Hydroxyl accessibility degradation

During the course of the alkaline degradations, both physical and chemical structures of the hydrocelluloses were monitored. Hydroxyl accessibility (13) was determined as a practical measure of the fraction of molecules accessible to the alkaline medium. The crystalline structure was characterized by x-ray diffraction (14). 

Alkaline Degradations - Change in Physical Structure. The hydroxyl accessibility of the fibrous hydrocellulose was initially 51.4 0.8%. In contrast, the amorphous substrate had an accessibility of 99.2 1.0%. Exposure of the fibrous hydrocellulose to the alkaline media caused the accessibility to decrease slightly to 50.7 1.0% and 49.1 1.2% at 60 and 80°C, respectively, but accessibility did not change significantly during the reaction periods (0-168 hr). 

Figure 1. Hydroxyl accessibility of the amorphous hydrocellulose during degradation in l.OM NaOH.

Exchange Reactions In Hydroxylic Media. Compounds 1 and 2 (Scheme 4) Interconyert readily at room temperature under acid catalysis. The equilibrium fayors the latter. Only 4.0% of 1 (R =Me) forms from 2 In excess MeOH. Unblocked aldehyde (Scheme 4) 1s observable (GC, NMR) under certain conditions as an unstable Intermediate In the aqueous hydrolysis of 1 to 2 (R=H). It Is not detectable In the IR or NMR spectrum of 2. Although k1net1ca lly accessible, the aldehyde Is thermodynamically disfavored. As a result, the degradative chain transfer and rapid a1r oxidation observed with unblocked aldehyde containing monomers and polymers (10) 1s avoided. 

Soil systems present even greater challenges. In these systems, hydro-phobic pollutants can be sorbed to soil in sites that have low accessibility for the hydroxyl radical. Dissolved iron is not likely to enter into hydrophobic soil pores and, therefore, pollutants in these sites can be extremely difficult to degrade. 

For example, the method potentially opens an access to compounds with a doubly chiral isopropyl unit (Scheme 14). In the pro-5-selective enzymatic hydroxylation of isobutyric acid (88) to (S)-/3-hydroxyisobutyric acid (89) the stereochemistry of the hydroxylation at C-3 is not known. It could be studied by preparing 88 in a doubly chiral form via stereocontrolled ami-SN2 reaction of dimethyl cuprate with the tosy-late 90 to give 91 which is then degraded by Lemieux- and then Baeyer-Villiger oxidation to 88. 

Chitosan has three kinds of functional groups (amino and hydroxyl groups) that are easily accessible for chemical modification and can yield derivatives with desirable characteristics for use in biomedical applications [219]. Chemical modifications to mask the cationic nature of chitosan said to be related to higher cytotoxicity have been proposed so far [220]. Furthermore, its cationic nature is mainly responsible for its bioactive properties, as it can ease interaction with the anionic proteins of the connective tissue and thus enhance cell adhesion of chitosan-based matrices [221]. It also permits a pH dependence, hence well-controlled degradation [222]. 

Clark et al [23] at the University of Durham, U.K. have applied XPS to the study of cellulose nitrate and the step by step nitration of cellulose. They have shown which hydroxyl group is most easily accessible for nitration. They have observed sulfate ester from the nitration process at the surface of the nitro cellulose, this is suspected as an initiator of the degradation of N.C. 

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