Solvolysis limiting cases

The points that we have emphasized in this brief overview of the S l and 8 2 mechanisms are kinetics and stereochemistry. These features of a reaction provide important evidence for ascertaining whether a particular nucleophilic substitution follows an ionization or a direct displacement pathway. There are limitations to the generalization that reactions exhibiting first-order kinetics react by the Sj l mechanism and those exhibiting second-order kinetics react by the 8 2 mechanism. Many nucleophilic substitutions are carried out under conditions in which the nucleophile is present in large excess. When this is the case, the concentration of the nucleophile is essentially constant during die reaction and the observed kinetics become pseudo-first-order. This is true, for example, when the solvent is the nucleophile (solvolysis). In this case, the kinetics of the reaction provide no evidence as to whether the 8 1 or 8 2 mechanism operates. 

All are tertiary halides so that attack by the S mode would not be expected to occur on (16) or (17) any more than it did on (8) (cf. p. 82). Sn2 attack from the back on the carbon atom carrying Br would in any case be prevented in (16) and (17) both sterically by their cagelike structure, and also by the impossibility of forcing their fairly rigid framework through transition states with the required planar distribution of bonds to the bridgehead carbon atom (cf. p. 84). Solvolysis via rate-limiting formation of the ion pair (SN1), as happens with (8) is... 

In all of these compounds solvolysis will lead to a tertiary ion. The series [10], [13], [11] clearly indicates the strain argument, and one may note that the difference in rates between [1] and [11] corresponds to an energy difference of only 1 1 kcal mole . The data do not prove that non-classical stabilization of the transition state in [1] and [12] is not partly responsible for the rate differences but rather suggests that relief of striiin could account for the results. Other factors, particularly differential solvation of the ground state and transition state and the possibility that solvolysis may not be of a limiting type but involve reaction with solvent, may also play a role but are difficult to evaluate. In any case the rate of solvolysis of exo-compounds does not appear to be unusually rapid when viewed in this light. 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

Their approach in looking into the problem further was to find structures in which specific covalent bonding to the back side of the carbon undergoing substitution is difficult or impossible. As models for reactions at tertiary carbon they chose bridgehead substitutions. We have seen in Section 5.2 that rates in these systems are retarded, in some cases by many powers of ten, because of the increase in strain upon ionization. But the important point in the present context is that it is impossible for a solvent molecule to approach from the back side of a bridgehead carbon the only possibilities are frontside attack, known to be strongly disfavored (Section 4.2), or limiting SW1 solvolysis with nonspecific solvation. 

In contrast to typical mono- or acyclic substrates (e. g.,isopropyl), 2-adaman-tyl derivatives are also found to be insensitive to changes in solvent nucleophilicity. A variety of criteria, summarized in Table 13, establish this point. In all cases, the behavior of 2-adamantyl tosylate is comparable to that observed for its tertiary isomer but quite unlike that observed for the isopropyl derivative. Significant nucleophilic solvent participation is indicated in the solvolysis reactions of the isopropyl system. The 2-adamantyl system, on the other hand, appears to be a unique case of limiting solvolysis in a secondary substrate 296). The 2-adamantyl/ isopropyl ratios in various solvents therefore provide a measure of the minimum rate enhancement due to nucleophilic solvent assistance in the isopropyl system 297). 

Let us now consider Scheppele s discussion of the solvolysis of 2-d-exo- and -endo-bicyclo[2.2.1] hept-2-yl p-nitrobenzoates and p-toluenesulfonates. A maximum a isotope effect of 1 22 is suggested for kH/kD for the limiting solvolysis of simple secondary sulphonates (solvolyses where the rate is not affected by the nucleophilicity of the medium). The endo-esters yield a similar value which might be taken to imply the absence of steric hindrance in their reaction. However, whereas the maximum isotope effect for the simple sulphonates is believed to arise partly from an activated complex in the conversion of the internal ion pair into a solvent-separated ion pair, B - C, the effect in the endo-norbomyl compounds is suggested to arise during the transition state involved in the formation of the intimate ion pair, A - B. The latter is deduced from the fact that polarimetric and titrimetric rate constants are the same in acetic acid, aqueous acetone and ethanol (Winstein and Trifan, 1949 and 1952). If this is the case, the endo-effect of 1-20 should be... 

Evaluation of solvent-sensitive properties requires well-defined referena i ran eis. A macroscopic parameter, dielectric constant, does not always give interpretable correlations of data. The first microscopic measure of solvent polarity, the Y-value, based on the solvolysis rate of t-butyl chloride, is particularly valuable for correlating solvolysis rates. Y-values are tedious to measure, somewhat complicated in physical basis, and characterizable for a limited number of solvents. The Z-value, based on the charge-transfer electronic transition of l-ethyl-4-carbomethoxy-pyridinium iodide , is easy to measure and had a readily understandable physical origin. However, non-polar solvent Z-values are difficult to obtain b use of low salt solubility. The Et(30)-value , is based on an intramolecular charge-transfer transition in a pyridinium phenol b ne which dissolves in almost all solvents. We have used the Er(30)-value in the studies of ANS derivatives as the measure of solvent polarity. Solvent polarity is what is measured by a particular technique and may refer to different summations of molecular properties in different cases. For this reason, only simple reference processes should be used to derive solvent parameters. 

Monoanions of A -substituted phosphoramidic acids (30) show an interesting solvolytic rate dependence on the basicity of the amine. For amines of pK, 8.0 the rate is independent of basicity, and for these cases it was suggested that the fraction of AT-protonated species became rate limiting. From a comparison of the Bronsted j8 values associated with the entering and leaving groups it was proposed that quinquecovalent intermediates are not involved in the solvolysis of (30) or in the solvolysis... 

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