Using Kamlet−Taft Solvent Descriptors To Explain the Reactivity of Anionic Nucleophiles in Ionic Liquids

Abstract

In this paper, we report the effect of ionic liquids on substitution reactions using a variety of anionic nucleophiles. We have combined new studies of the reactivity of polyatomic anions, acetate, trifuoroacetate, cyanide, and thiocyanide, with our previous studies of the halides in [C4C1py][Tf2N], [C4C1py][TfO], and [C4C1im][Tf2N] (where [C4C1im]+ is 1-butyl-3-methylimidazolium and [C4C1py]+ is 1-butyl-1-methylpyrrolidinium) and compared their reactivities, k2, to the same reactions in the molecular solvents dichloromethane, dimethylsulfoxide, and methanol. The Kamlet-Taft solvent descriptors (alpha, beta, pi) have been used to analyze the rates of the reactions, which were found to have a strong inverse dependency on the alpha value of the solvent. This result is attributed to the ability of the solvent to hydrogen bond to the nucleophile, so reducing its reactivity. The Eyring activation parameters (DeltaH++ and DeltaS++), while confirming the reaction mechanism, do not offer obvious correlations with the Kamlet-Taft solvent descriptors.

Hydrogen-bond acidity of ionic liquids: an extended scale

“…13,15,17,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][47][48][49][50][51] It should be highlighted that the hydrogen-bond acceptor strength of an IL is dominated by its anion while the hydrogen-bond donor ability is essentially controlled by the cation and only slightly depends upon the anion of the IL. 18,19 Thus, and since we were focused on developing an extended series for the hydrogen-bond acidity of ILs, ILs comprising a fixed anion ([NTf 2 ] − ) and a wide range of cations (imidazolium-, pyridinium-, piperidinium-, pyrrolidinium-, ammonium-, sulfonium-, and phosphonium-based) were selected.…”

Charge Screening in the SN2 Reaction of Charged Electrophiles and Charged Nucleophiles: An Ionic Liquid Effect

“…In spite of efforts to detect such a phenomenon, detailed quantitative investigations on the effects of ionic liquids on reactions of solutes have repeatedly demonstrated that the results can be correlated with, and hence rationalized in terms of, polarity behaviours originally derived to describe molecular solvents. 4,5 In particular, we have previously reported quantitative kinetic studies of nucleophilic substitutions by halides in reactions with neutral nucleophiles in a range of ionic liquids, showing that both cation and anion variation can change the nucleophilicity of the halide ion. 4 By comparing the kinetic data obtained against empirical solvent polarity measurements, 4 it became apparent that this could be understood by the application of the classical Hughes-Ingold predictions 4 of solvent polarity on reaction rates combined with an analysis of hydrogen-bonding effects, with the ionic liquid acting in the same way as polar molecular solvents.…”

Photochromic imidazolium based ionic liquids based on spiropyran

“…According to recent estimations, the number of potential anion-cation combinations available equates to around one trillion (10 12 ) different ILs. 1 Their proposed applications include recyclable solvents 2,3 , molecular solvent replacements for catalysis, 4 electrochemistry, 5 synthesis 6 and elemental analysis 7 . Past reviews have provided detailed insight into the synthesis and properties of ILs and have promoted their implementation as common laboratory solvents.…”