**4.2 Iso-solvation**

*Solvents, Ionic Liquids and Solvent Effects*

**4. Interactions and solvent effects**

aqueous solution [10, 46, 48].

**4.1 Preferential solvation**

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

It is well known that the chemical reactivity is determined by the ability of the solvent to interact with solute, intermediates, and transition state (TS) structures along the reaction pathway [1–3]. The main difference between COS and RTILs are the electrostatic solvent-solvent interactions between cation-anion and cationcation interactions [33]. These interactions in the COS are moderate dipole-dipole interactions; in the RTILs they become the leading term (ion-ion interactions) that are expected to outweigh the target solute-solvent interactions. Solute-solvent interactions contain the relevant information about catalysis, stabilizing/destabilizing effects affecting the electrophile/nucleophile pair (solute), TS structures, and the intermediate in a polar process [1]. Solvent effects can be split into two types: nonspecific interactions and specific interactions, including all the possible interactions

that can occur between solvent and the electrophile/nucleophile pair [5, 52].

Solvent effects can be split into two types: non-specific and specific interactions, including all the possible interactions that can occur between the solvent and solute [5, 39, 54]. Then, preferential solvent may be defined as the difference between local and bulk composition of the solute with respect to the various components of the solvent, usually mixtures of solvents [5, 55–57]. The "bulk of the solvent" is treated as the external shell, and it can be described using the classic theories of Kirkwood-Onsager, models of solvation based on reaction field theory or molecular dynamic [55, 58, 59]. Then, in a binary mixture of protic solvents, the "preferential solvation" may be cast into the form of specific solute-solvent interactions described as local solvation, which may be defined as a "first solvation shell." The local solvation may be classified as electrophilic or nucleophilic, respectively [17, 60–63]. Electrophilicity and nucleophilicity concepts are related to electron-deficient (electrophile) and electron-rich (nucleophile) species [39, 64, 65]. These concepts are based on the valence electron theory of Lewis [66] and the general acid–base theory of Brønsted and Lowry [67, 68] and introduced by Ingold in 1934. Then, for a mixture of polar solvents, the "electrophilic solvation" represents the specific interaction through a HB with the hydrogen atom of the solvent, whereas "nucleophilic solvation" describes a specific interaction through a HB between an acidic hydrogen atom of the solute and the heteroatom of the solvent [5, 60–63]. Mancini et al. have reported preferential solvation of 1-halo-2,4-dinitrobenzenes with amines in mixtures of dichloromethane with polar protic/polar aprotic solvents [7, 44, 45, 69, 70]. Ormazabal-Toledo et al. [5] reported an integrated experimental and theoretical study of 2,4,6-trinitrophenyl ether with a series of secondary alicyclic (SA) amines in ethanol/water mixtures at different compositions. In it only piperidine was sensitive to preferential solvation at high proportion of water. Piperidine increases its rate coefficient values suggesting a stabilization of the MC by HB displayed by the presence of more water molecules in the first shell at these proportions of water in the studied mixtures. This result shows that the environment of the MC changes for different solvent compositions. Then, for the remaining amines the environment showed to be similar being it attributed to polar nature of the substituent at position 4, suggesting that their kinetic responses are independent of the bulk properties of the reaction media. On

**234**

The concept of iso-solvation has been introduced to indicate the composition of a mixture in which the probe under consideration is solvated by an approximately an equal number of cosolvent molecules in the solvent mixture [48]. This effect has been extensively observed in COS mixtures [72–74]. Alarcón-Espósito et al. [48] studied the reaction between ClDNB with morpholine in a series of mixtures of ILs involving imidazolium cations. Iso-solvation effects were observed in the following mixtures: 1-ethyl-3-methyl imidazolium thiocyanate/1-ethyl-3-methyl imidazolium dicyanamide (EMIMSCN/EMIMDCN), 1-butyl-3-methyl imidazolium dicyanamide/1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMDCN/ BMIMBF4), BMIMBF4/1-butyl-3-methyl imidazolium hexafluorophosphate (BMIMPF6), and BMIMPF6/1-butyl-3-methyl imidazolium tris(pentafluoroethyl) trifluorophosphate (BMIMFAP), respectively. Iso-solvation regimes correspond to a solvent composition regime where the solute is being solvated by approximately the same number of different solvent molecules in the mixture. These results showed that for significant changes in composition, the rate coefficients remain approximately constant. On the other hand, for the solvent mixture BMIMBF4/BMIMPF6 at 0.9 molar fraction of BMIMBF4, a slightly better kinetic response is observed than the pure BMIMBF4 and BMIMPF6. Another interesting result was observed in the mixture of EMIMSCN/EMIMDCN; an increasing proportion of EMIMSCN with respect to EMINDCN results in a decrease of the rate coefficient within the range 0.1–0.75 in molar fraction of EMIMSCN. This result could be expressed as a competition between the anions toward the reaction center driven by the basicity of the reaction media.
