REACTIONS OF TERT-BUTOXYL RADICALS WITH ACYCLIC ETHERS STUDIED BY THERADICAL TRAPPING TECHNIQUE

1,3,3-Tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl has been used as a radical trap to investigate the pattern of hydrogen abstraction reacti ons occurring in a range of acyclic ethers by tert-butoxyl radicals. T he results confirm the high degree of selectivity for abstraction at C atoms adjacent to ethereal O. The presence of two alpha ethereal O at oms is less effective in enhancing abstraction than one. This is possi bly due to a stereoelectronic effect whereby the 1,3-oxygen orbital in teractions reduce the ability of either oxygen to interact effectively with the developing radical centre. Whereas the presence of an alpha- oxygen enhances the rate of hydrogen abstraction, the presence of a be ta-oxygen retards the rate of hydrogen abstraction. Abstraction at a m ethine C atom adjacent to ethereal O occurs at about the same rate or a little faster than at a methylene C atom, whereas abstraction at met hyl groups is much slower. A temperature study of the abstraction from methyl and methylene sites in dimethoxymethane shows that the differe nce in abstraction rates at these sites is predominantly an entropy ef fect. A lower entropy of activation due to the loss of the internal ro tational mode of the methyl group in the formation of the transition s tate is the probable reason. Ethers appear to be less reactive than al cohols in hydrogen abstraction reactions.