Mechanistic Descriptions and Borderline Mechanisms

Winstein concluded that two intermediates preceding the dissociated carbocation were required to reconcile data on kinetics and stereochemistry of solvolysis reactions. The process of ionization initially generates a carbocation and counterion in immediate 

Many specific experiments support this general scheme. For example, in 80% aqueous acetone, the rate constant for racemization of / -chlorobenzhydryl p-nifrobenzoate and the rate of exchange of the in the carbonyl oxygen can be compared with the rate of racemization. At 100 °C, = 2.3. 

If it is assumed that ionization results in complete randomization of the 0 label in the carboxylate ion, is a measure of the rate of ionization with ion pair return and k is a measure of the extent of racemization associated with ionization. The fact that the rate of isotopic exchange exceeds that of racemization indicates that ion pair collapse occurs with predominant retention of configuration. This is called internal return. When a better nucleophile is added to the system (0.14M NaNj), k is found to be unchanged, but no racemization of reactant is observed. Instead, the intermediate that can racemize is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the contact ion pair returns to the 

The ion pair return phenomenon can also be demonstrated by comparing the rate of racemization of reactant with the rate of product formation. For a number of systems, including 1-arylethyl tosylates, the rate of decrease of optical rotation is greater than the rate of product formation, which indicates the existence of an intermediate that can re-form racemic reactant. The solvent-separated ion pair is the most likely intermediate to play this role. 

Racemization, however, does not always accompany isotopic scrambling. In the case of 2-butyl 4-bromobenzenesulfonate, isotopic scrambling occurs in trifluo-roethanol solution without any racemization. Isotopic scrambling probably involves a contact ion pair in which the sulfonate can rotate with respect to the carbocation without migrating to its other face. The unlikely alternative is a concerted mechanism, which avoids a carbocation intermediate but requires a front-side displacement.  

SECTION 5.3. DETAILED MECHANISTIC DESCRIPTION AND BORDERLINE MECHANISMS 

The ionization and direct displacement mechanisms can be viewed as the extremes of a mechanistic continuum. At the 8 1 extreme, there is no covalent interaction between the reactant and the nucleophile in the transition state for cleavage of the bond to the leaving group. At the 8 2 extreme, the bond formation to the nucleophile is concerted with the bondbreaking step. In between these two limiting cases lies the borderline area, in which the degree of covalent interaction between the nucleophile and the reactant is intermediate between the two limiting cases. The concept of ion pairs is important in the consideration of 

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

Attack by a nucleophile or the solvent can occur at either of the ion pairs. Nucleophilic attack on the intimate ion pair would be expected to occur with inversion of configuration, since the leaving group would still shield the fiont side of the caibocation. At the solvent-separated ion pair stage, the nucleophile might approach fiom either fece, particularly in the case where solvent is the nucleophile. Reactions through dissociated carbocations should occur with complete lacemization. According to this interpretation, the identity and stereochemistry of the reaction products will be determined by the extent to which reaction occurs on the un-ionized reactant, the intimate ion pair, the solvent-separated ion pair, or the dissociated caibocation. 

If it is assumed that ionization would result in complete randomization of the 0 label in the carboxylate ion, fceq is a measure of the rate of ionization with ion pair return and fcrac is a measure of the extent of racemization associated with ionization. The fact that the rate of equilibration exceeds that of racemization indicates that 

This implies that ion pair formation and recombination is occurring competitively with ion pair formation and substitution. 

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