MECHANISMS OF PYROLYSIS OF POLYSACCHARIDES .7. PYROLYSIS OF SOME C-13-LABELED GLUCANS - A MECHANISTIC STUDY

An isotopic labeling study has been conducted to investigate the chemical mechanisms involved in the formation of certain pyrolysis products of glucans, specifically glycolaldehyde (GA), acetol (hydroxypropanone), acetic acid, and formic acid, which are the major non-aqueous components of the distillate fraction (-60-degrees-C condensate) of the pyrolyzate. C-13 labels at C-1, C-2, and C-6 of the glucose rings in synthetic glucans were used to reveal the origins of these compounds. In general, the results show that each compound is formed by several different mechanisms, but suggest that only a few mechanisms predominate in each case. Glycolaldehyde derives predominantly from the C-1-C-2 segment of the glucose monomers, with C-5-C-6 also contributing significantly. Evidence is presented supporting heterolytic mechanism which require a reducing end-group and base catalysis. Acetol derives mostly from three contiguous carbons that include a terminal carbon (C-1 or C-6), most often C-6 and most often appearing as the methyl carbon in the acetol. Acetic acid also arises most often from terminal carbons, the C-5-C-6 segment being the major source, with the methyl carbon usually deriving from C-1 or C-6. About half of the formic acid produced arises from C-1. Mechanisms derived from the chemistry of alkaline degradation and involving acylformylcarbinol intermediates are proposed.