Abstract
How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL (the “apparent” or “measured” dielectric constant ε of the IL is around 10–15) into a strongly polar molecular solvent (e.g., ε of DMSO: 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard. Toward this, we, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic–molecular binary systems on self-dissociation of C–H acid phenylmalononitrile PhCH(CN)2via pKa determination. As disclosed, in this category of binary media, (1) no linear correspondence exists between pKa and molar fractions of the respective solvent components; (2) only ∼1–2 mol% of weakly polar ILs in strongly polar DMSO make C–H bonds even more dissociative than in neat DMSO; (3) a small fraction of DMSO in ILs (<10 mol%) can dramatically ease acidic C–H-dissociation; and (4) while the DMSO fraction further increases, its acidifying effect becomes much attenuated. These findings, though maybe counterintuitive, have been rationalized on the basis of the precise pKa measurement of this work in relation to the respective roles of each solvent component in solvation.
Highlights
The primary difference between ionic liquids (ILs) and conventional media is that ILs contain entirely ions, while the counterpart has only molecules in composition
How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL into a strongly polar molecular solvent (e.g., 3 of dimethyl sulfoxide (DMSO): 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard
This is quite surprising, since among all kinds of chemical transformations, acidic R–H dissociation in conventional solvents,[16,17,18,19,20] recently in ionic media,[21,22,23,24,25] o en by means of pKa study, is the most extensively investigated process, which laid the foundation for many most in uential physical organic principles,[26,27,28,29] and provided some quantitative understandings of solvation effects on stabilizing various species generated upon heterolytic bond dissociation
Summary
The primary difference between ionic liquids (ILs) and conventional media is that ILs contain entirely ions, while the counterpart has only molecules in composition. We, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic–molecular binary systems on selfdissociation of C–H acid phenylmalononitrile PhCH(CN)[2] via pKa determination.
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