**5.2 Theoretical models of solvation**

*Solvents, Ionic Liquids and Solvent Effects*

(nucleophile).

**5. Solvent models**

**5.1 Kamlet-Taft model**

(THF) have a key role due to its ambiphilic character as an HB donor/acceptor that promotes a nucleophilic activation at the nitrogen center of the piperidine

Solvent effect studies have been focused mainly on the polarity of the reaction medium as a determinant of chemical reactivity properties. Experimentally, the most common way to measure the polarity of a solvent is through the ε. However, the measure requires that the reaction medium will be non-conductive, which does not happen in the RTLIs. The concept of polarity has been defined as the sum of all possible intermolecular interactions between the solvent and a solute, excluding those interactions that lead toward chemical reactions of the solute and including Coulombic interactions, HB interactions, dipole-dipole, dipole-induced dipole, electron pair acceptor-electron pair donor, and acid-base interactions [1, 33]. There are many empirical solvent polarity scales [75–83] that attempt to give quantitative estimates of solvent polarity, some of those were applied to RTLIs [84]. However, the high number of interactions in non-conventional reaction media cannot be incorporated in a measurement or polarity scale. The most used solvent polarity scale is the one developed by Kamlet and Taft [78] based on solvatochromism properties that show specific and non-specific interactions. Solvatochromism is solvent dependence of the electronic spectrum of chromophore. Intensity, position, and shape of absorption bands of dissolved chromophores are influenced by the change in solvents or mixture of solvents, according to their electronic and molecular structure, due to the different stabilization of their electronic ground and excited states. Therefore, any solvent-dependent property (SDP) in solution is normally expressed

as a linear solvation free energy relationship (LSER) as follows:

where *SDP* corresponds to any kinetic property, namely, selectivity or reaction rate coefficients, which is modeled as a linear combination of two H-bond terms, one for hydrogen bond donor (*a* ∝*s*) and hydrogen-bond acceptor (*b* β*s*) and a dipo-

ties of the solute [84]. In this approach, empirical solvatochromic parameters are introduced to describe specific HB interaction, ion-dipole, dipole-dipole, dipoleinduced dipole, solvophobic, dispersion London, and possible π-π and p-π stacking effects. The reason is that while empirical solvatochromic parameters in COS work reasonably well, for RTILs they consistently fail. The main reason seems to be the transferability of the response of a particular probe chromophore from some known SOC to RTILs. This transferability would warrant that the polarities of the RTILs and the SOC are the same and that the appropriate value of the parameter can then safely be assigned to the RTIL. The second implicit assumption is that the effect of transferring from a SOC to an RTIL is the same for all probes. They conclude that it is important to consider the nature of the chromophore as well as the solvent when establishing reliable solvent polarity parameters, mainly when this chromophore is transferred from a neutral molecular solvent to an RTIL. The main message, however, is that it cannot be a priori established if one solvent polarity scale with respect to another one is right or wrong: "each scale will turn useful in a given set of

), with SDP0 a constant describing intrinsic proper-

<sup>∗</sup> (1)

*SDP* = *SDP*<sup>0</sup> + *a* ∝*<sup>s</sup>* + *b* β*<sup>s</sup>* + *s πs*

∗

circumstances and in other ones they will not" [1, 12].

larity/polarizability term (*s πs*

**236**

In pure conventional solvents, the determination of properties and type of interactions has been reasonably achieved with the use of non-specific solutesolvent interactions, based on continuum dielectric models [85, 86]. In RTILs, the results are both scarce and not yet systematized [87, 88]. The super-molecule model provides a detailed synopsis of the field of solvation [71].

#### **5.3 Gutmann's donor and acceptor numbers**

Donor (DN) and acceptor (AN) numbers proposed by Gutmann [89–91] are used to describe acid-base solvent properties in RTILs based on a reformulation by Schmeisser et al. [92, 93]. On the original definition of Gutmann, DN and AN are a quantitative measure of Lewis basicity and acidity of a solvent, generally a nonaqueous media [4, 91]. These numbers can be measured by calorimetrical technique and by using the chemical shift in 31P NMR spectra [92, 94]. In COS these parameters are used in order to describe the ability of solvents to donate or accept electron pairs or at least electron density to the substrate. Then, DN represents a measure for the donor properties of a solvents, and AN is a measure of the electrophilic properties of a solvent. DN parameter in RTILs shows a strong dependence on the anionic component of the RTIL; however, AN is dependent on both anionic and cationic moieties of the RTIL [92].

Alarcón-Espósito et al. [4] studied three families of RTILs, based on 1-ethyl-3-methyl imidazolium (EMIM+ ), 1-butyl-3-methyl imidazolium (BMIM+ ), and 1-butyl-1-methyl-pyrrolidinium (BMPyr+ ) cations, respectively, with a wide series of anions in order to evaluate both models of solvent effects toward the reaction between 1-chloro-2,4-dinitrobenzene with morpholine by kinetic experiments. The first approximation of solvent effects was attributed to an "anion effect." This effect appears to be related to the anion size, polarizability, and its HB ability toward the substrate. The comparison between rate constants and Kamlet-Taft solvatochromic model systematically failed. However, the anion effect was confirmed by performing a comparison of the rate constants and DN emphasizing the main role of the charge transfer from the anion to the substrate.
