A FOURIER TRANSFORM-ION CYCLOTRON-RESONANCE STUDY OF STERIC EFFECTS ON PROTON-TRANSFER REACTIONS OF POLYALKYL BENZENES

The rates of deprotonation, MH+ + B --> M + BH+ (M = 1, 2), and the rates of protonation, M + BH+ --> MH+ + B (M = 2, 3, 4), have been measured by Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry for the increasingly sterically crowded alkyl benzenes ethylbenzene, 1; 1,3,5-trimethylbenzene (mesitylene), 2; 3,5-di-(t-butyl)toluene, 3; 1,3,5-tri(t-butyl) benzene, 4; and a series of n-bases B. In addition, the gas phase basicities of 3 and 4 were determined by proton transfer equilibrium measurements using FT-ICR. The efficiencies of deprotonation and protonation are always large (> 50%) for exoergic reactions and increase with increasing exoergicity, but never reach unit efficiency. For each of the alkylated benzenes 1-4 the efficiency decreases steeply at the border between exoergic and endoergic reactions. This is the "normal" behaviour of proton transfer reactions and shows the absence of steric effects for proton transfer to and from alkylated benzenes even in the case of 3 and 4 substituted by bulky t-butyl groups, and is in complete contrast to the behaviour of t-butylated pyridines. This difference is explained by different geometries of the transition state for the proton transfer reactions in the case of the alkylated pyridines and alkylated benzenes, and by the critical role of pi-complexes in proton transfer reactions of benzene derivatives.