ESR studies of hindered phenoxy radicals

Abstract

Oxidation of 2,6-di-tert.-buty1-4-methyl phenol, 2,6-di-tert.- buty1-4-ethyl phenol and 2,6-di-tert.-buty1-4-n-propy1 phenol with lead dioxide or iodine produces the corresponding 2,6-di-tert.- buty1-4-alkyl phenoxy radicals. Decay of these radicals in tetrahydrofuran and benzene has been studied by optical and electron spin resonance techniques. Decay of the 2,6-di-tert.-buty1-4- methyl phenoxy radical is second order with respect to the radical with a specific rate constant K (subscript -d) 4.59 x 10 M² sec�¹ , in benzene and 3.33 x 10² M�¹ sec�¹ in tetrahydrofuran at 24.0 ± 1.0°. The specific rate constant is independent of phenol concentration and PbO₂, but is extremely sensitive to oxygen. Isolation of dimeric products 1,2-bis(3,5-di-tert.-buty1-4-hydroxyphenyl)ethane and 3,3',5,5'-tetra-tert.-butyl-4,4'-stilbenequinone and observation of the quinone methide intermediate in both solvent systems suggest that disproportionation of phenoxy radicals to yield quinone methide and the parent phenol is the mechanism for phenoxy decay. Decays of the 2,6-di-tert.-buty1-4-ethyl phenoxy radical and the 2,6-di-tert.-buty1-4-n propyl phenoxy radical are clearly second order with respect to radical at low concentrations of hindered phenol., As the concentration of hindered phenol is increased, the observed rate constant k (subscript --ob), decreases and deviation from linearity of the second order kinetic curves is observed. Changes in both esr linewidths and hyperfine splitting constants as functions of phenol concentration suggest the existence of a radical-phenol complex. A kinetic scheme involving such a complex is postulated, and data from the 2,6-di-tert.-buty1-4-ethyl phenol- 2,6-di-tert.- butyl-4-ethyl-phenoxy- benzene system support this postulate. The complex possesses 1:1 stoichiometry, with K (subscript eq) 2.85 ± 0.28 at 24.0 ± 1.0°. There is strong evidence of similar complex formation for other phenoxy radical- phenol systems, but experimental difficulties obviate calculation of equilibrium constants.