The Acidity of the HBr/AlBr3 System: Stabilization of Crystalline Protonated Arenes and Their Acidity in Bromoaluminate Ionic Liquids. Issue 20 (25th March 2015)

• Record Type: Journal Article
• Authors: Scholz, Franziska; Himmel, Daniel; Eisele, Lea; Unkrig, Wiebke; Martens, Arthur; Schlüter, Peter; Krossing, Ingo
• Is Part Of: Chemistry. Volume 21:Issue 20(2015)
• Page Start: 7489
• Page End: 7502
• Publication Date: 2015-03-25
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201405952
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Bulk protonated mesitylene, toluene, and benzene bromoaluminate salts were stabilized and characterized in the superacidic system HBr/<italic>n</italic> AlBr₃ with NMR spectroscopy and X‐ray analysis of [HC₆H₃(CH₃)₃]⁺[AlBr₄]⁻ (<bold>1</bold>), [HC₆H₅(CH₃)]⁺[AlBr₄]⁻ (<bold>2</bold>), and [C₆H₇]⁺[Al₂Br₇]⁻⋅C₆H₆ (<bold>3</bold>). Protonation attempts in bromoaluminate ILs led to a complete protonation of mesitylene, and a protonation degree of up to 15 % for toluene in the IL BMP⁺[Al₂Br₇]⁻. Benzene could only be protonated in the more acidic IL BMP⁺[Al₃Br₁₀]⁻, with a degree of 25 %. Protonation attempts on aromatics provide evidence that the bromoaluminate ILs tolerate superacidic environments. On the basis of the absolute Brønsted acidity scale, quantum chemical calculations confirmed the superacidic properties, and rank the acidities in ILs down to a pH_abs value of 164 with an error of less than one pH unit compared with experimental findings. The neat AlBr₃/HBr system even may reach acidities down to pH_abs 163.</p> </abstract>