Solvent effects mechanisms

If reliable quantum mechanical calcnlations of reactant and transition state stnictures in vacnnm are feasible, treating electrostatic solvent effects on the basis of SRCF-PCM rising cavity shapes derived from methods... 

The GB equation is suitable for the description of solvent effects in molecular mechanics and dynamics [16], as well as in quantum mechanical calculations (17,18]. An excellent review of implicit solvation models, with more than 900 references, is given by Cramer and Truhlar [19]. 

Thus, solvent effects are consistent with z 8 2 mechanism in which... 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

E. S. Amis, "Solvent Effects on Reaction Rates and Mechanisms." Academic. New York, 1966. 

Solvent Effects on the Rate of Substitution by the S l Mechanism Table 8 6 lists the relative rate of solvolysis of tert butyl chloride m several media m order of increasing dielectric constant (e) Dielectric constant is a measure of the ability of a material m this case the solvent to moderate the force of attraction between oppositely charged par tides compared with that of a standard The standard dielectric is a vacuum which is assigned a value e of exactly 1 The higher the dielectric constant e the better the medium is able to support separated positively and negatively charged species 8olvents... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

For condensed species, additional broadening mechanisms from local field inhomogeneities come into play. Short-range intermolecular interactions, including solute-solvent effects in solutions, and matrix, lattice, and phonon effects in soHds, can broaden molecular transitions significantly. 

Chemical reactions are undoubtedly the most important issue in theoretical chemistry, where electronic structure plays an essential role. However, as will be demonstrated in this section, solvent effects also often play a crucial role in the mechanism of a chemical reaction in solution. 

We will discuss shortly the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theoiy allows discussion of reactions proceeding through transition states of intermediate character in terms of the limiting mechanistic types. The most important structural features to be considered in such a discussion are (1) the nature of the leaving group, (2) the nature of the base, (3) electronic and steric effects of substituents in the reactant molecule, and (4) solvent effects. 

The reactions of enamines as 1,3-dipolarophiles provide the most extensive examples of applications to heterocyclic syntheses. Thus the addition of aryl azides to a large number of cyclic (596-598) and acyclic (599-602) enamines has led to aminotriazolines which could be converted to triazoles with acid. Particular attention has been given to the direction of azide addition (601,603). While the observed products suggest a transition state in which the development of charges gives greater directional control than steric factors, kinetic data and solvent effects (604-606) speak against zwitterionic intermediates and support the usual 1,3-dipolar addition mechanism. 

Photoinitiation of polymerization of MMA and styrene by Mn(facac)3 was also investigated, and it was shown that the mechanism of photoinitiation is different [33] from that of Mn(acac)3 and is subject to the marked solvent effect, being less efficient in benzene than in ethyl acetate solutions. The mechanism shown in Schemes (15) and (16) illustrate the photodecomposition scheme of Mn(facac)3 in monomer-ethyl acetate and monomer-benzene solutions, respectively. (C = manganese chelate complex.)... 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

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