Reactions involving solvational change

Solvent exchange processes are the most fundamental substitution reactions that characterize the lability and reaction mechanism of a metal center within a coordination geometry. For these reactions the interpretation of the volume of activation becomes rather straightforward since these reactions do not involve major changes in solvation due to changes in dipole moments and electrostriction, which can in... 

Figure 1. Constancy of AV for solvent exchange reactions where AV = —6.5 cm6/mol for Cr(DMF)63+-DMF solvent exchange at 338 K (a) and pressure dependence of AV for reactions involving charge development and, hence, solvational change where AV0 = —18.5 err /mol for Co(NH3)sSO/ direct aquation at 298 K (b).

Sec. 4.2.1). The interpretation of reactions involving charges is more difficult, because of the importance of ion solvation changes. 464 types Qf reactions have been studied and will be encountered in Part II. Representative examples are shown in Table 2.4. 5-i69 jt jg worth reemphasizing the value of determining both AK and AF for a reaction system. The construction then of volume profiles enables a detailed description of the mechanism. The reaction steps for interaction of CO2 with Co(NH3)50H2 are considered to be... 

Nucleophilic addition of phenolic nucleophiles to l,l-dicyano-2-arylethenes in the gas phase and in water has been studied theoretically" using the semiempirical AMI method and the Cramer-Truhlar solvation model SM2.1. The difference between the Brpnsted coefficients (a" = 0.81 and P" =0.65) determined for the gas-phase reaction is indicative of a small positive transition state imbalance of / = 0.16. For reaction in water the estimates (a" = 0.61 and P" = 0.36, giving I = 0.25) are close to the experimental values (a" = 0.55 and P" = 0.35) obtained with amine bases, and the small imbalance is as expected for a reaction involving no hybridization change at the incipient carbanion site. 

Substituted benzaldehydes have proved useful as acid-base indicators for reactions involving the addition of hydroxide ions n strongly alkaline aqueous media (19). It seemed logical to extend their use to solutions of sodium hydroxide in water-ethanol and water-DMSO mixtures. In ethanol-water, it was of interest whether the competition between addition of hydroxide and ethoxide ions will be reflected in the dependence of the J- function on ethanol concentration. In water-DMSO mixtures, it was important to investigate whether the radical change at higher DMSO concentrations, observed for H values and attributed to changes in solvation of the hydroxide ion, will be observed for the addition reaction as well. 

Changes in solvation pattern on activation, and the effect on A factors for reactions involving charges and charge-separated species in solution... 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

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