Substituent effects on the kinetics of acid-catalyzed hydrolysis of methyl cellulose

The kinetics of the hydrolysis of methyl cellulose (MC, DS 1.27 and 1.95) was studied by a two-step procedure, comprising partial hydrolysis in 1 M TFA in water and water/acetone at 120 °C for various time periods, labeling of generated reducing ends by reductive amination, complete depolymerization by methanolysis followed by trimethylsilylation, and gas chromatographic analysis of the two sets of partially O-methylated glucose derivatives. Rate constants of MCs were all in the order of 10−4 s−1. In aqueous TFA, overall rate of hydrolysis of the MC with lower DS was faster than of the MC with higher DS. When substituting half of the water by acetone, reaction was slowed down while selectivity regarding different O-methyl glucosyl residues increased. Compared to the parent glucosyl unit methylation at O-2 and at O-6 decreased rate of hydrolysis, while 3-O-methyl favored it especially in the early stage of the conversion of the macromolecules. Beside slight differences between the two MCs and reaction conditions, rate constants ki (i = position of methyl) followed the order k36 ≈ k3 > k0 ≈ k23 > k6 > k2 ≥ k236 > k26. For the higher substituted MC2 an initial slow phase with more pronounced differences of ki, followed by a faster less selective period was observed. Regioselectivity of hydrolysis with respect to methyl positions was expressed as standard deviation of ki and was between 16 and 46% depending on MC and conditions. Findings are discussed with respect to electronic effects, solvent-effect, H-bonding pattern and solution state.