Solvent effects, reactive mechanisms

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). 

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. 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

The Diels-Alder reaction is among the most useful tools in organic chemistry. It has been the object of a great number of theoretical studies95-131 dealing with almost every one of the experimental aspects reactivity, mechanism, selectivity, solvent effects, catalysis and so on. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Despite the impressive knowledge accumulated on some of the more common organic reactions, the question of true intrinsic reactivity is still at large in most cases. It is known that rates and mechanisms can be influenced by solvent effects, and that such changes can seldom be accommodated by rigorous theoretical treatments which are only applicable to isolated species. Thus, the concept of intrinsic reactivity should be, in principle, derived from chemical behaviour in a solvent-free environment. This statement is particularly relevant for reactions involving ionic species which are subject to strong electrostatic interactions with the solvent. 

Studies with p-ni troperbenzoic acid, a reactive peracid, favor a different mechanism. The high degree of retention observed in oxidation of epimeric cycloalkanes and (—)-2,6-dimethyloctane [Eq. (9.30)], as well as substituent and solvent effects, agree with a cyclic transition state (3) originally suggested by Bartlett79 to interpret epoxidation by peracids ... 

Mechanism. The following types of evidence ere pertinent in selecting on acceptable mechanism for olefin epoxidation by means of peroxy acids (1) the nature of the peroxy acid and the electronic effect of eubBtituents on its reactivity (2) the electronic effect of substituents on the reactivity of the olefin component (3) stereochemical factors affecting the reactivity of the olefin (4) the possibility of acid dialysis (5) solvent effects and (6) neighboring group effects. 

L abb6 and co-workers have found that 4-alkyl-5-sulfonylimino-A2-l,2,3,4-thiatriazolines 165 can be easiliy prepared by reaction of the highly reactive arylsulfonyl isothiocyanates 166 with alkyl azides (Equation 15). Kinetic experiments indicate that the reaction between picryl isothiocyanate and alkyl azides is probably concerted since no significant solvent effect has been observed <1973JOC2916, 1974JA3973>. This is in support of the postulated mechanism involving the intermediate 5-imino-A2-l,2,3,4-thiatriazolines 164 in the reaction of isothiocyanates with hydrazoic acid. 

Here we give an overview of the current status and perspectives of theoretical treatments of solvent effects based on continuum solvation models where the solute is treated quantum mechanically. It is worth noting that our aim is not to give a detailed description of the physical and mathematical formalisms that underlie the different quantum mechanical self-consistent reaction field (QM-SCRF) models, since these issues have been covered in other contributions to the book. Rather, our goal is to illustrate the features that have contributed to make QM-SCRF continuum methods successful and to discuss their reliability for the study of chemical reactivity in solution. 

What is clear, is that solvent effects may play a substantial, complex, and yet often subtle role in the solvolytic reaction mechanism, and that further study is required to increase the limited understanding which has been achieved to date. The original reactivity-selectivity relationships obtained by Sneen et al. (1966a) and Raber, Schleyer et al. (1971a) are now seen to be entirely fortuitous. The N+ correlation has demonstrated that the selectivities of cations are independent of their stability. Hence, the observed relationships are a result of the averaging of selectivities of attack at the different ion pair stages, and are not simple reactivity-selectivity relationships at all. 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

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