Radical rearrangements 3-hydroxyalkyl radicals

The different behavior of the alcohols probably arises from differences in bond dissociation energies. Experiments show that radical attack on methanol (4) and ethanol (27) leads to rupture of the C—H rather than the O—H bond. There appear to be no direct measurements of C—H bond energies in alcohols. However, D(R—OH) has been determined as 102 kcal. and does not appear to vary greatly with changes in R, provided R is a simple alkyl radical (16). Moreover, the heat of rearrangement of alkoxy radicals to hydroxyalkyl radicals has been determined from electron impact data (12). Considering, for example, 2-propanol and the following reactions... 

Even though the radical attacking ethyl alcohol in the above reaction generated a-hydroxyethyl rather then ethoxy free radical, there seems to be little or no tendency for alkoxy free radicals to rearrange to a-hydroxyalkyl radicals. Thus in the reaction... 

Elimination and rearrangement reactions of the primary radicals (see Sect. 11,1) that are slower than 2 x 10s s-1 are, therefore, suppressed at ordinary concentrations of oxygen (air-saturated [02] 2 x 10-4 M). Similarly, radical-radical reactions (see Sect. 11,2) cannot compete effectively with reaction 36, even at the high dose-rates of pulse radiolysis. Because the hydroxyalkyl radicals are nearly planar, two different peroxyl radicals are generated at optically active centers. 

Oxidation of cyclohexene by peroxydisulfate in the presence of copper(II) salts results in the formation of cyclopentanecarboxaldehyde as the main product in an aqueous acetonitrile solution (equation 261), and 2-cyclohexenyl acetate in an acetic acid solution (equation 262). Reaction (261) has been interpreted as the formation of a radical cation (186) by oxidation of cyclohexene with SaOg , followed by hydrolysis of (186) to the yS-hydroxyalkyl radical (187), which is oxidized by copper(II) salts to the rearranged aldehydic product (188 equation 263). ... 

The Thd C(3 ) radical is an a-hydroxyalkyl-P-alkoxyl radical that can rearrange and yields after reduction 2>4-didcoxy-pcnlos-3-ulosc, whereby Thy is released (Dizdaroglu et al. 1976) [reactions (277) and (278) for the yield, see Table 10.27]. 

---