Bond Dissociation Energies In Room-Temperature Ionic Liquids

The most fundamental process in chemistry is the cleavage and formation of chemical bond. Bond dissociation energy (pKa, BDE, etc.) is the key factor to determine if or how such process can happen. It has played a central role in the development of chemical research and other related fields. Scientists have focused on measuring the bond dissociation energy for decades, and thousands of data have been reported. However, most of these works were based on the conventional molecular solvent, with only a few cases in Ionic Liquids (ILs), the emerging solvent system that has aroused a booming research enthusiasm in recent decade.Not long ago, we developed a method to precisely measure the absolute pKa values (sd≤±0.05pK) in ILs and established the indicator platform (as board as13pK units). In the present work, we extended our continued effort in establishing the pKa scales of benzoic acid and benzenethiol, which is helpful to understand the midea effect in ILs and determine the applicability of the fundamental theory established in molecular solvent. Furthermore, efforts were also spent into attempting to develop the method to measure the BDEs of several carbon acids in ILs.(1) Absolute equilibrium acidity scales of fifteen benzoic acids in four pure RTILs were established. A modified indicator overlapping method was introduced to the present acidity study in order to overcome the complication caused by the homo-hydrogen bonding interaction between the parent O-H acid and the corresponding oxanion. A regression analysis confirms the intrinsic nature and high quality of the measured data. Comparisons of the acidity values in ILs carrying different anion or cation directly indicate that the pKa variation of the same acid in different ionic liquid is brought in by the varying ability of acid-strengthening effect of the ionic liquid. Furthermore, the regression analyses between the slope of Hammett plot and acidity of benzoic acid in both molecular solvent and ionic liquid shows three lines with similar slopes, suggesting that these solvent can be divided into three kinds (molecular solvents with high polar and basity, molecular solvents with low polar and basity and Ionic Liquids). However, there is a common trend, the acidity of benzoic acid stronger, the substituent effect less obvious.(2) Acidity scales of fifteen substituted benzenethiols in four RTILs were established. Hammett plots in all four pure ILs show two distinct lines with similar slopes, which could be rationalized with the established SSAR effects. The regression analyses demonstrate that all the pKa scales obtained in four RTILs are not only correlated excellently with each other, but also with these in molecular solvents. In addition, the comparison of acidity among S-H, O-H and C-H acids discloses that the solvation dynamic of ILs on the acidic bond dissociation is affected by not only the resonance, substituent and homo-hydrogen effect of the substrates, also the structure and composition of RIILs. The pKa of seven commonly seen aromatic thiols were also measured in this work.(3) The classic method of estimating the BDEg of H-A bonds developed by Bordwell and Cheng was introduced in ILs, and the equation was obtained basing on a themodynamic cycle. Oxidation potentials of12conjugate anions of carbon acids (Eoox(A-)) were measured by the cyclic voltammetric instrument. As the solvation power of ILs becomes stronger, the EooX(A-) value becomes more positive. The gas phase BDEg(BmimNTf2) was obtained by coupling the pKa and Eoox(A-) values. Comparison of BDEg values obtaining from data in BmimNTf2and DMSO shows only a few deviations for these C-H acids. Furthermore, the RSE for benzene malononitriles in both ILs and DMSO are all similar, meaning that RSE are also solvent independence.