EPR Spectroscopic Study of Radicals Formed from Aliphatic Carboxylic Esters and Related Compounds

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Abstract
A long‐range coupling across the alkoxycarbonyl group of about 1.4–1.5 G has been observed in the EPR spectra of radicals formed in the hydroxyl‐radical‐initiated homopolymerization of methyl acrylate, ethyl acrylate, and methyl methacrylate within an aqueous flow system at 25°±1°C. This coupling has been further investigated by way of a comprehensive EPR study of a series of simpler, related radicals prepared under similar conditions from methyl formate, ethyl formate, methyl acetate, ethyl acetate, methyl propionate, and methyl isobutyrate by hydrogen‐atom abstraction with hydroxyl radicals. For ester radicals other than those produced from the formates there was a long‐range coupling of 1.4–1.5 G with protons δ to the unsaturated carbon atom when this unsaturated carbon atom was adjacent to the oxycarbonyl group, i.e., for radicals with the following functional structures, Ċ–CO–O–C and C–CO–O–Ċ. The two formates yielded only the radicals HCOOĊH2 and HCOOĊHCH3. These displayed a γ coupling with the formyl proton of 2.4–2.5 G. All these results are shown to be compatible with a simple model based on a delocalized π‐bond system. Comparison of the observed proton coupling constants with the available data for similar unsubstituted aliphatic radicals suggests that the value of Qα in the McConnell equation aα=Qαρ is significantly lower in a conjugated radical than in a similar nonconjugated radical. In support of the data from this study of hydroxyl‐radical attack on esters, the substrates formic, acetic, propionic, n‐butyric, and isobutyric acids and methanol, ethanol, and acetone have been examined in a like fashion. Many of the esters and acids investigated underwent hydrogen‐atom abstraction at more than one site. For those substrates, it was possible to estimate qualitatively the selectivity of hydroxyl‐radical attack from the relative intensities of the EPR absorptions of the different reaction products. The results so obtained are reckoned to generally support the expected electrophilic character of the hydroxyl radical in these reactions.