Role of solvents

The role of solvent in substitution reactions may be tu ofold since it can act both as a reagent in solvolytic processes and as a reaction medium. S nI reactions in which the solvent competes with azide ion are well known and have been discussed previously. In this section consideration is given to the use of solvents as reaction media. 

In common S nI reactions involving heterolysis to form ions, the differential solvation between protic and aprotic solvents for the anion X produced in the rate-limiting step is generally greater than for the cation R+ (equation 18). Hence such reactions typically proceed faster in protic solvents in which X is better solvated. 

Some rate ratios ( DMp/ MeCH) for reaction in dimethylformamide compared with methanol are azide ion with -iodonitrobenzene at 0 , 8-7 X 10 azide ion with methyl bromide at 0°, 1-6 x 10-. These may be compared with the solvent rate ratios for acetone acetone-methanol (75 25 v/v) methanol, for reaction ofp-nitrobenzyl bromide with azide ion at —20% viz. 4 5 x 10 47 1, or approximately 10 1 for acetone acetone-methanol . Similarly the ratio acetone acetone-methanol (75 25 v/v) for reaction of azide ion with n-butyl bromide at —20° is 2 35 x 10 . 

In addition to changes of rate in different solvents, solvent rate ratios have been shown to vary on changing the leaving group. In a simple approach the order of solvation energy (enthalpy) in both protic 

3 - R- Br, and with the differences between protic and aprotic solvents decreasing in the same order. In such a pattern the solvent rate ratio A dmp/ mcoh should therefore be lai er for displacement of bromine than of chlorine and this is apparent from the work of Parker and his colleagues and of Miotti which indicates that the solvent ratio for bromo compounds is about 3-5 times greater than that of the chloro compounds. 

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Extraction, a unit operation, is a complex and rapidly developing subject area (1,2). The chemistry of extraction and extractants has been comprehensively described (3,4). The main advantage of solvent extraction as an industrial process Hes in its versatiHty because of the enormous potential choice of solvents and extractants. The industrial appHcation of solvent extraction, including equipment design and operation, is a subject in itself (5). The fundamentals and technology of metal extraction processes have been described (6,7), as has the role of solvent extraction in relation to the overall development and feasibiHty of processes (8). The control of extraction columns has also been discussed (9). 

The correct choice of solvents is essential to the success of radiation-induced graft copolymerization. Their influence on radiation grafting has been the subject of many studies. It has been established that solvents play an important role in grafting because of the significance of polymer swelling. The grafting patterns to PP are solvent-dependent, thus, it is essential to examine the role of solvents. It was found that grafting in the alcohol is better than when other polar solvents such as dimethyl-... 

The Role of Solvent Viscosity, Ionic Radii, and Solvation... 

A reference to the role of solvents in biocatalysis was made in Section 4.5.2. 

Desai SK, Neurock M. 2003. First-principles study of the role of solvent in the dissociation of water over a Pt-Ru alloy. Phys Rev B 68 075420. 

Fundamental studies of coal liquefaction have shown that the structure of solvent molecules can determine the nature of liquid yields that result at any particular set of reaction conditions. One approach to understanding coal liquefaction chemistry is to use well-defined solvents or to study reactions of solvents with pure compounds which may represent bond-types that are likely present in coal [1,2]. It is postulated that one of the major routes in coal liquefaction is initiation by thermal activation to form free radicals which abstract hydrogen from any readily available source. The solvent may, therefore, function as a direct source of hydrogen (donor), indirect source of hydrogen (hydrogen-transfer agent), or may directly react with the coal (adduction). The actual role of solvent thus becomes a significant parameter. 

Our earlier studies [2,3] have measured the reactivity of both hydrocarbon and nonhydrocarbon acceptors with good donor solvents (Tetralin, hydrophenanthrenes), and poor donors (mesitylene). Although the primary role of solvents was... 

Carbonyl compounds are also suited to the investigation of the role of solvent reorganization in the dynamics of intramolecular dissociative electron transfer as observed in a series of phenacyl derivatives bearing various leaving groups.199... 

At this stage, the important (but still not well-understood) role of solvent in SCO compounds properties should be pointed out indeed, the nonsolvated [Fe(qsal)2] [Ni(dmit)2] compound was reported 2 years later [103]. The latter does not exhibit any cooperative spin-transition, probably due to the lack of n-interactions observed in its structural arrangement. Therefore, it seems that, whatever the counter-ion, the control of Ji-stacking is one of the key points to enhance cooperativity in SC complexes. 

Role of Solvent in Ligand Binding in the Active Site of... 

ROLE OF SOLVENT IN LIGAND BINDING IN THE ACTIVE SITE OF DHFR... 

