Ambiguous Acidity

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Science  29 Oct 2004:
Vol. 306, Issue 5697, pp. 781
DOI: 10.1126/science.306.5697.781b

Acidic strength is often broadly defined as the ease with which an acid loses H+ to a base. Stoyanov et al. show that in nonpolar media, where acid and base stay closely associated after reacting, the relative strength of two acids varies with the base. They use infrared spectroscopy to compare two similar acids, both common synthetic reagents: TfOH and Tf2NH (where Tf is a CF3SO2 group). When water acts as the base in benzene or dichloroethane solution, TfOH is the better H+ donor, forming a (H3O+)(TfO) ion pair; Tf2NH forms only a weak hydrogen bond to water. In contrast, trioctylamine base accepts H+ more readily from Tf2NH than from TfOH.

The data suggest that the discrepancy results from extra hydrogen bonding between H3O+ and the SO2 group in TfO, which geometry precludes in the Tf2NH complex. Thus, the acidity of HX depends not only on the stability of X but on direct molecular interactions between X and the base. — JSY

J. Phys. Chem. A 108, 9310 (2004).

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