Random cleavage of glycosidic bonds of cellulose under alkaline pulping conditions is the degradation pathway responsible for the loss of pulp viscosity.Until recently, an intermolecular nucleophilic substitution with attack at the C(1) carbon by the conjugate base of OH-2, SNicB( 2 ), had been thought to be the only mechanism causing this type of bond cleavage. The carbohydrate model phenyl 3-Dglucopyranoside, which forms 1,6-anhydro-P-D-glucopyranose (levoglucosan) and phenolate anion by the SNicB(2) mechanism at 100°C, was examined under high temperature alkaline conditions to determine whether the reaction mechanism or product ratios changed with increased temperature.Heating phenyl P-D-glucopyranoside at 100°C in alkali and monitoring reactant and product concentrations by gas chromatographic techniques yielded a degradation rate constant of 5.0 x 10 -5 sec -' and an 88% yield of levoglucosan that were consistent with literature values. Additional experiments were performed at the typical wood chemical pulping temperatures of 170°C using a modified fast flow reactor. Levoglucosan and phenol yields at 170°C were approximately 82% and 100%, respectively.The extent of oxygen-aglycon cleavage determined by 180 incorporation was negligible; indicating the lack of an SN2Ar mechanism. Addition of a strong nucleophile (NaSH) did not greatly affect the reaction rate constants but affected the levoglucosan yield; thus the SN 2 pathway was eliminated. A 1300 times decrease in the reaction rate constant due to blocking the C(2)-OH reaction site indicated an SNi type mechanism. The apparent thermodynamic functions of activation (AHt 28.3 kcal mol-land ASt -1.3 kcal mol ' 1 °K-1) were indicative of an SNicB( 2 ) pathway rather than the SNicB(2)-ro or SN 1 mechanisms. The 2% decrease in reaction rate constants with a fivefold increase of the reaction medium's ionic strength and a kinetically derived linear relationship of reaction rate constants with varying the hydroxide ion concentration at constant ionic strength were also consistent with the SNicB(2) mechanism.Thus, phenyl P-D-glucopyranoside degradation proceeds by an SNicB(2) pathway at both 100 and 170°C. The fact that the expected SNicB(2) product, levoglucosan, was formed in less than quantitative yields was probably due to competing reactions which occur after the SNicB( 2 ) rate-determining step. From the observed levoglucosan yields, it may be possible to determine the extent of SNicB(2) reactions occurring with other, slower reacting carbohydrate models at 170°C.
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