Kinetics of the Hydroxyethylation of Starch in Aqueous Slurries

Kinetic results are presented for the hydroxyethylation of potato starch with ethylene oxide. The reactions were carried out at 303.0 K in alkaline aqueous starch slurries containing sodium sulphate as swelling inhibitor. A new side reaction was discovered: the formation of hydroxyethyl hydrogen sulphate from ethylene oxide and sulphate dianions. Part of the ethylene oxide is hydrolysed to ethylene glycol. Selectivities were determined by analysis of the products hydroxyethyl starch and ethylene glycol. Since hydroxide ions are formed in the production of hydroxyethyl hydrogen sulphate, the rate of formation of this compound could be measured by the amount of acid added to keep the pH constant. The hydrolysis of ethylene oxide towards ethylene glycol can be described by two parallel reactions: (a) by water, and (b) catalyzed by hydroxide ions. The reaction of ethylene oxide with starch is more enhanced by alkali than the base catalyzed reaction of ethylene oxide with water to ethylene glycol, causing the selectivity of the hydroxyethylation to increase from 7 to 68% if the sodium hydroxide dose is increased from zero to 0.060 mole per mole of anhydroglucose unit