Quantitative study of the acidity of HCl in a molten chloroaluminate system (AlCl3/1-ethyl-3-methyl-1H-imidazolium chloride) as a function of HCl pressure and melt composition (51.0-66.4 mol % AlCl3)

• Published in: Journal of the American Chemical Society Vol. 111; no. 14; pp. 5075 - 5077
• Main Authors: SMITH, G. P, DWORKIN, A. S, PAGNI, R. M, ZINGG, S. P
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 05.07.1989
• Subjects: 400000 - Chemistry; 400201 - Chemical & Physicochemical Properties; ALKYLATED AROMATICS; AROMATICS; BENZENE; CHEMICAL ANALYSIS; CHEMICAL REACTION KINETICS; Chemistry; CONDENSED AROMATICS; DATA; DATA ANALYSIS; Exact sciences and technology; EXPERIMENTAL DATA; HYDROCARBONS; HYDROCHLORIC ACID; HYDROGEN COMPOUNDS; INFORMATION; INORGANIC ACIDS; Inorganic chemistry and origins of life; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; KINETICS; Kinetics and mechanism of reactions; LEWIS ACIDS; MEASURING INSTRUMENTS; MEASURING METHODS; NAPHTHALENE; NUMERICAL DATA; ORGANIC COMPOUNDS; QUANTITATIVE CHEMICAL ANALYSIS; REACTION KINETICS; TOLUENE; Lewis acid; Chemical equilibrium; Hydrogen Chlorides; Metal Compounds; Trivalent metal Compounds; Aluminium III Chlorides; Medium effect; Content; Protonation; Pressure
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja00196a009

The acidity of HCl in Lewis acid mixtures of AlCl{sub 3} and 1-ethyl-3-methyl-1H-imidazolium chloride (EMIC) has been determined as a function of HCl pressure (P{sub HCl}) and melt composition at ambient temperatures. The equilibrium constant (K{prime}{sub b}) for the protonation of arene bases (B) according to the reaction HCl + B {r reversible} BH{sup +} + Cl{sup {minus}} was determined from the relation log K{prime}{sub B} = H{prime} + log ((BH{sup +})/(B)) with the protonation ratio measured spectrophotometrically and the acidity function, H{prime} = log ((Cl{sup {minus}})/P{sub HCl}), evaluated by using the thermodynamic model of Dymek et al. to calculate (Cl{sup {minus}}). Values of log K{prime}{sub B} were determined for chrysene, fluorene, 2-methylnaphthalene, and mesitylene over a range of HCl pressures and melt compositions, while estimates for benzene, toluene and naphthalene were obtained at a single pressure and composition (1 atm, 66.4 mol % AlCl{sub 3}). The correlation between log K{prime}{sub B} for these arenes in HCl/AlCl{sub 3}-EMIC and log K{sub B} for the same arenes in HF/BF{sub 3} suggests that H{prime} {minus} H{sub 0} {approx equal} 0.4, where H{sub 0} is the Hammett acidity function. According to this criterion H{sub 0} for 0.01 atm HCl in 51 mol % AlCl{sub 3} has a value of {minus}12.6 (a superacid comparable to 100% H{sub 2}SO{sub 4}). At 1 atm HCl, a melt saturated with AlCl{sub 3} at ambient temperatures ({approximately} 67 mol % AlCl{sub 3}) is a much stronger superacid with a value of H{sub 0} on the order of {minus}18.