Tert-Butyl chloride solvolysis

When a scale of YC1 values (17) was used together with N0Ts values within equation 2, an l value of 0.30 was determined for tert-butyl chloride solvolysis. An Sn2 (intermediate) mechanism, involving both an intermediate and covalent interaction by a nucleophilic solvent molecule, was proposed... 

Because the solvolysis of adamantyl chloride has been used as a standard reaction, we list here the results of applying the conversion equations to this standard, using the equation given by Swain to represent the adamantyl rates. R of equation 5 was again set equal to 1 for acetic and formic acids. The yA value (equation 5) was found to be 0.04684, compared with 0.0627 when tert-butyl chloride solvolysis was the standard reaction for obtaining Y. 

Gajewski, J.J., Is the tert butyl chloride solvolysis the most misunderstood reaction in organic chemistry Evidence against nucleophihc solvent participation in the tert butyl chloride transition state and for increased hydrogen bond donation to the 1 adamantyl chloride solvolysis transition state, /. Am. Chem. Soc., 2001, 123(44), 10877-10883. 

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. ... 

Kevill and co-workers first address the much-debated issue of nucleophilic involvement in solvolysis of tert-butyl derivatives. Interestingly, the tert-butyl sulfonium salt shows more rate variation with solvent changes than does the 1-adamantyl salt. In particular, the tert-butyl salt shows a rate increase in aqueous TFEs (where both Y and N increase) that is not found for 1-adamantyl. Because a variation in Y cannot explain the result, Kevill argues that the tert-butyl derivative is receiving nucleophilic solvent assistance. On the basis of the available evidence, Harris et al. (Chapter 17) propose that tert-butyl chloride is inaccurately indicated by some probes to receive nucleophilic solvent assistance because the model system (1-adamantyl chloride) has a different susceptibility to solvent electrophilicity. Kevill and coworkers disagree with this proposal, noting that essentially the same tert-butyl to 1-adamantyl rate ratio is found for the chlorides and the sulfonium salts if solvent electrophilicity were important in one case but not the other, then the rate ratio should vary. 

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. 

Figure 2. Correlation of logarithms of solvolysis rates for 1-adamantyl chloride versus tert-butyl chloride at 25 °C (7). E represents ethanol, T represents trifluoroethanol, and HF represents hexafluoro-2-propanoL...

The initial objective of our work was to quantify solvent effects (particularly solvent nucleophilicity) by adapting the Grunwald-Winstein equation (2) (5). In equation 2, k is the rate of solvolysis of a substrate (RX) in any solvent relative to 80% v/v ethanol-water (k0) and Y is the solvent ionizing power defined by m = 1.000 for solvolyses of tert-butyl chloride at 25 °C. In this chapter, a discussion of equation 2 and similar free-energy relationships is presented. At the time our work began (1969), in collaboration with Schleyer, mechanisms of solvolytic reactions were close to a high in controversy (6-8). More recent mechanistic developments (9-13) are not reviewed in detail here, but increased recognition of the importance of nucleophilic solvent assistance should be noted. 

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... 

Supportive of the suggestion that ionization is not a major pathway in the Claisen rearrangement of the parent compound is the fact that the SDKIEs in aqueous solution are comparable to those in the gas phase and in m-xylene. Furthermore, attempts to solvolyze 1,1-dideuterioaUyl mesylate in aqueous methanol resulted in no ionization to an allyl cation instead, the direct displacement product was formed exclusively. Finally, determinations of a solvent kinetic isotope effect in deuterium oxide resulted in values around unity 10%. ° In the solvolysis reaction of tert-butyl chloride the value is 40% at room temperature. " It is possible to cause allylvinyl ethers to ionize by providing cation stabilizing substituents and Lewis acids or Lewis acidic solvents. ... 

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 ... 

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