Solvolysis of tert-butyl chloride

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... 

Relative Rate of SnI Solvolysis of tert-Butyl Chloride as a Function of Solvent Polarity ... 

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. " ... 

Results such as these have led to recurring discussions about the extent of stabihzation of the transition state for heterolytic cleavage at tertiary carbon by nucleophilic assistance from solvent. The difficulty lies in reconciling studies that suggest that there is a small dependence of obsd (s ) for solvolysis of tert-butyl chloride,and some cumyl derivatives, on solvent nucleophilicity with other work that shows there is no detectable stabilization of the transition state for these reactions by interaction with the strongly nucleophilic azide and hydroxide ions, and the strong neutral nucleophile propanethiol. ... 

We first applied the solvatochromic equation (SCE) to solvolysis of tert-butyl chloride (t-BuCl) to determine if the method could give a reasonable result for this much-studied reaction (7). Abraham et al. (11) had previously attempted correlation of these rates with the SCE without the cavity term, but as Bentley and Carter (12) have noted, an unsatisfactory result was achieved (7). First, TFE and hexafluoroisopropyl alcohol (HFIP) did not fit the correlation. Second, no rate dependence on solvent nucleophilicity 0 was found, despite other works indicating a weak dependence on this parameter (12, 13). Also, different correlations were observed for hydroxylic and nonhydroxylic solvents Bentley considered this finding to indicate that the dehydrohalogenation transition state (in nonhydroxylic solvents) and the solvolysis transition state (in hydroxylic solvents) were significantly different and thus concluded that the two types of reactions should not be included in the same correlation. 

In an analysis of the solvolysis of tert-butyl chloride in terms of equation 3, Abraham et al. (9) found that the specific rates could be correlated against tt and a, without the need to include a term relating to basicity (P) values. However, a new analysis with inclusion of the cavity term has indicated (II) a minor contribution from the fop term. Conversely, an analysis (12) in terms of the Koppel-Palm equation, with inclusion of a cavity term, did not find any dependence on nucleophilicity (basicity). 

The superscripts and subscripts are solvent designations. The term Ahepsl refers to the difference in A values between heptane, a zero point on the Swain scale, and solvent SI, for example. In our first investigated conversion, we base the Y values on the rates of solvolysis of tert-butyl chloride, as calculated from the A, B, a, and b parameters. As shown in the Appendix, making the additional assumption that the nucleophilicities of two solvents, Srefl and SrefS, have the ratio R that allows the calculation of the constants needed to apply equations 3 and 4. The results are... 

In the conversion of A and B to N and Y, the Y values represent the best fit of equation 1 to the rates of solvolysis of tert-butyl chloride. Clearly, none of the N solvent property should be added to Y to give a better fit, because the fit is already optimized. Therefore, s = 0 for the reactions of this compound. Equation 8 with s = 0 leads to... 

Kinetics so closely related to the solvent effeet as those of the Menschutkin reaction between triethylamine and ethyl iodide [eq. (10.3.27)], the solvolysis of tert-butyl chloride [eq. (10.3.28)] or the deearboxylation of 3-carboxybenzisoxazole [eq. (10.3.29)], are ac-eeptably described by our scales. 

The rate of this reaction between triethylamine and ethyl iodide, whieh varies by five orders of magnitude fromn-hexane (1.35xl0MmoT s ) toDMSO (8.78xlO lmoT s ), is aceurately described by solvent polarity and acidity -the sensitivity to flie latter is somewhat imprecise. The equation for the kinetics of solvolysis of tert-butyl chloride is ... 

In the light of the previous reasoning, describing the solvolysis of tert-butyl chloride or the decarboxylation kinetics of 3-carboxybenzisoxazole in mixed solvents in terms of SPP, SB and SA for the mixtures appeared to be rather difficult owing to the differences between the processes concerned and the solvatochromism upon which the scales were constructed. However, the results are categorical as judged by the following facts ... 

In fact, the solvolysis of tert-butyl chloride is one of the cornerstones of physical organic chemistryThus, some quantitative approaches to the kinetics of spontaneous reactions in various solvents -and, more interesting, solvent mixtures- are based on linear Ifee-energy relations such as that of Grunwald and Winstein or its extensions. These equations allow one to interpolate or extrapolate rate constants that caiuiot be readily mea-smed, and also to derive mechanistically significant information in the process. 

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