**3.2 Binary mixtures based on ionic liquids**

The use of RTILs or ionic liquid binary mixtures could give variations in the structure of the ionic lattice of neat ILs after mixing [43–45]. This fact may have significant repercussions on the nature and strength of the interactions that contribute mainly to coulomb interactions that determine the 3D structure of ILs [46, 47]. Studies of binary mixtures with common anions, for instance, the same cation but different anions, have shown how the presence of random co-networks or block co-networks depends on the size of the anions [4, 47, 48]. Seddon suggests the use of IL mixtures to expand the range of room temperatures in ILs [49]. Initially, the hygroscopic nature of the ILs was a problem; however the high capacity of the ILs to solubilize water opens a wide spectrum of reaction media, mainly based on the role of the hydrogen bond (HB) and electrostatic interactions between molecules in the mixture. Reports have shown that the addition of COS to ILs may affect significantly the density, viscosity, and conductivity with respect to pure ILs. For instance, the direct relationships between the viscosity of the IL/COS mixtures with the solvent dielectric constant (ε) of the COS pure [50, 51]. It may be attributed to the difference in the ion-dipole interactions between the ions and solvents. The addition of water to ILs may change the molecular structure of pure ILs probably due to HB between the water molecules and the anions of the ILs [52, 53]. Sanchez et al. studied solvent mixtures between 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4)/water at different molar fractions, observing on the studied range of compositions, a border line located close to χ = 0.2. Before this value indicates that the added RTIL promotes the reactivity of the substrate by preferential solvation. After this value, the rate coefficients remain approximately constant. At low concentrations the water begins to break down the 3D

*Solvents, Ionic Liquids and Solvent Effects*

effect may lead to a nucleophilic substitution (NS) product or a SNAr product or both [12–16]. A SNAr reaction occurs in activated aromatic compounds bearing good leaving groups (LG). In general, it is widely accepted that the mechanism of the SNAr reactions involves the formation of a σ-complex (also called Meisenheimer complex (MC)) that occurs after the nucleophilic attack step at the ipso atom of aromatic moiety. Next, the departure of LG with re-aromatization of the aromatic ring closes the set of steps to give the desired product. Commonly, the LG departure step is faster than the nucleophilic attack; therefore, the addition of the nucleophile to the ring moiety appears as the rate-limiting step in these processes [13, 17–24]. In the last time, a concerted reaction mechanism could be prevailing [25–27]. A lot of work has been carried out to clarify whether the concerted mechanism is an excep-

Bernasconi et al. [23, 30] postulated the existence of an intramolecular hydrogen bond between a hydrogen atom of the nucleophilic center in the nucleophile and the orto-NO2 group of the substrate in order to explain the reactivity trends in ortohalonitrobenzenes to respect para-halonitrobenzenes toward amines. Ormazábal-Toledo et al. [31] carried out computational studies about the role of HB effects along the intrinsic reaction coordinate profile, demonstrating that it promotes the activation of both the substrate and nucleophile, respectively. Note that the analysis was performed in transition state (TS) structures, because the reactant states hide most of the information about specific interactions that characterize the SNAr reactions. Recently, Gallardo-Fuentes and Calfumán et al., respectively, showed that the HB not only determines the reactivities, but also it could be involved in concerted

Ionic liquids or room temperature ionic liquids (RTILs) are defined as molten salts (composed entirely of cations and anions) that melt below 100°C [33] with remarkable physicochemical properties: non-flammable, non-corrosive, nonvolatile, and bulk physical constant, which can be tuned by the combination of different cations and anions [34–38]. RTILs are composed by bulky organic cations usually imidazolium or pyridinium derivatives substituted with alkyl chains and an inorganic or organic anion (usually a halide, tetrafluoroborate, hexafluorophosphate, and others). The high combinatorial flexibility has converted these materials into "designer solvents" or "task-specific" solvents [33, 35, 38] whose properties can be specified to suit the requirements of a particular reaction [2, 4, 12, 39]. For these reasons, RTILs have gained importance in the solvent effects field being recognized

A series of reaction have been studied in RTILs and mixtures of them with water or COS. The criteria to select the RTILs were based on the following: (i) the solubility of substrates and nucleophiles; (ii) to have a reasonable number of anions and cations to assess anion and cation effects; and (iii) to ensure that these RTILs do not interfere with the reaction [12]. Solvent effects in RTILs are a complex problem, because the solute-solvent interactions will be masked by the leading solvent-solvent interactions that are coulombic in nature. Some strategies to study

tion or the dominant pathway in these processes [28, 29].

**2.1 Hydrogen Bond in SNAr Reactions**

routes in SNAr reactions [1, 26, 32].

**3.1 SNAr reactions in ionic liquids**

**3. Room temperature ionic liquids**

as very promising reaction media with green features.

**232**

structure of the ILs, which then goes on to form ionic clusters as the concentration of water increases until eventually ion pairs form, which are the dominant species in the aqueous solution [10, 46, 48].
