Solvation properties, ionic liquids solvent polarity

A number of studies have attempted to characterize ionic liquids through their dielectric constant, and all have observed inconsistencies between the measured dielectric constant and the solvation properties of the liquid. Recent experiments making use of dielectric reflectance spectroscopy [214] indicate dielectric constants in the range of 10-15 for a series of imidazolium-based ILs, substantially lower than those for molecular solvents observed to possess comparable polarities as estimated by solvatochromism. Weingartner [215] has recently published a series of static dielectric constants obtained from dielectric reflectance spectroscopy, and compared them with those of common molecular liquids. The analysis includes comparison with the Kamlet-Taft ji parameter for the liquids from Eq. (11) we have prepared a plot of n versus dielectric constant in Fig. 6. The relationship between n and e for molecular liquids... 

Owing to their unique physico-chemical properties (solvation, polarity, structure), ILs have been proved to be more than physical solvents, providing an unusual coulombic environment where kinetic constants, activation energies and thermodynamic parameters may be strongly modified with respect to their values in traditional molecular solvents. Indeed, tailored ILs with supplementary functionalities, usually referred to as task-specific ionic liquids (TSILs), can chemically interact with the catalyst, for example acting as an activator, a ligand, etc. 

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

The widest application of ionic liquids in microwave chemistry has been as polar solvents. They heat very rapidly under the action of microwave irradiation and have excellent solvating properties. Many chemical reactions benefit from this combination of rapid heating and an ionic reaction medium. 

The polarity of the solvents is often thought to be influential in selectivity and activity of catalytic processes. The polarity of ILs is difficult to measure. Hydrogen-bond properties have often been described as playing an important role in the structure and properties of these solvents. The polarity of ionic liquids has been measured using solvatochromic dyes [50, 51] (see also Section 5.2.1.2). It is often assumed that the polarity of ionic liquids is much lower than that of water and closer to that of ethanol. However, their ability to undergo multiple solvation interactions with many molecules places them among the most complex solvents [52]. 

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