined58 for the hydrolysis of a-amino acid esters in mixed ligand complexes (Table 6). For base hydrolysis of the complex [Cu(nta)-(NH2CH2C02Et)], AH = 20.5 kJ mol-1 and AS = -138 J K 1 mol-1. Catalysis in this system is primarily due to a substantial decrease in AH (by ca. 21 kJ mol1) compared with the free ligand. A detailed discussion of the activation parameters is available.58... 

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