Hydroxyl group, ionization carbohydrate

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

Table I. Ionization Constants (Hydroxyl Group) of Carbohydrates in Water ...

Aldonic, uronic, and ascorbic acids, lactones, and N-acetylated amino sugars were separated on sulfonated polystyrene-divinylbenzene, a strong polyanion exchanger (Wheaton and Bauman, 1953). This method is adaptable to neutral carbohydrates without complexation or adsorption, by immersion in strong alkali to ionize the hydroxyl groups (ion chromatography). 

Furthermore, tiie products wo% characterized by electrospray ionization mass spectrometry (ESI MS). The ESI MS analyses of the organosilicon carbohydrate products (Rgures 2-3) support the selective formation of an ester bond between one hydroxyl group of the o,P-ethyl glucoside molecules and the diacid siloxanes. For clarification in Figure 2, the non-reactive cyano functionality was an impurity in the commercial diacid disiloxane starting... 

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