MECHANISMS OF PYROLYSIS OF POLYSACCHARIDES - CELLOBIITOL AS A MODEL FOR CELLULOSE

The pyrolysis of cellobiitol has been studied as a model for cellulose. The products were determined from pyrolysis at 350° in vacuo and at 350° and at 171° at atmospheric pressure. In vacuo, a 32% yield of 1,6-anhydro-β-d-glucopyranose (levoglucosan, 5) and a 70% yield of d-glucitol and its anhydrides were obtained, but under atmospheric pressure these yields were much lower. A mechanism is postulated in which the first step in the pyrolysis is a heterolytic scission of the glucosidic linkage to produce a glucosyl cation and a glucitol anion. The former yields the volatile levoglucosan by intramolecular attack of O-6 on the C-1 cation and this reaction is favored in vacuo because of the resultant efficient removal of 5. The gluticol is obtained predominantly as anhydrides, which are volatile in the vacuum pyrolysis. Lower temperature (171°) pyrolysis gave no 5. The glucosyl cation under such conditions is believed to undergo intermolecular nucleophilic substitution with hydroxyl groups from unreacted starting material and from glucitol to produce oligosaccharides. © 1990.