Oxidation and Fragmentation of Some Phenyl-Substituted Alcohols and Ethers by Peroxydisulfate and Fenton's Reagent

Abstract

The mechanisms of the reactions of the hydroxyl radical (generated from Fenton's reagent) and the sulfate radical anion (generated thermally from S2O82-), with various phenyl-substituted alcohols have been examined in some detail. The reaction of 1-phenylalkanols (PhCHOHR) with XO· (X = H or SO3-) at pH 1.8 yields some PhCOR, but benzaldehyde is the major product when R is isopropyl or tert-butyl and a minor product when R is ethyl. The total yield of carbonyl products is 70-100%. From competition experiments it was determined that all the 1-phenylalkanols are 5-6 times more reactive than 2-propanol toward HO·, and PhCHOHCH3 is 22 times more reactive than 2-propanol toward SO4· -. Para-substituted 1-phenylethanols show relative reactivities toward HO increasing in the sequence: CN< Br < H < C2H5. The tertiary alcohol, PhC(CH3)2OH, is more reactive toward XO· than 2-propanol and is converted in good yield to PhCOCH3, but tert-butyl alcohol is relatively unreactive and gives only a small amount of acetone. Ethers of 1-phenylalkanols are also more reactive toward XO· than 2-propanol and yield both fragmentation and ketone products. Secondary and tertiary alcohols of structure Ph(CH2)nC(OH)(CH3)R show high reactivity toward XO · and give fragmentation products when = 0-2 and R = H or CH3; when = 3 and R = CH3, however, the alcohol has a reactivity similar to tert-butyl alcohol. 1-Deuterio-1-phenylalkanols show the same relative reactivities toward HO· as their hydrogen analogs but PhCDOHCH- (CH3)2 gives a higher ratio of benzaldehyde (completely PhCDO) to ketone than does PhCHOHCH(CH3)2. Product studies indicate that the fragmentations occur by cleavage of alkyl radicals. The electrophilic radicals (XO·) generated by several different methods, in the presence or absence of transition metals, give essentially the same reaction characteristics. The presence of Fe(III), Cu(II), Hg(II), or complexing agents does not affect the yields, relative reactivities, or product ratios obtained with Fenton's reagent at pH 1.8, but increasing the pH to 4.6 does; the pH effects were shown to be largely due to the phenyl-substituted alcohol acting as a catalyst for the conversion of Fe(II) to Fe(III) at higher pH's. The characteristics of the reaction of thermally generated SO4 ·- are unchanged from pH 1.5 to 11. The results are interpreted in terms of a mechanism involving a resonance stabilized cation radical as an intermediate in the reactions of these reactive phenyl-substituted alcohols and ethers with XO· the results require that the cation radical be stabilized by delocalization over the aromatic ring and oxygen functions. © 1974, American Chemical Society. All rights reserved.