Aqueous-phase solvation

Consequently, this reaction is endothermic in the gas phase. Also, AG was calculated to be 4-5.3 kcal moP in the gas phase. However, in the aqueous phase, solvation by water of the H—O2 radical provides some driving force for this reaction and, using a known solvent model, it is estimated that AGaqueous will drop to 1.6 kcalmop due to solvation effects. Substituent effects on the Q —H could make AG negative and accelerate the reaction even further. Consequently, a very weak phenolic O—H bond (BDE < 60 kcalmop ) can cause the phenolic antioxidant to turn into a pro-oxidant, since the H02 formed will start new oxidation chains. 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

Figure 1 shows the mechanistic picture developed by C. M. Starks (1,2) for Hquid—Hquid PTC in a graphical form. The catalyst cation extracts the more hpholilic anion Y from the aqueous to the nonpolar organic phase where it is present in the form of a poorly solvated ion pair Y ]. This then reacts rapidly with RX, and the newly formed ion pair X ] returns to the aqueous phase for another exchange process X — Y . In practice most catalyst cations used are rather lipophilic and do not extract strongly into the aqueous phase so that the anions are exchanged at the phase boundary. 

Phase-transfer catalysis (Section 22.5) Method for increasing the rate of a chemical reaction by transporting an ionic reactant from an aqueous phase where it is solvated and less reactive to an organic phase where it is not solvated and is more reactive. Typically, the reactant is an anion that is carried to the organic phase as its quaternary ammonium salt. 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

At electrode potentials more negative than approximately - 2.8 V (SHE), free solvated electrons appear in the solution as a result of (dark) emission from the metal. At this potential the electrochemical potential of the electrons according to Eq. (29.6) is about —1.6 eV, which is at once the energy of electron hydration in electron transfer from vacuum into an aqueous phase. 

The results here clearly demonstrate some of the important differences between reactions in the vapor phase and those in the aqueous phase. Water solvates the ions that form and thus enhances the heterolytic bond activation processes. This leads to more significant stabilization of the charged transition and product states over the neutral reactant state. The changes that result in the overall energies and the activation barriers of particular elementary steps can also act to alter the reaction selectivity and change the mechanism. 

Figure 4.11 Optimized structures of CH cO species, as indicated, over aqueous-solvated Pt(lll) as determined by DFT in Cao et al. [2005]. Horizontal and vertical arrows indicate C—H and O—H cleavage steps, respectively. Reaction energies are included for the aqueous phase [Cao et al., 2005] and the vapor phase (in parentheses) [Desai et al., 2002]. The thermodynamically preferred aqueous phase pathway is indicated by bold arrows (in blue). (See color insert.)...

This dependence is fundamental for electrochemistry, but its key role for liquid-liquid interfaces was first recognized by Koryta [1-5,35]. The standard transfer energy of an ion from the aqueous phase to the nonaqueous phase, AGf J, denoted in abbreviated form by the symbol A"G is the difference of standard chemical potential of standard chemical potentials of the ions, i.e., of the standard Gibbs energies of solvation in both phases. 

Extractive alkylation is used to derivatize acids, phenols, alcohols or amides in aqueous solution [435,441,448,502]. The pH of the aqueous phase is adjusted to ensure complete ionization of the acidic substance which is then extracted as an ion pair with a tetraalkylammonium hydroxide into a suitable immiscible organic solvent. In the poorly solvating organic medium, the substrate anion possesses high reactivity and the nucleophilic displacement reaction with an alkyl halide occurs under favorable conditions. 

The hydration of polyoxyethylene (POE) is dramatically affected by the anion present(J 0) in the aqueous phase. The adsorption of HEC (both 2.0 and 4.3 M.S.) was therefore studied in Na SO and Na PO, at equivalent normalities. The multivalent anions are more effective in precipitating POE than is the chloride ion. The amounts adsorbed and the interlayer expansions at normalities below precipitation conditions are given in Table III. The influence of multivalent anions on the intrinsic viscosity of variable M.S. HECs is illustrated in Figure 6. The increased amounts adsorbed are within experimental error, but the decrease in d. with the 4.3 M.S. HEC is notable. The d. changes in the absence of increased adsorption are not explainable in terms of solvation effects. 

The chaotropic properties of many chemical compounds prevent the H2O cage structures necessary for the formation of solvates and thus facilitate the transfer of nonpolar molecules between nonaqueous and aqueous phases. Water is incombustible and nonflammable, odorless and colorless, and is universally available in any quality important prerequisites for the solvent of choice in catalytic processes. The DK and d can be important in particular reactions and are advantageously used for the analysis and control of substrates and products. The favorable thermal properties of water make it highly suitable for its simultaneous dual function as a mobile support and heat transfer fluid, a feature that is utilized in the RCH/RP process (see below). 

B. C. Garrett and G. K. Schenter, Nonequilibrium solvation for an aqueous-phase... 

C. S. Callam, S. J. Singer, T. F. Fowary, and C. M. Hadad, Computational analysis of the potential energy surfaces of glycerol in the gas and aqueous phases Effects of level of theory, basis set, and solvation on strongly intramolecularly hydrogen bonded systems. J. Am. Chem. Soc. 123, 11743 11754 (2001). 

The thermodynamics of the extraction mechanism is extremely complex. In the initial equilibration of the ion pairs (Scheme 1.6) account has to be taken not only of the relative stabilities of the ion-pairs but also of the relative hydration of the anionic species. Assuming the complete non-solvation of the ion-pairs, the formation of the ion-pair [Q+Y] will generally be favoured when the relative hydration of X- is greater than that of Y. However, in many cases, the anion of the ion-pair is hydrated [8-11] (Table 1.1) and this has a significant effect both on equilibrium between the ion-pairs in the aqueous phase and the relative values of the partition coefficients of the two ion-pairs [Q+X ] and [Q+Y ] between the two phases. 

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