Activation parameters solvent effects

The allyl-silyl exchange reactions are clearly more complicated than the 100% intramolecular rearrangements of silyl silylmethyl ethers. Nevertheless, the present author believes that they are closely related. It is rather striking that both processes (in the fluorenyl series) have nearly identical activation parameters. Solvent effects are small in both cases. Thus, it appears reasonable to assume similar transition states, namely rate-determining coordination between silicon and oxygen. For the fluorenyl substrate a mechanistic dichotomy evolves (Scheme 3). The... 

Catrina IE, Hengge AC. Comparisons of phosphorothioate and phosphate monoester transfer reactions activation parameters, solvent effects, and the effect of metal ions. JAm Chem Soc. 1999 121 2156-2163. 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

Table XIII gives typical examples of 7t-barriers for planar heterocycles comparing the electron-attracting or electron-donating moiety to push-pull ethylenes. Steric effects destabilize the ground state and thus greatly reduce the 71-barriers to rotation. In these push-pull ethylenes the large entropy of activation and solvent effects hampered easy comparison of the barriers without high-quality determination of the activation parameters.

The mechanistic dichotomy (Eq. 50 and 52) that obtains in the thermolysis of 2,3-dioxabicyclo[2.2. l]heptane 9 has no equivalent with 2 and accordingly the abnormally large solvent effects found in the thermolysis of 9 were not observed for 2. In fact, the [2.2.2] system 2 is considerably more stable thermally than the [2.2.1] system 9. While 9 decomposes quite rapidly in cyclohexane at 60-80 °C, for 2 temperatures as high as 120-150 °C are needed to promote comparable rates the activation parameters are AH = 21kcalmor1and AS = — 19e.u.for2,3-dioxabieyclo[2.2.1]-heptane, and AH = 33 kcal mol-1 and AS = +3 e.u. for 2,3-dioxabicyclo[2.2.2]-octane 67). 

The reaction goes faster in more polar solvents (a range of 106 in the rate constant) and parallels carbonium ion rearrangements in that respect. The effect of substituents in the para position of the benzoate group also suggests that the rate-determining step is the formation of an initial ion pair. The reaction is faster with the nitro than with the methoxyl substituent.819 820 The Hammett p value is 1.34. The activation parameters are not known for any but the unsubstituted member of the series however, and hence it is not known to what extent the relative rates depend upon the temperature. 

On Main Group Metal Ions by Al3+ Effect of Non-leaving Ligands on the Rate Constants and Activation Parameters for Solvent Exchange on Al3+... 

The pseudothermodynamic analysis of solvent elfects in 1-PrOH-water mixtures over the whole composition range (shown in Figure 7.3) depicts a combination of thermodynamic transfer parameters for diene and dienophile with isobaric activation parameters that allows for a distinction between solvent elfects on reactants (initial state) and on the activated complex. The results clearly indicate that the aqueous rate accelerations are heavily dominated by initial-state solvation effects. It can be concluded that for Diels-Alder reactions in water the causes of the acceleration involve stabilization of the activated complex by enforced hydrophobic interactions and by hydrogen bonding to water (Table 7.1, Figure 7.4). °... 

The early experiments concerned with proton transfer from hydrogen-bonded acids did not provide information which permitted a choice between the two mechanisms (Kresge, 1973). These experiments included the measurement of kinetic isotope effects (Haslam et al., 1965b Eyring and Haslam, 1966 Haslam and Eyring, 1967), activation parameters (Haslam et ai, 1965a), the effect of different solvents (Jensen et al., 1966) and substituent effects in the intramolecularly hydrogen-bonded acid (Miles et al.,... 

Valuable information on mechanisms has been obtained from data on solvent exchange (4.4).The rate law, one of the most used mechanistic tools, is not useful in this instance, unfortunately, since the concentration of one of the reactants, the solvent, is invariant. Sometimes the exchange can be examined in a neutral solvent, although this is difficult to find. The reactants and products are however identical in (4.4), there is no free energy of reaction to overcome, and the activation parameters have been used exclusively, with great effect, to assign mechanism. This applies particularly to volumes of activation, since solvation differences are approximately zero and the observed volume of activation can be equated with the intrinsic one (Sec. 2.3.3). 

The acid-catalysed hydrolysis of the acylal, 1-phenoxyethyl propionate (13), has been studied using the PM3 method in the gas phase. The kinetics and mechanism of the hydrolysis of tetrahydro-2-furyl and tetrahydropyran-2-yl alkanoates (14) in water and water-20% ethanol have been reported. In acidic and neutral media, kinetics, activation parameters, isotope-exchange studies, substituent effects, solvent effects and the lack of buffer catalysis pointed clearly to an Aai-1 mechanism with formation of the tetrahydro-2-furyl or tetrahydropyran-2-yl carbocation as the rate-limiting step (Scheme 1). There is no evidence of a base-promoted Bac2 mechanism up to pH 12. ... 

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