A New Model Compound for Studying Alkaline Cellulose Chain Cleavage Reactions

A conformationally rigid cellulose model, the 4′,6′-O-benzylidene derivative of 1,5-anhydrocellobiitol, has been studied to learn more about the mechanisms of chain cleavage reactions under alkaline pulping conditions. Heating the model at 170°C in 2.5N NaOH gave 55% glycon-oxygen (G-O) bond cleavage, and ～45% oxygen-aglycon (O-A) bond cleavage. The amount of observed O-A bond cleavage is significantly higher than that for 1,5-anhydrocellobiitol. The benzylidene model also degraded about ～35% faster than 1,5-anhydrocellobiitol; much of this rate increase can be attributed to a faster rate of O-A bond cleavage for the benzylidene model. The greater amount of O-A bond cleavage in the benzylidene case may be attributable to a more highly substituted glycosyl ring (making the ring a better anion leaving group) and/or to a more conformationally inflexible glycosyl ring. The inflexibility restricts one of the standard G-O bond cleavage mechanisms, namely the S_NicB(2′) mechanism. The results point out the value of choosing appropriate cellulose models.