Hydrogen bonding interactions solvent effects

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

The.effect of the entropy of activation was noted above for the quaternary pyridine salts (280 and 281). In future work, it may also be found to reflect the electrostatic or hydrogen-bonding interactions in transition states of amination reactions and the effect of reversible cationization of an azine-nitrogen. Brower et observed a substantial rate difference between piperidino-dechlorinations of 2-chloropyrimidine in petroleum ether and in alcohol due partly to the higher entropy of activation in the latter solvent (Table III, lines 3 and 4). 

Water has physical hemical properties that are very different from those of other solvents [1] and its role in enhancing the reactivity and selectivity of some organic reactions is still a debated question. Recent experimental studies [3e, 9] and computer simulations [10] seem to indicate, at least with respect to the rate enhancement of aqueous Diels Alder reactions, that the main effects are due to the enforced hydrophobic interactions and hydrogen bond interactions. 

In other systems, similar kinetic laws were observed when studying the effect of added pyridine, although differentiation with the dimer nucleophile mechanism is made in the interpretation of the experimental results (see below). Rationalizations of the involved phenomena are based on the strong hydrogen-bond interactions between the nucleophile and the pyridine, and on the catalytic effect of a third amine molecule in the decomposition of the zwitterionic intermediate in non-polar solvents. 

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