Watching wavelengths move

There has always been a problem in our ability to properly evaluate Brønsted acids in non-polar organic solvents. This is because acidities are commonly measured in water or DMSO. These solvents are quite polar and their high dielectric constants make them far from ideal in organic synthesis. As a corollary to that, anything measured in these polar media is of limited use.

Sherif Kaldas, who is one of the PhD students in my lab, brought up a nice paper at our journal club about 2 weeks ago. This JACS manuscript provides the best (in my view) way to experimentally determine the pKa of a molecule in your solvent of choice.

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http://pubs.acs.org/doi/abs/10.1021/jacs.5b01805

While pKa is an overwhelmingly useful metric in organic chemistry, I think we all have an example or two that will demonstrate how misleading it is to use tabulated pKa’s for acids in non-polar media. Now Steven Kass and co-workers of the University of Minnesota report an ingenious tool to measure acidities in non-aqueous media. In their work, IR spectroscopy provides a convenient analytical method. For instance, the IR spectra of dilute solutions (5 mM) of phenols in carbon tetrachloride and 1% deuterated acetonitrile result in a sharp band for the “free” O−H stretch around 3600 cm-1. In a series of derivatives, the frequency reduction of the band correlates with the formation of an ROH···acetonitrile hydrogen bond. Some really useful findings reported in this paper are transferable to proton-mediated catalysis in organic transformations. In my view, this paper fills an important gap in the literature and should find many applications.

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