摘要：

The rates of solvolysis of representative 3-aryl-2,3-dimethyl-2-chlorobutanes and 3-aryl-2-methyl-2-chloropropanes were measured in 80% aqueous ethanol in order to examine the importance of aryl participation in such systems. In the former series, the range of rates was only a factor of 8 (p-H, 1.00; m-Me, 1.44; p-Me, 2.51; p-OMe, 7.86). In the latter series, the effect of the substituents was even smaller p-H, 1.00; m-Me, 1.11; p-Me, 1.21; p-OMe, 1.51). These relative rates, analyzed with the aid of the usual free energy treatments, reveal that the transition states for these solvolyses must involve only a minor amount of charge delocalization from the carbonium ion center into the aromatic ring. It has long been an accepted position in this area that the transition state must closely resemble the first intermediate, i.e., the carbonium ion produced in the solvolysis. On this basis, the carbonium ion itself must involve only minor charge delocalization into the aromatic ring. The question then arises whether the intermediates in these reactions are best represented as symmetrical aryl-bridged cations (arylonium ions), or as rapidly equilibrating cations involving minor interactions between the π cloud of the aromatic ring and the carbonium center. In an attempt to answer this question, the tagged compounds, 1,1-dimethyl-d 6-2,2-dimethylpropanol, 1,1-dimethyl-d 6-2-methyl-2-phenylpropanol, and 1,1-dimethyl-d 6-2-methyl-2-p-anisylpropanol, were prepared and subjected to a number of representative carbonium ion reactions. It is evident that the formation of symmetrically bridged intermediates requires complete scrambling in the products, whereas rapidly equilibrating cations offer the possibility that they may be trapped and converted to products before complete scrambling has occurred. No difficulty was encountered in converting the first derivative (β-Me) into unscrambled product. In view of the small rate effects observed, the marked tendency for the second (β-Ph) and the third derivative (β-An) to yield fully scrambled products was unexpected. However, even here, reactions were found which yielded essentially unscrambled products from the β-Ph compound, while resulting in completely scrambled products from the β-An compound. Finally, dehydration of the alcohols over alumina, addition of hydrogen chloride to the olefins, and deoxideation of the alcohols with bromoform and alkali (Skell reaction) permitted the formation of largely unscrambled products from all three derivatives. These results are not compatible with the formation of symmetrically bridged arylonium intermediates. They are compatible with the formation of rapidly equilibrating unsymmetrical cations (or ion pairs), (H 3C) 2RCC +(CD 3) 2 (H 3C) 2C +CR(CDs) 2 in which there may be more or less interaction between R and the carbonium center, with a rate of equilibration that increases in the order R = Me < Ph < An. It is proposed that the present data support the view that we must be dealing with an essen-tially continuous spectrum of cations, varying from static classical, to equilibrating classical, to equilibrating π bridged, to static symmetrically bridged species, rather than the sharp dichotomy of static classical and static bridged species which has been the main basis for interpreting the data in this field.

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