Gas Phase Basicities of Phenylacetylenes. Intrinsic Resonance Demand of the 1-Phenylvinyl Cation

Gas-phase basicities (GB) of a series of substituted phenylacetylenes were determined by measuring the standard free energy changes of proton transfer equilibria using a pulsed ion cyclotron resonance mass spectrometer. The LArSR analysis of the substituent effect on the GB values gave an r+ value of 1.18 and a ρ value of −14.0. The r+ value is significantly larger than that obtained for the GB of α-methylstyrenes but is smaller than that for chloride ion affinities of benzyl cations, while the ρ value is similar in the three systems. This means that the 1-phenylvinyl cation has an intermediate resonance demand between those of the α-cumyl cation and the benzyl cation. Further, the r+ value of 1.18 for 1-phenylvinyl cation obeys the same “resonance demand vs. carbocation stability" relationship observed for a series of ordinary sp2-hybridized benzylic carbocations. This reveals that the π-delocalization mechanism in the vinyl cation system has no unique feature but it is a part of a continuous spectrum of resonance 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. This suggests that the π-delocalization in the transition state differs considerably from that of the stable phenylvinyl cation intermediate. This is in contrast to the situation in the SN1 solvolysis where the r+ value is identical to the value characteristic for the formation of the fully developed carbocation.