Solvent-cationic intermediate interactions in fluorinated alcohol-water systems. Effects on the Selectivity in solvolysis

Abstract

Fluorinated alcohols, pure or in aqueous mixtures, are known to exhibit peculiar solvating properties which are particularly pronounced in the case of hexafluoroisopropyl alcohol (HFP). As a result, unexpected behaviour appear when the effect of such media on the rate of solvolysis is examined. In the present paper we will study the influence of the composition of binary HFPH 2O systems on solvolysis products, i.e. on the selective attack on the cationic intermediate by the two components of the medium. ▪ The results will be compared to those observed in EtOH-H 2O and trifluoroethanol (TFE)-H 2O mixtures (owing to its solvating properties TFE is representative of R fCH 2OH or R fCHOHCH 3 fluorinated alcohols). The main purpose of such a study was to determined the various factors which govern the ▪ interactions of a cationic intermediate with these different media. In a solvolytic reaction, like that illustrated in (1), the selectivity is defined by the equation: ▪ For a given medium, S can therefore be determined from the relative abundance of reaction products. As in previous studies 2-adamantyl tosylate has been used here as substrate. Owing to its peculiar structure this compound undergoes product formation exclusively via collapse of solvent separated ion pair. Thus the observed variations of the selectivity cannot originated from a change in the nature of the intermediate (as it is generally observed in solvolysis of other substrates) and are directly related to the solvating properties of the medium. In binary HFP-H 2O systems selectivity is nearly independent of water content and HFP is always shown to be more efficient than water during the attack on the cationic intermediate. This result appears very surprising since water exhibits considerably higher nucleophilicity than HFP. In this case, the order of ‘apparent’ nucleophilicity is contrary to the intrinsic nucleophilicity of HPF and water. Such an invesion has already been observed in aqueous ethanol systems, but the latter results appeared in nucleophilicity between EtOH and H 2O. TFE-H 2O mixtures exhibit an entirely different behaviour. In water poor mixtures the selectivity value is an agreement with the intrinsic nucleophilicities (H 2O more efficient than TFE). However the selectivity decreases as the water content increases and an inversion is observed in water rich mixtures. As a consequence of the mechanism of 2-adamantyl tosylate solvolysis, the selectivity value must depend, both on the rate of collapse of the solvent separated ion pairs a and b (relative proton transfer ability of each component of the medium) and on the stability of those in pairs. The stability of the ion pairs will be mainly related to the following properties of the two components of the solvent: ( i) ability to solvate the leaving group via hydrogen bond, ii) ability to solvate the cationic center, ( iii) bulkiness of the solvent molecule (which enforces the minimal value of charge separation in the ion pair). The latter factor will be more important in the less dissociating medium. We will show how the interactions of the components of the solvent with the cationic center and the leaving group (coupled with their dielectric constant and acidity) account for the selectivity observed in the three considered mixtures and also for its variations with respect to water content.