Activation parameters acetates

The trends of variation of the activation parameters are correlated with the solvation mechanism and dielectric behavior of the medium. Thus, AH, AG and A5 for the acidic resin-catalyzed hydrolysis of isopropyl acetate were calculated using the Wynne-Jones and Eyr-... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

Activation parameters that have been measured for Aal1 reactions are generally consistent with a unimolecular rate-determining step. The volume of activation for the hydrolysis of f-butyl acetate in 0.01 M HCI at 60°C is zero, within experimental error70. No significant change in rate is observed from atmospheric pressure up to 2 kbar, although this increase in pressure almost doubles the rate of hydrolysis of ethyl acetate in 0.1 M HCI at 35°C. 

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. 

The kinetics of the reaction between bromopropionate and thiosulfate ions have been studied at 10-40 °C in various ethanol-water mixtures.107 Activation parameters were evaluated as a function of ionic strength and dielectric constant of the medium. The medium effect of mixed solvents on the rate constants of the Menshutkin reaction of triethylamine with ethyl iodide has been studied for binary mixtures of cyclohexane with benzene or ethyl acetate,108 and with chlorobenzene or dimethoxyethane.109 Rates were measured over the temperature range 293.1-353.1 K, and activation parameters were determined. 

The acetolysis of a number of tetraalkylleads was studied by Robinson2 who used an excess of acetic acid as the solvent. He found that the acetolyses were first-order in tetraalkyllead rate coefficients and activation parameters for reaction (31) are given in Table 9. Robinson also studied the cleavage of tetraalkylleads by perchloric acid in solvent acetic acid. 

The kinetics of the oxidation of aromatic aldehydes by A-chloronicotinamide in aqueous acetic acid are first order in both reactants and in proton.339 The effect of substituents has been studied, and data at different temperatures yield activation parameters. 

Picosecond absorption spectroscopy studies of the contact ion pairs formed in the photo-initiated, S N 1 reaction of three substituted benzhydryl acetates (18) provided the rate constants for the k and k2 steps of the reaction (Scheme 10), in acetonitrile and DMSO.83 The activation parameters for the k and k2 steps were obtained from the temperature dependence of these steps and the transition state energies were calculated from the rate constants. This allowed the energy surfaces for three substituted substrates to be calculated in each solvent. The effect of solvent reorganization on the reactions of the unsubstituted and methyl-substituted benzhydryl contact ion pairs (CIP) was significant, causing a breakdown of transition state theory for these reactions. The results indicated that it will be very difficult to develop a simple theory of nucleophilicity in, S N1 reactions and that Marcus theory cannot be applied to SnI processes. 

The activation parameters for more complex reactions show similar patterns to that outlined above. For example, in the alkaline hydrolysis of ethyl acetate in aqueous mixtures (Tommila et al.t 1952), the second-order rate constant decreases with increasing mole fraction of ethyl alcohol, while AG increases, and AH falls to a minimum near x2 = 0-1 (Fig. 52), 5m AS having a minimum in the same region. A similar trend is observed in aqueous acetone and in... 

Rate coefficients and activation parameters of the acid catalyzed hydrolyses of cyclic acetals and ketals in various solvents... 

Special attention must be given to acetals and ketals in which the central carbon and the two oxygens are part of a five- or six-membered ring. A large amount of experimental work in this field has been done recently, mainly by Fife and his associates. Rate coefficients and activation parameters for the hydrolyses of some cyclic acetals and ketals can be found in Tables 12 and 13. For the 1,3-dioxolanes (= ethylene acetals or ketals), the rate coefficient is 1.5 to 4.4 powers of ten lower than for the corresponding diethyl acetals or ketals, respectively. The entropies of... 

Table 4. Rate constants and activation parameters of hydride transfer from linear and cyclic ethers and acetals to the (CeH5)3C cation... 

Simply changing the solvent in the Pd-based catalytic system from water to a mixture of water and a perfluorocarboxylic acid (some water is necessary for the reaction see Scheme 6) had no significant effect on product composition formic acid was still the principal product from methane. However, the addition of Cu or Cu chloride to the reaction mixture had a dramatic effect. Methanol and its ester now became the preferred products, with virtually no acetic and little formic acid being formed [40 b]. The activation parameters for the overall reaction determined under the condition when the rate was first order in both methane and carbon monoxide were A = 2 X [O sfa = 15.3 kcal mol . Since methyl trifluoro-acetate is both volatile and easily hydrolyzed back to the acid and methanol, it should be possible to design a system where the acid is recycled and methanol is the end product. Lee and co-workers have recently reported on the further characterization of the catalyst in this bimetallic Pd/Cu system [41]. 

The activation parameters (Table 3) are characterized by positive values of H koA negative values of and indicate that bond-formation plays an important role in forming the transition state. This is in agreement with previous work which showed that oxidative addition proceeds via an associative mechanism." For 2 in ethyl acetate, the oxidative addition rate constants could not be determined accurately, but /cr could be used for the calculation of the activation parameters. The values suggest less ordered transition states in which significant solvent interaction may occur, but it is clear that additional research is still required. 

Quantitative data are available for the reaction of solvolysis of 1(2-tellurienyl)ethyl acetates in 30% ethanol.38,59 The first-order rate constants for the reaction at 60° of all the four congener systems, the activation parameters, the rates relative to the thiophene derivative (k/kTb), and the rates of the 3-methyl-substituted compounds relative to the corresponding unsubstituted derivatives (k Jkf,) are summarized in Table XIX. 

Rate Constants, Activation Parameters, and Relative Rates for the Solvolysis of I(2-Aryl)Ethyl Acetates in 30% Ethanol... 

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