Solvent nucleophilic constant

Line No. Pyridine substituents Nucleophile (solvent) Rate constant" (temp. °C) 10 fc liter mole- seo-i Activation energy kcal mole-1 Entropy of activation cal mole i deg-i Frequency factor logioA Ref. 

Line No. Triazine substituents Nucleophile + catalyst Solvent Rate constant (temp. °0) 106 liter mole i sec i Kin param Ex etic eters > JSt Ref. 

Line No. Benzene substituents Nucleophile Solvent Rate constant (temp. °C) 106 jc liter mole-1 see-i Energy of activation keal mole-1 Frequency factor logioA Ref. 

There is an ongoing controversy about whether there is any stabilization of the transition state for nucleophilic substitution at tertiary aliphatic carbon from interaction with nucleophilic solvent." ° This controversy has developed with the increasing sophistication of experiments to characterize solvent effects on the rate constants for solvolysis reactions. Grunwald and Winstein determined rate constants for solvolysis of tert-butyl chloride in a wide variety of solvents and used these data to define the solvent ionizing parameter T (Eq. 3). They next found that rate constants for solvolysis of primary and secondary aliphatic carbon show a smaller sensitivity (m) to changes in Y than those for the parent solvolysis reaction of tert-butyl chloride (for which m = 1 by definition). A second term was added ( N) to account for the effect of changes in solvent nucleophilicity on obsd that result from transition state stabilization by a nucleophilic interaction between solvent and substrate. It was first assumed that there is no significant stabilization of the transition state for solvolysis of tert-butyl chloride from such a nucleophilic interaction. However, a close examination of extensive rate data revealed, in some cases, a correlation between rate constants for solvolysis of fert-butyl derivatives and solvent nucleophicity. " ... 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

Similar changes in nucleophilic (or dipole) solvation (Scheme 2.8A) provide a simple explanation for the observation of other correlations between rate constants for solvolysis and solvent nucleophilicity. This interpretation does not require that there be stabilization of the transition state for solvolysis of tertiary derivatives by a partial covalent interaction between nucleophile and electrophile. We have defined this latter interaction as nucleophilic solvent participation (Scheme 2.8B) and have argued that the results of simple and direct experiments to detect stabilization of the transition state for reaction of simple tertiary derivatives by... 

For lack of a better system, the ratio of rate in an ethanol-water mixture of the same Y value as acetic acid to rate in the much less nucleophilic acetic acid, ( Etoii/ acoh) yj has served as a measure of sensitivity to solvent nucleophilicity. More recently, the problem has received renewed attention, and two groups have proposed possible approaches.114 Of the two proposals, that of Bentley, Schadt, and Schleyer is easier to apply. Their scheme defines the solvent nucleophilicity, N, by Equation 5.21, where k is the solvolysis rate constant of methyl tosylate in... 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

Line No. Quinoline substituents Nucleophile (solvent) Bate constant (temp. C) 10 lfc liter mole-1 seo i Activation Entropy of energy activation koal mole-1 oalmole-ideg -1 Ref. 

Line No. Naphthalene substituents Nucleophile (solvent) Rate constant/ (temp. C) 10 ifc liter mole-t sec Activation energy kcal mole-t Entropy of activation cal mole-1 deg Frequency factor 1 logioA Ref. ... 

Naively, one would expect that solvolysis rates constants, and ig, in solvents Si and S2 would be related as in (16). This would not be the case however, if one of the solvents were specifically involved in the transition state, i.e. if the solvolysis were not a pure S l process, but depended on solvent nucleophilicity (Winstein et ah, 1965). 

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