THE CHEMISTRY OF PROTONS IN AMBIENT-TEMPERATURE IONIC LIQUIDS - SOLUBILITY AND ELECTROCHEMICAL PROFILES OF HCL IN HCL-IMCL-ALCL3 IONIC LIQUIDS AS A FUNCTION OF PRESSURE (295 K)

The liquidus region of the HCl:ImCl:AlCl3 ionic liquid system has been determined through manometric measurement of HCl solubility as a func tion of pressure for 0 < P-HCl < 1 atm at 295 +/- 1 K. Analysis of the pressure-composition isotherms thus obtained, in conjunction with spe ctroscopic data, reveals that dissolution of HCl in these systems is a ssociated with reactions of the general form HCl(g) + X(-)(1) reversib le arrow HCIX(-)(1), where X(-) drop Cl-, HCl2-, H2Cl3- AlCl4-, or Al2 Cl7-. In the binary HCl:ImCl system, the HCl-derived proton exists as HCl2- H2Cl3-, and H3Cl4-; the equilibrium constants for the reaction a bove are 715 +/- 76, 3.28 +/- 0.05, and 0.74 +/- 0.02 atm(-1) for X(-) drop Cl-, HCl2-, and H2Cl3- respectively. In Lewis acidic HCl:ImCl:Al Cl3 melts (X(AlCl3), > X(ImCl)), HCl is complexed as the species AlCl4 . HCl- and Al2Cl7 . HCl-. The corresponding equilibrium constants wer e determined as 0.135 +/- 0.001 and 0.109 +/- 0.001 atm(-1) for X(-) = AlCl4- and Al2Cl7-, respectively. In Lewis basic HCl:ImCl: AlCl3 syst ems, all species with the exception of Al2Cl7 . HCl- contribute, thoug h the equilibrium constants associated with the reaction above are les s well-defined, being dependent on the specific composition of the mel t. Liquid phase equilibria such as reported earlier from this laborato ry are obtained by combining two or more of the dissolution equilibria above such that molecular HCl does not appear in the result. Electroc hemical profiles of HCl dissolved in HCl:ImCl:AlCl3 ionic liquids are reported as a function of HCl pressure for 0 < P-HCl < 1 atm at 295 +/ - 1 K. In Lewis basic systems, the electroactivity of HCl2-, H2Cl3-, a nd AlCl4 . HCl- is observed, while in Lewis acidic systems, two electr oactive protic species are present: AlCl4 . HCl- and Al2Cl7 . HCl-. Th e reduction potential of Al2Cl7 . HCl is ca. 0.7 to 1.0 V more positiv e than that of other protic species, confirming that the proton of Al2 Cl7 . HCl- is exceptionally labile. Based on this observation, we conc lude that Al2Cl7 . HCl- is the entity responsible for Bronsted superac idity observed in the Lewis acidic melts. It is demonstrated that the reproducibility of voltammograins requires strict control of the HCl p ressure.