Solvent effects solvolysis

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... 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

Brown-Hammett correlation of 1-aryl-1-cyclobutylethyl p-nitrobenzoate gave a p+ value of -3.94 which is more positive than that of a model compound, l-aryl-l,2-dimethylpropyl p-nitrobenzoate (p+ = -4.65), which indicates substantial neighboring group participation of the cyclobutyl ring even in tertiary carbocationic systems.12 Solvent effects on the kinetics of the solvolysis... 

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... 

Problem 7.29 In terms of transition-state theory, account for the following solvent effects (a) The rate of solvolysis of a 3° RX increases as the polarity of the protic nucleophilic solvent ( SH) increases, e.g. 

The preceding sections have shown the complexity of solvent effects in the solvolysis of acyl chlorides, and how ambiguities in the role of the solvent, particularly in its apparent reaction order, critically affect the assignment of detailed mechanism. It is the intention in this brief section to point to some of... 

The application of correlation analysis of solvent effects to mechanistic studies of solvolysis has been reviewed by Takeuchi in Japanese.110 The article mainly covers die behaviour of tertiary chloro compounds. Tins author s research group has continued experimental studies in this area.111-113 Rates of solvolysis of 2-chloro-2,4-trimethylpentane have been measured in 17 solvents and analysed through the extended Grunwald-Winstein equation, which includes a term for nucleophilic participation.111... 

To assess the accuracy and precision of these other factors influencing relative reactivity. Recently, solvent effects were shown to produce important variations in the relative reactivity of very similar molecules. Thus the relative influence of m-methyl and m-t-butyl substituents on the rate of solvolysis of benzhydryl chloride depends on the solvent (Shiner and and Verbanic, 1957). Less remarkable but equally important variations in reactivity were detected among the p-alkylated benzhydryl chlorides (Shiner and Verbanic, 1957 Berliner and Chen, 1958). A full analysis of solvent influences (Clement et al., 1960) requires much detailed... 

Nevertheless, the demonstrated variations in solvolytic reactivity made a study of solvent effects mandatory. The solvolysis of eighteen substituted phenyldimethylcarbinyl chlorides was examined in two or more solvents (Okamoto et al., 1958). Anhydrous methanol, ethanol, and propan-2-ol were employed in addition to 90% aqueous acetone. The results presented in terms of the a—constants are summarized in Table 21. 

The o -values evaluated from the solvolysis study are in the inverse order (Table 27). As noted in Section IV, solvent effects are apparently significant to the minor extent necessary to effect these small changes. 

Roberts, D. D. Solvent effects. The solvolysis rates of cyclopropylcarbinyl, 1-methylcyclopro-pylcarbinyl, and 1-phenylcyclopropylcarbinyl arenesulfonate derivatives./. Org. Chem. 1964, 29, 294-297. 

An analysis of these results in terms of solvent effects leads to the observation of similarities with Ritchie s work on the N+ relation. Thus the constant selectivities obtained in the solvolysis reactions of certain methyl derivatives (Table 9) may indicate the existence of a basic similarity between the rate-determining process in these reaction and in the electrophile-nucleophile combination reactions correlated by the IV+ relation. The failure of the methyl halides to conform to this pattern might suggest that their substitution reactions are fundamentally different, and that the free energy of activation is dependent on factors other than desolvation. 

Winstein, S. and Fainberg A.H., Correlation of Solvolysis Rates. 1 V.l Solvent Effects on Enthalpy and Entropy of Activation for Solvolysis of Butyl Chloride2. J. Am. Chem.Soc. (1957) 79 5937-5950. 

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