Hammett plots permanganate oxidations

Figure 2. Hammett plots for the oxidation of unsaturated compounds by permanganate in aqueous dioxane (a), and methylene chloride (b) and (c).

In general, the radicals dimerize rapidly unless inhibited by pyranyl ring substitution 2-(4-nitrophenyl)benzopyranyl is reported not to dimerize, however.The kinetics of the formation of a series of 36 by reduction of the precursor flavylium ions by Cr(II) have been investigated a Hammett plot of the second-order rate constants versus has p = 1.03. Most work has been concerned with the polarography of the parent cations there appear to be no ESR results for benzopyranyl radicals. The mediation of benzopyranyl radicals in the oxidation of 2-phenyl-2//-benzopyrans by potassium permanganate has been suggested.(See Section II,B,5,b for radicals from anthocyanidins.)... 

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

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