The Study Of Absolute Acidity Of Compounds And The Solvation Effect In Ionic Liquids

Ionic liquid is a new kind of reaction medium,which is entirely composed of anion and cation.Ionic liquids have many advantages such as low volatility,recyclability,and can be designed,which make ionic liquid as the third types of solvent after water,organic solvents.In the recent decade,the number of research papers on ionic liquids has been increasing year by year,most of these studies have focused on the basic physical parameters and applications of ionic liquids.However,the bond energy of compounds in ionic liquids is severely lagged behind,which impedes the rational d evelopment of chemistry in this new solvent.pK_a is an important physical parameter and reflects the heterolytic bond dissociation energies of compounds.The pK_a value is influenced by the properties of the compound and the solvent,therefore pK_a is an important probe for understanding the solvation properties of ionic liquids and it can guide the reaction in ionic liquids.In recent years,we have developed a method to measure precisely the absolute p Ka values in pure ionic liquids and have established the indicator acidity scale for this purpose.So far we have investigated the absolute pK_a values of benzoic acids,thiophenols,sulfonamide and ylide compounds.In order to further reveal the special solvation effect of ionic liquids,the acidities of different types of compounds were accurately measured in ionic liquids and compared with molecular solvents.(1)The pK_as of the protic amines in ionic liquids.We measured the absolute p Ka of 21 protic amines in ionic liquids and found there is a good linear correlation between the acidity of these amines in ionic liquids and acetonitrile.The countering anion has no or neglected effect on the p Ka values of protic amine salts,which is indictive of no long term ion-pair formation in the ionic liquids involved in current study and ionic liquids behave as a super-diisociating solvent towards the solvation of salts.(2)The pK_as of phenols in ionic liquids.We measured the absolute pK_a of a series substituted phenols through modified indicator overlapping method in ionic liquids.We found the pK_a scales were correlated well with the Hammett substituent parameters and the data in DMSO.(3)The acidity study in the mixture of ionic liquids and DMSO.The acidity of PhCH(CN)2 and its derivatives were measured in the mixture of ionic liquids and DMSO through the autoprotolysis processes.We found the acidity of these carbon acids has a maximum value due to the synergistic effect of ionic liquid and DMSO,which is supported by further theoretical study.(4)The second order rate constants of the aromatic nucleophilic substitution(SNAr)reaction between 4-fluoronitrobenzene(4-FNB)and 5 primary and scondary amines with the measured accurate p Kbs were measured in ionic liquid.The relatively large Br?nsted ?nuc than in molecular solvents indicates that large amount of positive charge is located on the N atom in the transition state(TS),correspondingly,large amount of negative charge is dwelled on the nitro group,possibly through solvent assisted resonance delocalistion(CSAR).The TS of the reaction is a charge seperated MC with the proton attched on the incoming N atom.These obsevations indicate that the reaction experience a late-type nucleophilic attacking as the RDS in IL.