KINETICS OF THE HYDROXYPROPYLATION OF POTATO STARCH IN AQUEOUS-SOLUTION

Kinetic results are presented for the hydroxypropylation of Potato sta rch with methyloxirane in aqueous solution. Reaction temperatures were varied from 303 to 362 K. Sodium hydroxide was used as a catalyst. Th e overall conversion rate of methyloxirane in alkaline starch solution is determined by the rate of four parallel reactions: the catalyzed a nd uncatalyzed hydroxypropylation of starch and the catalyzed and unca talyzed hydrolysis of methyloxirane. All reactions appear to be first order in both reactants. The temperature dependency of all rate consta nts can be described by an Arrhenius equation, as follows: for catalyz ed hydroxypropylation, k1,1 = 42.8 X 10(3)e-66390/RT; for uncatalyzed hydroxypropylation, k1,1 = 39e-71200/RT; for catalyzed hydrolysis of methyloxirane, k1,1* = 32.8 x 10(4)e-71400/RT; for uncatalyzed hydrol ysis 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 dissociat ion constant of starch hydroxyl groups could be calculated from the ex perimental results. The temperature dependency of the pK(a) of the sta rch hydroxyl groups could be fitted with the equation pK(a) = 2174(1/T ) + 6.06 (T in K).