Selective oxidation and determination of the substitution pattern of hydroxypropyl guar gum

Abstract

The substituent distribution in hydroxypropyl guar gums was characterized by chemical methods in this study. A new method based on periodate oxidation combined with probability analysis was proposed to determine the degree of substitution at secondary hydroxyl groups. The periodate oxidation was investigated at pH = 4.3 and at the temperature of 25 °C. The oxidized products were characterized by IR and NMR. It was shown that carbonyl groups in oxidized products were in the form of hemiacetal. The degree of substitution at primary hydroxyl groups was determined by the reaction of 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO)-mediated selective oxidation. Therefore, the total degree of substitution and the average length of hydroxypropyl substituents were calculated. Results showed that the values of the total degree of substitution (DS) were practically the same as the values of molar substitution (MS) in the range of 0.2–1.5. The average chain length of hydroxypropyl substituents was almost 1.0.