Wednesday, August 05, 2009

Our attempt to reproduce an oxidation by NaH

Yesterday I was discussing with my students the controversy over the claim that NaH can act as an oxidant for secondary alcohols (Wang, JACS09). There has been a lot of discussion and an attempt to reproduce one of the experiments has appeared on Totally Synthetic.

Khalid Mirza and Marshall Moritz thought it would be worthwhile to see if we can shed any light on the situation. I was also curious to see what the reaction did over time, before quenching.

We had 1-phenylethanol on hand, for which the Wang paper claims a 75% conversion (by GC) to acetophenone. All the details can be found on the notebook page UC243.

The reaction was monitored by taking aliquots of the solution then adding benzene-d6 to lock during NMR acquisition without quenching the reaction. After 19 hours at room temperature there was no change in the spectra, except for loss of the alcoholic proton. Acetophenone would have been easily detected at 7.9 and 2.6 ppm (ChemSpider spectrum)

In a comment on Carbon-Based Curiosities, European Chemist points to a 1965 paper where oxidants on the surface of NaH are likely responsible for oxidative behavior (Lewis JOC). This would certainly explain why some researchers are reporting some oxidation products but with widely divergent yields. For example, Totally Synthetic reported a 15% NMR yield for the conversion of the 4-chloro derivative to the corresponding ketone while Wang isolated the product in 86% yield.

As many have pointed out, this is a very good example of the way Web2.0 tools and community can complement traditional publishing to move science forward.

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