Solvation effects in the heterolyses of 3‐X‐3‐methylpentanes (X = Cl, Br, I)

Abstract

A comparative study of the heterolysis reactions of 3‐X‐3‐methylpentanes (X = Cl, Br, I) in a set of protic and aprotic solvents was performed at 25.00°C. Rate constant values were correlated with solvent descriptors using the TAKA multiparametric equation. Our results point towards a decrease in both hydrogen bond donor acidity (electrophilicity) and hydrogen bond acceptor basicity (nucleophilicity) contributions, and towards an increase in the dipolarity/polarizability term on going from the chloride substrate to iodide. These features suggest the formation of an increasingly early transition state, in contrast to the classical Hughes–Ingold rationale, but in agreement with the Hammond postulate. Furthermore, there seems to be no evidence for a shift in solvation from an electrophilic (or anionic) mode in the chloride substrate to a nucleophilic (or cationic) mode in the iodide, as claimed by some authors. Copyright © 2004 John Wiley & Sons, Ltd.

Revisiting the Reactions of t‐BuX (X = Br, I) with Monoalcohols: A Mechanistic Analysis through Numerical Integration and Nonlinear Regression Methods

“…As briefly described elsewhere , all kinetic experiments were followed by conductimetry using a Wayne‐Kerr B905 conductance bridge, equipped with a RS‐232C serial interface linked to an in‐house built multiplexer to which up to nine conductivity cells with two bright platinum electrodes, also in‐house manufactured, could be connected. Cells were covered with aluminum foil to prevent any photochemical reaction and then immersed in a Braun Thermomix 1400 silicon oil bath associated with a Braun Frigomix R refrigerating unit.…”

“…Since the pioneering work by Hughes and Ingold in 1935 , the reactions of tertiary alkyl halides, and in particular of tertiary butyl halides, (CH 3 ) 3 CX (or t– BuX, where X stands for Cl, Br, or I) with hydroxylic solvents have been thoroughly investigated and commonly considered to follow first‐order kinetics . However, most of the published studies were limited to the only generally acknowledged meaningful reaction step, viz .…”

The Influence of Carbon-Carbon Multiple Bonds on the Solvolyses of Tertiary Alkyl Halides: a Grunwald-Winstein Analysis

“…As Mayr et al [ 14 ] pointed out, in spite of the good correlation of these two scales with each other, as well as with some other solvent nucleophilicity scales, there is a still a vivid ongoing discussion on the role of nucleophilic solvent participation (NSP) in solvolysis reactions [ 17 , 18 ]. This is certainly related to the indiscriminate use of the terms nucleophilic solvent participation and nucleophilic solvation (NS) as recently claimed by Richard et al and other authors [ 3 , 19 ].…”

“…The ongoing interest over these studies not any longer resides in the possibility of making predictions of rate constants for other solvents, but especially in its potential to assist us in the understanding of the true nature of substrate - solvent - solvent interactions, at a molecular level. In this context, there has been a continuing interest from the scientific community in the study of the solvolysis reactions of tertiary alkyl halides, which are still considered good model systems to monitor solvent and solvation effects [ 1 – 3 ]. One of the most used linear solvation energy relationships (LSER) to study solvent effects, in particular in aqueous organic media, has been the Grunwald-Winstein (G-W) equation, proposed for the first time in 1948 [ 4 ].…”

“…The motivation for the research here presented came from our interest on the solvolysis reactions of tertiary alkyl halides [ 3 , 28 ]. In the context of our systematic work on solvent effects, we came across the need to isolate and rationalize certain solvent contributions and for that reason we had to select some particular substrates.…”