A kinetic, spectral and theoretical investigation on the role of oxygen in the radiolytic oxidation of a sorbityl cyclic acetal

摘要

The oxidation process of the cyclic acetal sorbitylfurfural (SF) has been thoroughly examined from the kinetic, spectroscopic and theoretical point of view. Oxidation has been initiated by the radiolitically produced OH radical in the presence of variable oxygen amounts. Two competing reaction pathways are evidenced which lead to quite different products, although they do not affect the acetal ring integrity. The peroxidation of the hydroxylated furanic ring (k4 = (6.1 0.9) x 108 M-1 s(-1)) maintains the ring structure via HO2 center dot elimination (k(6) = (1.9 +/- 0.4) x 105 S-1). Unlike that, the peroxidation of the pseudo-allylic radical (k(5) = (1.9 +/- 0.9) x 109 M-1 s(-1)), formed via P-cleavage, fixes the destructured intermediate, leading to a tetroxide, which slowly decomposes through a Russell mechanism (k(8) = (2.3 +/- 0.6) x 102 S-1). It is confirmed that the steady state concentration of the tetroxide is very low, which suggests a molar absorption coefficient for it around 1.2 x 104 M-1 cm(-1) at 265 nm. The end products of the latter pathway have been characterized as carboxylic and butenal D-sorbitol derivatives. The kinetic and spectral data of every step of the process have been fitted by the above outlined mechanism. The energetics of the mechanism has been detailed by ab initio computations as well, carrying further substantiation to it. Semi-empirical calculations were also employed to describe the spectral properties of each intermediate.