Oxidation of potato starch with NO2: characterization of the carboxylic acid salts

Oxidation of primary hydroxyl groups of potato starch to the corresponding carboxylic acids was performed in a fluidized bed using N₂-diluted NO₂ as oxidant. FT-IR spectrum of the oxidized starch show the presence of two absorption bands at 1743 and 1379 cm^?1. The absorption at 1743 cm^?1 which normally attributed to carbonyl absorption vanishes after washing the oxidized starch by water. Then two new absorption bands appeared at 1650 and 1400 cm^?1 corresponding to the ionized carboxyl groups.

The presence of nitrate favor the un-ionized form of carboxyl groups and then the band at 1743 cm^?1 can be observed. Furthermore, the ionized carboxyl absorption (1600, 1400 cm^?1) are replaced by carbonyl absorption at 1743 cm^?1 by adding DCl. By means of ¹³C NMR analysis, oxidized starch, show a band around 200 ppm attributed to carboxylic acid, no formic acid was detected indicating that no cleavage took place under the applied reactions conditions.