Nitrogen dioxide oxidation polysaccharides

Nitrogen dioxide oxidizes the primary hydroxyl groups of polysaccharides to uronic acids (161). Side reactions occur to an extent of 10 to 15%, presumably to produce carbonyl groups. These oxidized polysaccharides have some applications, based principally on their conversion to a soluble form by salt formation. 

The use of nitrogen dioxide for the selective oxidation of polysaccharides to polyuronic acids was introduced by Kenyon and his coworkers13,63 in 1941. By this means extensive oxidation of the primary alcohol groups in cellulose was obtained, through the mechanism of preferential nitration followed by decomposition of the nitric acid ester with carboxyl forma-tion.68(0< > Apparently some undissociated nitration products also were formed, since infrared absorption studies54 indicated the presence of nitrate radicals in the polyuronic acid. Side reactions produced carboxyl,... 

Carbohydrates. Mild oxidants such as chlorine, bromine, or iodine readily convert the aldehyde end groups in the wood polysaccharides to aldonic acid end groups. Nitrogen dioxide selectively converts the primary hydroxyl groups on C-6 in cellulose to carboxyl groups (J3). Periodic acid is a specific oxidant for vicinal diols and yields formaldehyde from primary hydroxyl groups and aldehydes from secondary. 

Polysaccharides containing free primary alcohol groups can be oxidized to the corresponding carboxylic acids with nitrogen dioxide, or oxygen and a platinum catalyst hexose units are thus converted into hexuronic acids. 

Full oxidation of monosaccharides by nitric acid to aldaric acids has been an established technique for more than a century. Analogously, oxidation of polysaccharides such as cellulose or starch by nitrogen dioxide (N2O4) yields 6-carboxy starch and 6-carboxycellulose respectively [84]. Subsequent hydrolysis at rigorous conditions (0.5-2M HCl at 150°C) of these materials yields D-glucuronic acid. A drawback of the oxidation with nitrogen dioxide is that depolymerisation may be an important side reaction. An improvement of the process with respect to this aspect can be achieved by conducting the reaction when the polysaccharide is dissolved In 85% phosphoric acid and with sodium nitrite as the oxidant better yield versus a more Important depolymerisation [85-86]. 

The diversity of polysaccharides at the molecular level is due to the simple basic elements from which they are built. Abundant and ubiquitously available carbon (dioxide), oxygen, hydrogen, and H2O, with contributions from phosphorus (P), sulfur (S), and nitrogen (N) at different states of oxidation (global distribution of C 9.5%, H 63%, O 25.5%, N 1.4%), are the building blocks for on-demand formation and transformation of natural carbohydrates, primarily by green plants and algae. The polysaccharides... 

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