Hughes-Ingold solvent theory

Excellent discussions of solvent effects on reactions of this type have been given by Popovici and Pop (1957), by Palit (1947) and by Caldin and Peacock (1955). The qualitative Hughes-Ingold solvent theory (Ingold, 1953) predicts that reaction (29), in which uncharged reactants pass to... 

The rate of reaction (31) increases with decreasing solvent polarity. This observation agrees with the Hughes-Ingold solvent theory for reactions of this charge type. 

Spontaneous decarboxylations of carboxylate ions and hydrolyses of aryl phosphate dianions and aryl sulfate monoanions are much faster in organic solvents than in water (Thomson, 1970 Kemp and Paul, 1970 Bunton et al., 1967 Kirby and Varvoglis, 1967). This solvent effect is consistent with the Hughes-Ingold qualitative solvent theory because these reactions involve dispersion of charge in forming the transition state. 

HUGHES-INGOLD THEORY FOR SOLVENT EFFECTS ON REACTIVITY... 

HUGHES-INGOLD THEORY EOR SOLVENT EEEECTS ON REACTIVITY HUMMEL-DREYER TECHNIQUE HYALURONATE LYASE HYALURONIDASES HYBRIDIZATION HYDRATION ATMOSPHERE HYDRATION NUMBER Hydrazone reduction,... 

The reaction of 9-cyanofluorenyl anion (9-CFA, prepared from 9-cyanofluorene and sodium ethoxide) with alkyl and benzyl halides in ethanol is shown to be an Sn2 reaction from a kinetic study (Bowden and Cook, 1968). Qualitative application of the Hughes-Ingold theory of solvent effects predicts that, for an SN2 reaction of this charge type, an increase in the dielectric constant of the medium will cause a small rate decrease. That should be the case if the reaction is studied in ethanol (dielectric constant, D, 24 2) and DMSO (D, 48-9) is added. The data for the 9-CFA alkylation are in accord with this prediction, as seen in Table 10. 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

In spite of these limitations, the electrostatic Hughes-Ingold theory remains a good guide in predicting the solvent influence on chemical reactions, at least in a qualitative way. Exceptions can be safely assumed to involve strong specific solute-solvent interactions. 

Finally, because the interpretation proposed for specific MW nonthermal effects is rather similar to that for solvent effects (Hughes-Ingold theory), one can expect that aprotic polar solvents can be removed and replaced by MW activation requiring operation with a nonpolar solvent or, much better, under solvent-free conditions. In this situation we add also advantages of green chemistry conditions. In... 

It is only more recently that reactions of Charge type 4 have been known and studied, but the theory proposed by Hughes and Ingold predicted the results accurately here too, for the reaction of trimethylamine with trimethylsulfonium ion (Equation 4.12) proceeds more rapidly in nonpolar than in polar solvents.18... 

Apart from the selection of reactions involving dipolar, isopolar, or free-radical activated complexes used to demonstrate the qualitative theory of solvent effects by Hughes and Ingold [16, 44] in the preceding sections, further illustrative examples can be found in the literature e.g. [14, 15, 18, 21, 23, 26, 29, 460, 468]). 

In the past, transition state theory(l) was typically used in a qualitative manner to explain the effect of medium changes on reaction rates. This approach reached its quintessence in the systematic prediction by Hughes and Ingold(2) of the effect of increased solvent polarity on the rates of various types of nucleophilic substitution reactions. Their classification of S 2 reactions... 

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