Maitre, A., et al., Systemic sclerosis and occupational risk factors role of solvents and cleaning products, J. Rheumatol., 31, 2395, 2004. 

Fe3+X6...Fe2+X6, which is the reactant of the outer-sphere electron transfer reaction mentioned above when X = Y. Clearly the ground state involves a symmetric linear combination of a state with the electron on the right (as written) and one with the electron on the left. Thus we could create the localized states by using the SCRF method to calculate the symmetric and antisymmetric stationary states and taking plus and minus linear combinations. This is reasonable but does not take account of the fact that the orbitals for non-transferred electrons should be optimized for the case where the transferred electron is localized (in contrast to which, the SCRF orbitals are all optimized for the delocalized adiabatic structure). The role of solvent-induced charge localization has also been studied for ionic dissociation reactions [109],... 

We now turn to the electronically adiabatic ET reaction problem (cf. Sec. 2.2). There has been a spate oftheoretical papers [8,11 28,33,35,36,50] dealing with the possible role of solvent dynamics in causing departures from the standard Marcus TST rate theory [27,28] (although many of these deal with nonadiabatic reactions). The ET reaction considered is a simplified symmetric model, A1 2 A1/2 A1/2 A1/2, in a model solvent similar to CH3C1. The technical and computational... 

Murphrey, T. H. and Rossky, P. J. The role of solvent intramolecular modes in excess electron solvation dynamics, J.ChemPhys., 99 (1993). 515-522... 

P. Kumar, A. Mehta, S.M. Mahurin, S. Dai, M.D. Dadmun, B.G. Sumpter, and M.D. Barnes, Formation of oriented nanostructures from single molecules of conjugated polymers in microdroplets of solution the role of solvent, Macromolecules, 37 6132-6140, 2004. 

Chapter 18 by C. Chiappe focuses on the mechanism of bromination of alkenes, exploring the role of solvent on the formation of cyclic bromonium ion versus P-bromocarbemium ion, as key intermediates. In Chapter 19, H. P. A. Mercier et al. discuss the utility of a novel class of noble-gas onium salts as oxidants for generation and isolation of various trihalomethyl cation salts. 

ROLE OF SOLVENT IN ION-RADICAL CLEAVAGE AND IN STEPWISE VS. CONCERTED COMPETITIONS... 

Hydrogen-bonding can modify the aromatic character, and the role of solvents has been demonstrated by changes in the HOMA index caused by differences in the hydration of sodium and magnesium salts of... 

In recent years, there have been numerous studies examining the dynamics of proton transfer within the context of recently developed theoretical models. Reactions in the gas phase, in the solution phase, and in matrices have been examined [59-72]. Few of these studies, however, have addressed the issue of how the rate of proton transfer correlates with the thermodynamic driving force, which is an important correlation for discerning the validity of the various theoretical models. However, there have been two series of investigations by Kelley and co-workers [70, 71], and by Pines et al. [65, 66] that have sought to elucidate the role of solvent dynamics on the rate of proton transfer. 

Apart from the chemical effects of solvent as hydrogen donor, being heterogeneous systems, an important role of solvent is to provide appropriate reaction sites on the balance of solvent interactions with base polymer and growing graft chain. For the study of solvent effects, AM is not an adequate monomer since it is scarcely soluble in non-polar solvents. Instead, acrylic acid(AA) was employed and photografting was conducted as shown in Figure 8. 

In all cases the dielectric constant used is that of the pure solvent. Neglect of the solute is usually justified by its low concentration and the assumption that any necessary correction would be additive. In at least a few cases where the first two expressions have been employed the linearity of the results is to some extent dependent on how closely the refractive index of the solute meets the conditions n2 = 2.0 or 2.5 a situation not always recognized by the investigators. In one instance attempts have been made to clarify the role of solvent reaction field by examining solutes with different dipole moment orientations relative to the bonds involving coupled atoms. 

The role of solvent dynamics in the electron transfer reaction at a Pt electrode was discussed utilizing the theory of Zusman. In this work, ° the solvent-dependent rate constant for the electron transfer reaction at an electrode was given as... 

In comparison with the surface layer chemistry on active cathode materials where both salt anions and solvents are involved, a general perception extracted from various studies is that the salt species has the determining influence on the stabilization of the A1 substrate while the role of solvents does not seem to be pronounced, although individual reports have mentioned that EC/DMC seems to be more corrosive than PC/DEC. Considering the fact that pitting corrosion occurs on A1 in the polymer electrolytes Lilm/PEO or LiTf/PEO, where the reactivity of these macromolecular solvents is negligible at the potentials where the pitting appears, the salt appears to play the dominant role in A1 corrosion. 

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