Kinetic Studies on Solvolysis. VIII. Phenolysis of Optically Active α-Phenethyl Chloride in Phenol-Benzene Solven—Three-stage Mechanism in the SN1-Type Solvolysis

Abstract

Abstract 1. Titrimetric and polarimetric rate constants (Kt and kα) for phenolysis of α-phenethyl chloride in 50wt.% phenol-benzene at 25°C were measured, and satisfactory first-order behavior was observed irrespective of presence or absence of anilinium phenolate. From this the SN1-characteristics of this phenolysis was concluded. 2. The values of kα’s were fairly constant at any phenolate concentrations (0.0∼0.6 n). The kt’s were also fairly constant at higher phenolate concentrations. However, the Kt’s gave smaller values than the Kα’s, and showed a sudden decrease in the vicinity of zero phenolate concentration. The gap between the kα line and kt line was not diminished and remained fairly constant as the phenolate concentration increased. 3. In the presence of anilinium phenolate the main product was α-phenethyl phenyl ether with predominantly retained configuration, whereas in the absence of the phenolate only a minor amount of α-phenethylphenol was obtained. 4. From the constant gap observed between kα and kt it was generally concluded that the phenolysis should proceed by way of a twointermediate (I1 and I2) three-stage mechanism. In view of the predominant retention of configuration in the product, it was postulated that the intermediates should be optically active. Hence the mechanism was divided into parallel series of the three-stage processes for the optically antipodal chlorides and intermediates, namely, (+)RCl\ightleftarrows(+)I1\ightleftarrows(+)I2→(+)RY and (−)RCl\ightleftarrows(−)I1\ightleftarrows(−)I2→(−)RY These parallel series of the processes were connected with each other by fairly rapid equilibration between the first antipodal intermediates ( −I1 and +I1). For the first intermediate was suggested an ion-pair surrounded by a solvation shell and for the second a kind of quadrupolar associates which consist of the ion-pair and the reactant dipole (phenol or anilinium phenolate). 5. It was pointed out that a necessary condition for the existence of three-stage mechanism is a kinetically-definite stabilization of the two kinds of intermediates. This stabilization was verified by the absence of commonion depression for this phenolysis rate. It was also pointed out that unstable ion-pair intermediates, like “solvent separated” and “intimate” ion-pair, could not be kinetically distinguished among themselves, and that in this case any three-stage formulation actually converged to two-stage treatment. This was shown in the case of acetolysis in 2-p-anisylpropyl system to which Winstein and Fainberg5) assumed their three-stage mechanism to be applicable. 6. It was shown that Winstein’s three-stage mechanism with the “intimate” and “solvent separated” ion-pairs is generally unapplicable to any cases where a three-stage is required, e. g., not only to this phenolysis but also to the acetolysis in threo-2-p-anisy3-butyl system. For the latter a three-stage mechanism similar to the one suggested for this phenolysis in α-phenethyl system was presented. Fairly stable, two discrete intermediates, namely, a phenonium ion-pair for the first intermediate and a quadrupolar associate between the first intermediate and acetic acid molecule for the second were suggested.