Mirror, Mirror on the Wall

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Science  03 Jan 2014:
Vol. 343, Issue 6166, pp. 8
DOI: 10.1126/science.343.6166.8-b

What's the strongest acid of them all? In the case of Brønsted acidity, associated with the tendency to lose a proton, it depends on the surrounding medium. The pKa scale was originally grounded in reactivity toward water, but many common solvents are inherently less basic, which allows discrimination of acids that would otherwise all dissociate fully under aqueous conditions. Halogenated carboranes have emerged as the weakest class of conjugate bases in this context; unlike antimony pentafluoride-derived superacid conjugates, they lack a highly Lewis acidic component. Nava et al. report the synthesis and isolation of an acid with the fluorinated carborane CHB11F11 as its conjugate, following up on a report several years ago of its preparation and the vibrational spectrum of an ethyl-substituted analog. Because this compound is sufficiently acidic to cleave alkyl C-H bonds by protonolysis, it was crucial to exclude organic impurities (as well as water) during the steps prior to its ultimate purification by sublimation. Suspension of the solid in hexane was observed to liberate hydrogen at room temperature in 50% yield within 2 hours, a substantial rise in reactivity relative to the previous Brønsted champion, which bore a chlorinated carborane conjugate; butane reacted similarly.

Angew. Chem. Int. Ed. 10.1002/anie.201308586 (2013).

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