Kinetics of the hydroxypropylation of potato starch in aqueous solution

Abstract

Kinetic results are presented for the hydroxypropylation of Potato starch with methyloxirane in aqueous solution. Reaction temperatures were varied from 303 to 362 K. Sodium hydroxide was used as a catalyst. The overall conversion rate of methyloxirane in alkaline starch solution is determined by the rate of four parallel reactions: the catalyzed and uncatalyzed hydroxypropylation of starch and the catalyzed and uncatalyzed hydrolysis of methyloxirane. All reactions appear to be first order in both reactants. The temperature dependency of all rate constants can be described by an Arrhenius equation, as follows: for catalyzed hydroxypropylation, k1,1 = 42.8 X 10(3)e-66390/RT; for uncatalyzed hydroxypropylation, k*1,1 = 39e-71200/RT; for catalyzed hydrolysis of methyloxirane, k1,1** = 32.8 x 10(4)e-71400/RT; for uncatalyzed hydrolysis of methyloxirane, k1,1*** = 449.4e-77150/RT; with k in m3/(mol.s), R in J/(mol.K), and the energy of activation in J/mol. The dissociation constant of starch hydroxyl groups could be calculated from the experimental results. The temperature dependency of the pK(a) of the starch hydroxyl groups could be fitted with the equation pK(a) = 2174(1/T) + 6.06 (T in K).