GAS-PHASE BASICITIES OF PHENYLACETYLENES - INTRINSIC RESONANCE DEMANDOF THE 1-PHENYLVINYL CATION

Gas-phase basicities (GB) of a series of substituted phenylacetylenes were determined by measuring the standard free energy changes of proto n transfer equilibria using a pulsed ion cyclotron resonance mass spec trometer. The LArSR analysis of the substituent effect on the GB value s gave an r(+) value of 1.18 and a rho value of -14.0. The r(+) value is significantly larger than that obtained for the GB of alpha-methyls tyrenes but is smaller than that for chloride ion affinities of benzyl cations, while the rho value is similar in the three systems. This me ans that the 1-phenylvinyl cation has an intermediate resonance demand between those of the alpha-cumyl cation and the benzyl cation. Furthe r, the r(+) value of 1.18 for 1-phenylvinyl cation obeys the same ''re sonance demand vs. carbocation stability'' relationship observed for a series of ordinary sp(2)-hybridized benzylic carbocations. This revea ls that the pi-delocalization mechanism in the vinyl cation system has no unique feature but it is a part of a continuous spectrum of resona nce demands of varying benzylic carbocations characterized by r(+) = 1 .18. A comparison of the present GB values with the substituent effect for the acid-catalyzed hydration of phenylacetylenes in solution has shown that the r(+) value is perceptibly smaller in the latter case. T his suggests that the pi-delocalization in the transition state differ s considerably from that of the stable phenylvinyl cation intermediate . This is in contrast to the situation in the S(N)1 solvolysis where t he r(+) value is identical to the value characteristic for the formati on of the fully developed carbocation.