Clarification in the mechanism for Molisch's test for glucose
YUSUF HASAN: 5-Hydroxymethyl furfural (5-HMF) 1 when protonated on the carbonyl oxygen becomes a reactive electrophile. Addition of phenol 2 at the reactive para-position affords 3 which deprotonates at the para-position, rearomatizing the ring to a phenol and liberating a proton. Secondary alcohol 4 is protonated on oxygen with loss of water to form stabilized cation 5. A second equivalent of phenol adds to the cation as previously described leading to the triarylmethane 6. The methane hydrogen is susceptible to oxidation by atomspheric oxygen, which is in a triplet state (unpaired electrons behaving as a free radical). Resonance stabilized radical 8 is formed along with the hydroperoxy radical 7, which can abstract the hydrogen from triarylmethane 6 forming more of radical 8 and hydrogen peroxide. Oxidation of radical 8 with oxygen gives the resonance stabilized carbocation 9 and superoxide anion, aka, superoxide radical anion 10. This species can be protonated by the the strong acid conditions of the Molisch test to form more of the hydroperoxy radical 7.