Solvolysis in dipolar aprotic media. I. Production of water-extractable bromide vs. olefin distribution in the course of the solvolysis of 2-bromo-2-methylpentane in dimethylformamide

Abstract

Selectivity parameter measurements have been made on a series of 2 M mixtures of metal bromides and fluorides in HBr and HF. The Lewis acids have been ranked in the order AlBr3 > GaBr3 > TaFg > BBr3 > BF3, TiF4, and HfF4 in HBr and TaF5 SbF5? > BF3 > TiF4 > HfF4 in HF.The order is based on the ability of the system to support isomerization relative to proton exchange. It is subject to the unknown effect of comparing homogeneous with heterogeneous systems, and the fact that the selectivity parameter for SbFs and TaFs in HF inverts as the concentration of the Lewis acid increases is an unresolved puzzle.The Lewis acids tend to exhibit a higher selectivity parameter in HF than in HBr. This is attributed mainly to the enormous difference in dielectric constant and hence dissociative tendencies in the solvents. This overcomes the fact that HBr is more acidic than HF and therefore is a more discriminating rather than a leveling solvent. The (I/E)i-c5 scale was found to be related to (I/E)mcp, with measurements being compared in different Br<j>nsted acids. This suggests that these scales may provide a basis for comparing the acidity or at least the catalytic activity of Lewis acids in varied media like HBr, HF, and HS03F.The I/E scale in any one acid appears to correlate with H0 but since the correlation depends on the solvent one cannot use I/E to predict H0 in an unknown solvent or, vice versa, one cannot use Hq to infer the ion stabilizing properties of the acid.

Kinetic parameters of ionizing power of solvents. The nature of solvation effects in heterolysis of 2-aryl-2-chloroadamantanes

“…Це стабілізує інтермедіат і ускладнює розділення іонів у перехідному стані. Нуклеофільна сольватація контактної іонної пари ускладнює видалення нуклеофугу за механізмом S N 1 [45,46].…”