Gy. Pan et al., MODEL-COMPOUND STUDIES ON THE CLEAVAGE OF GLYCOSIDIC BONDS BY OZONE IN AQUEOUS-SOLUTION, Research of chemical intermediates, 21(3-5), 1995, pp. 205-222
Kinetic expressions have been developed for the initial phase of the r
eactions between ozone and methyl-beta-D-glucopyranoside, a model comp
ound for cellulose, in aqueous solutions at moderately low pH (pH 3) a
nd ambient temperature. Ozone is reasonably stable under these conditi
ons. The reaction is first order in the glucoside and ozone, second or
der overall. Kinetic and analytical data suggest that the glucoside is
directly attacked by ozone to form polyoxide intermediates. The alpha
-glucoside was more stable and was attacked by ozone exclusively at th
e acetal oxygen in the initial phase of the reaction. In contrast, the
beta-anomer was ozonized nearly twice as fast, and was attacked by oz
one mainly at the C-H bond of the anomeric carbon. The latter reaction
resulted in the formation of delta-gluconolactone, probably by way of
a hydrotrioxide intermediate. The fragmentation of the intermediate c
ould be assisted by properly oriented non-bonding orbitals of the two
acetal oxygens. These stereoelectronic rules were previously proposed
by Deslongchamp and coworkers on the ozonolysis of acetals in non-aque
ous media. The activation energy for the reaction was 69.5 kJ/mole in
both acetic and sulfuric acid solutions at pH 3. The presence of trans
ition metal-ion, in particular Co(II) and Fe(II) ions, resulted in enh
ancement of both ozone decomposition and degradation of the glycoside,
apparently caused by free radical reactions. In some cases, addition
of acetic acid led to substantial suppression of the degradation of th
e glucoside.