Solvent molecules and carbocation intermediates in solvolyses
Electrophilic or hydrogen bonding assistance of hydroxylic molecules can be verified by the rate acceleration in SN1 acetolysis of 1-phenylethyl chloride, by retentive outcomes of the substitution products in the reaction of an optically active trifluoroacetate in benzene, and by kinetic partial resolution of 1-phenylethyl chloride in optically active carboxylic acid solvents. Nucleophilic solvation of carbocation intermediates is another driving force for SN1 ionization and can be verified by rear-side shielding of the intermediate by nitriles and hindered phenols in the solvolyses in these solvents. As a solvolysis solvent, phenol has an amphiphilic, electroand nucleo-philic, driving force, which has been proved by stereochemical outcomes of phenolyses of 1-phenylethyl systems with vanous leaving groups and by those of competitive methanolysis and phenolysis of optically active 1-phenylethyl chloride. There are two types of phenolysis pathways for simple alkyl and aralkyl systems; both of them afford retained phenyl ether from optically active substrate in the presence of a base. One type has single ionpair intermediate, while the other has two. In the latter most of the phenolysis products are derived from the second intermediate. The number of intermediates has been determined by use of Winstein's kp-kt plot against concentration of added salt, such as LiOPh, NaOPh, 2,6-di-tert-butyl-4-methylpynidinium phenoxide, LiC1O4, and Bu4NC1O4. Examination of the stenic outcomes of three products, ROPh, 0and pRC6H4OH, suggests a variety of ion-pair structures, including four-center quadrupole and rear-side shielded ion-pair, as the first and second intermediates, respectively, in phenolyses. The usefulness of the methanol perturbation method for examination of the intermediate structure has been elucidated in the retentive phenolysis of l-p-anisyl-2,2-dimethylpropyl p-nitrobenzoate. INTRODUCTION The intermediacy of the carbocation in the solvolyses of alkyl and anal kyl halides was suggested in the mid 1920's (Ref. 1). Ten years later the mechanism was designated as SN1 by Ingold, and a free carbocation was suggested as the intermediate (Ref. 2). In 1940 the role of solvent molecules in carbocation formation was first described by Hammett, and the reaction was depicted as polymolecular solvolysis (Ref. 3). He pointed out the inipient solvation of the leaving group ion in the transition state as the driving force of the solvolytic reaction. Eighteen years later Winstein presented irrefutable experimental evidence for two kinetically distinguishable ion-pairs as the successive intermediates (Ref. 4), and since then the ionpairing and ion-pair return has been shown to be of critical importance in every solvolysis. Although many experimental methods have been designed and refined, they have mostly been used to add detailed and additional examples to Winstein's mechanistic scheme, and mechanistic interpretation of solvolytic displacement has not changed much in principle. Reviews of some of the developments have recently been published (Ref. 5). Despite such a long history some problems of importance still remain, mainly because of the experimental difficulty of direct observation of carbocation intermediates with a very short life in a very low concentration. Hence microscopic features of ionization, especially, the role of solvent molecule as an electrophilic (electron-accepting) and a nucleophilic (electron-donating) molecule have not been entirely disclosed. The structure of ion-pair intermediates has also been a subject for debate, although Winstein's intimate and solvent-separated ion-pair theory has prevailed. In addition, the molecularity of solvent in connection with "borderline case" solvolysis is still open for future study (Ref. 6). Phenol has remained relatively unfamiliar among the traditional solvolysis solvents. However, it has high ionizing power (vide infra) and low polarity, and has three reaction centers towards the carbocation intermediate. In addition, phenolysis generally gives the phenyl ether with predominantly retained configuration, starting from optically active substrates which do