Substituent Effects on Chloride Transfer Equilibrium in the Gas Phase. Intrinsic Resonance Demand of Benzyl Cation

Abstract

Abstract Relative chloride ion affinities of 48 carbocations have been measured by the chloride ion transfer equilibrium of benzyl chlorides in the gas phase. The substituent effects on the chloride affinity of benzyl cation have been analyzed in terms of the LArSR Eq., giving a ρ = −14.0 and an r+ = 1.29. The r+ value higher than the corresponding value of unity for the α-cumyl(2-phenyl-2-propyl) cation clearly indicates that the benzyl cation has an enhanced resonance demand in the intrinsic carbocation stability compared with that of the α-cumyl one. Most importantly, the unique r+ value of 1.29 for the gas-phase benzyl cation stability has been found to be identical with the value observed for the kc ionization process of the solvolysis of the activating substituent range of benzyl tosylates. The identity of the r + value, which is consistent with previous observations for the relevant benzylic carbocation systems, suggests that the intramolecular charge-delocalization in the transition state of the kc solvolysis should be quite close to that of the carbocation intermediates. It is concluded that the exalted r+ value observed for the kc solvolysis of benzyl tosylates is not a correlational artifact arising as a result of the non-linearity caused by the kc–ks mechanistic transition but is due to the enhanced resonance stabilization of the benzyl cation compared with the α-cumyl cation.