# Method of allylic and benzylic oxidation

I am told that $\ce{MnO_2}$ oxidizes allylic and benzylic alcohols. However, what is the mechanism of this oxidation? All the professor had to offer me was that the metal ion, manganese 2+, chelates the substrate; there is some sort of interaction between a pi bond, the metal ion, and the electrophilic carbon atom bearing the hydroxyl group. No further explanation was given, and a Google search hasn't unearthed anything of use. What, therefore, is the mechanism of this oxidation? I would like to know more about the mechanism to better understand why $\ce{MnO_2}$ only touches allylic and benzylic alcohols rather than any vanilla alcohol.

This is one of those heterogenous reactions occurring on a surface which are mechanistically messy. As expected for a reaction that occurs on the surface of a reagent, factors such as surface area and reagent preparation (freshness, surface contamination) are significant variables. A free radical mechanism is generally accepted and a reasonable mechanism is posted below.

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When an allylic alcohol with stereochemistry (cis, trans) around the double bond is oxidized, the stereochemistry around the double is preserved.

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This suggests that we are not really dealing with a free, long-lived radical intermediate, but rather a free radical held in close association with the manganese ion.

• FYI, Your link doesn't seem to work. – Geoff Hutchison Sep 18 '14 at 21:49
• Hmm, it works for me. Let me put the url here again in case anyone else has a problem. citab.utad.pt/refbase/files/1890_JI-DONGLOU2011.pdf – ron Sep 18 '14 at 22:58
• looks fine now - I was getting a link back to Chemistry.SE not the PDF. Thanks! – Geoff Hutchison Sep 19 '14 at 12:41
• The link does not work – Schwarz Kugelblitz Mar 2 at 15:24
• @SchwarzKugelblitz I've added the mechanism directly to the answer. – ron Mar 3 at 18:20