Carbohydrate pyrolysis mechanisms from isotopic labeling. Part 5. The pyrolysis of D-glucose: The origin of the light gases from the D-glucose molecule

摘要

Flash pyrolysis in air of the complete set of C-13(1) isotopologs of D-glucose, monitored by GC/MS using an efficient column for separating the light gases, allowed us to determine the sources within D-glucose for a range of light hydrocarbons and carbon oxides. These include carbon monoxide (CO), carbon dioxide (CO2), ethyne, ethene, ethane, propadiene, propene, propane, various isomers of butene, 1,3-butadiene, 1,3-cyclopentadiene and benzene. Inasmuch as the pyrolysis product was swept into the chromatographic column as formed, changes in isotopic incorporation with temperature rise could be qualitatively observed as changes of isotopic content across the chromatographic peak. There was significant divergence in labeled origin of CO and CO2 , suggesting substantial mutual independence of formation. For both, however, composition was dominated by the first four carbons of D-glucose. The high-temperature range of formation of these may reflect the composition of the underlying char undergoing combustion. Similarities in isotopic content of ethene and ethane, or of allene, propylene and propane or of the various C-4 species suggest that the least saturated versions are formed initially, and then undergo free-radical chain induced hydrogenation. Concerted electrocyclic fragmentations were invoked to explain the dominant formation of ethene, ethyne, propadiene and 1,3-butadiene. CO formation was postulated to arise in part from the fragmentation of glyoxal. CO and CO2 showed strong evidence of preferential ionization of the C-13 isotopologs relative to the C-12 ipsologs under our conditions, due to the magnetic isotope effect. Overlaid on this was a partial chromatographic enrichment of (CO2)-C-13 in the leading edge of the chromatographic peak. The data were normalized to adjust for both effects.