Activation entropy oxidation

Habid and Malek49 who studied the activity of metal derivatives in the catalyzed esterification of aromatic carboxylic acids with aliphatic glycols found a reaction order of 0.5 relative to the catalyst for Ti(OBu)4, tin(II) oxalate and lead(II) oxide. As we have already mentioned in connection with other examples, it appears that the activation enthalpies of the esterifications carried out in the presence of Ti, Sn and Pb derivatives are very close to those reported by Hartman et al.207,208 for the acid-catalyzed esterification of benzoic and substituted benzoic acids with cyclohexanol. These enthalpies also approach those reported by Matsuzaki and Mitani268 for the esterification of benzoic acids with 1,2-ethanediol in the absence of a catalyst. On the other hand, when activation entropies are considered, a difference exists between the esterification of benzoic acid with 1,2-ethanediol catalyzed by Ti, Sn and Pb derivatives and the non-catalyzed reaction268. Thus, activation enthalpies are nearly the same for metal ion-catalyzed and non-catalyzed reactions whereas the activation entropy of the metal ion-catalyzed reaction is much lower than that of the non-catalyzed reaction. 

A suitable model for the oxygen carrier protein hemerythrin is [Fe2(Et-HPTB)(OBz)](BF4)2 (Et-HPTB = AWAT,iV -tetrakis[(N-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-l,3-diaminopropane, OBz = benzoate). It can mimic the formation of a binuclear peroxo iron complex in the natural system (101). The measured value of -12.8 cm3 mol1 for the activation volume of the oxidation reaction together with the negative value of the activation entropy confirm the highly structured nature of the transition state. 

In two earlier studies (106, 107), the oxidation of two Schiff base complexes were studied at room temperature, but in these cases only activation parameters for the overall process could be obtained since it was not possible to detect the formation of an intermediate species which could be attributed to a peroxo species. Nevertheless, the kinetic measurements provided indirect evidence for the existence of this intermediate. In both studies negative values for the activation entropies and the activation volumes were obtained. The oxidation of [Cu2(H-BPB-H)(CH3CN)2](PF6)2 (H-BPB-H = l,3-bis[iV-(2-pyridylethyl)-formidoyl]benzene) is accompanied by an activation entropy of -53 11 J K-1 mol-1 and an activation volume of -9.5 0.5 cm3 mol-1. In... 

Substrate binding is supported by activation entropies (AS ) that are generally in the range of -20 to -35 eu for both oxidative and reductive processes [232],... 

The activation energies, calculated from both thermal and photo-oxidation, were found to be identical. The difference in the rate constant values was attributed to the difference in the activation entropies. A similar study has also been performed in our group for isothermal crystallization of PEO [72]. 

Oxidative addition reactions of dihydrogen , iodine ", alkyl halides and Hg(CN)2 to carbonyl, olefin or phosphine substituted derivatives of rhodium(I) and iridium(I) have been described. In order to determine the effect on the rate of the reaction, the kinetics of the oxidative addition of Hg(CN)2 to Rh(dik)(P(OPh)3)2 has been studied . A second-order rate law coupled to large negative values of the activation entropy suggest an associative mechanism which probably proceeds via a cyclic three-centred transition state (equation 58). Analogous results were obtained with Ir(dik)(cod) . ... 

A kinetic study by Harrod and Smith of oxidative addition to a square planar cationic iridium complex also supports the three-center mechanism (258). The rate law is first order in the iridium complex and first order in hydrosilane. Determination of the activation parameters indicated a moderate activation enthalpy (AH — 5-6 kcal/mol) and a large negative activation entropy (AS — -47 e.u.) No variations were observed on changing the solvent. Harrod and Smith concluded that oxidative addition proceeds via a concerted three-center transition state in which little bond-making or bond-breaking had occurred. The activation enthalpy was attributed to a deformation of the square planar complex on its approach to the transition state. 

Other than giving a very good account of the experimental qualitative behaviour, the above results probably offer a reasonable quantitative evaluation of the leacbon parameters. The apparent activation free enthdpy of the reaction of 1-pyrroline-1-oxide and 5,5-dimethyl-1-pyrroline-l-oxide with acrylonitrile have been meas-ured 2 in cyclohexane (20.4 0.4 and 21.7 0.4 kcal mol, respectively) and in di-chloromethane (22.4 0.2 and 23.010.8 kcal mol ) at 298K the rate constants for the reactions of H-nitrone have not been measured, but it can be stressed that N-monosubstituted nitrone, e.g, t-Bu-nitrone, exhibits a high reactivity in 1,3-dipolar cycloadditions, which is similar to that of 1-pyrroline-l-oxide so that the evaluations of Table 6 emerge to be surprisingly good. Moreover, the activation entropy of the reaction of 5,5-dimethyI-1-pyrroline-l-oxide with acrylonitrile in cyclohexane has been estimated -31,9 eu, a value which is well reproduced by our calculations in the gas-phase (-33.4, -31.1, footnote of Table 6). 

This conclusion was supported by the observation that pyrolysis of 3-butenol has a AS of —8.8 eu, which is similar to the activation entropy values reported for pyrolysis of ethyl formate and for 3-butenoic acid, and the activation energies for all three pyrolyses are also similar (about 40 kcal/mol). Another well-known concerted syn elimination is the Cope elimination, which involves the thermal elimination of an alkene from an amine oxide (Figure 10.53). Unlike the reactions discussed above, all of which have... 

The variation of rate constant with ionic strength was measured and values of ZaZb of —2.00 in accord with a transition state involving the reactants were observed when the distance of closest approach of the reactants was 0.5 nm. Comparison of the present data with those for oxidation of free thiocyanate shows the latter to have a lower activation energy (12 compared with 15 kcal mol ) but amore negative activation entropy (-32 compared with —18.5 cal mol ). 

The activation energy for oxidation of the amine to complex, X2 was 14.7kcal/mole and activation entropy was — 35.3 entropy units. A mechanism similar to that postulated by Worthrich and Fallab was proposed, equation (126). 

The combination of kinetic isotope effects, activation parameters, and established rate law are consistent with the mechanism proposed in Scheme 18. The nature of intermediates R and S are unable to be probed since they occur after the rate-limiting step. Intermediates P and Q are inferred from the inhibitory effects of product and PPhs on the rate of reaction. The relatively neutral activation entropy is in agreement with intermediate P as a resting state and oxidative addition being the rate-limiting step in that little change in the overall molecular organization would be expected. 

Cobalt(in) oxidizes 2-mercaptoethylamine (HMea) in [Co(en)2(Mea)] to the corresponding co-ordinated disulphide complex by pathways involving Co + and [CoOH] +. An outer-sphere mechanism is suggested by the activation entropy (—3.1 cal K mol ) for reaction with Co and the reaction with [CoOH] + is substitution controlled. Redox proceeds by formation of a co-ordinated radical complex [Co(en)2(Mea)] +,... 

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