Ionic liquid structure-induced effects on organic reactions.

Understanding the ways in which the constituents of ionic liquids, i.e. the type of cation, its substitution, and the type of anion chosen, interact with reactants is prerequisite to deliberately designing an ionic liquid solvent with optimum performance. Several approaches, including physico-chemical and spectroscopic measurements and computational studies of binary ionic liquid-substrate mixtures have been presented that investigate the strength of interactions.The qualitative order of the basicity (hydrogen bond acceptor potential) of anions as most prominent force is already reasonably well understood, and reliably determined using, e.g. selective solvatochromic dyes. In certain reactions, the relative order of basicity correlates well with the reactivity of substrates. However, the determination of a relative order for the cations is still in its infancy. Owing to the fact that potential cation-derived interactions may not solely be due to hydrogen bond interactions, but also to ion pair interactions (electron pair donor/acceptor properties), the relative magnitudes of interactions between the anion and cation vary considerably - even in the absence of solutes - depending on the experimental method. In addition, it appears that the basicity of the anion superimposes in many instances on the effects exhibited by the cation and/or the cation's substituent. Hence, understanding the effect of the cation on the activation of substrates is still a challenge.This chapter aims at summarising the trends observed for binary model systems in experimental and computational investigations, and drawing conclusions about ionic liquid structure-induced effects relevant to organic reactions, in particular nucleophilic substitution reactions.