Accessible hydroxyl groups

At low relative humidities, adsorption is due to interaction of water with accessible hydroxyl groups. These are present on the lignin and on the carbohydrates ia the noncrystalline or poorly crystalline regions. The high differential heat of adsorption by dry wood, - 1.09 kJ/g (469 Btu/lb) water. 

The thallous derivative so formed was found to undergo metathesis with methyl iodide and to yield thallous iodide and methyl cellulose. Analyses of the latter product for methoxyl content after suitable purification yielded a measure of the accessible hydroxyl groups. The reactions were all conducted in non-aqueous systems. 

Partially protected carbohydrates can be selectively oxidized at the primary hydroxy group to uronic acids at the nickel hydroxide electrode. At the same electrode, in polyhydroxy steroids, a preferential oxidation of the sterically better accessible hydroxyl groups is achieved [142]. By way of the mediator, TEMPO, carbohydrates that are only protected at the anomeric hydroxyl group are selectively oxidized at the primary hydroxyl group (Fig. 27) [143-145]. 

If this acetylation system does not include a strong catalyst or cosolvent, only the easily accessible hydroxyl groups will be acetylated. We developed an acetylation system that uses no strong catalyst or cosolvent and probably acetylates only easily accessible hydroxyl groups (27). 

To arrive at racemic coriolin, Danishef sky and coworkers chose to add an acetonyl fragment to a bicyclic enedione by Diels-Alder chemistry (Scheme LXXIII) Treatment of the resulting adduct 695 sequentially with a series of conventional reagents produced the key intermediate 696. Suitable aldolization deUvered 697, the functionality in which was adjusted by deconjugation and reduction. Fiuther reduction of dPSiwith lithium in liquid ammonia and methanol followed by epoxidation afforded 699. Selective oxidation of the more accessible hydroxyl group and phenyl-sulfenylation gave 700 which experiences smooth elimination to 701 after conversion to the sulfoxide. As before, epoxidation completed the sequence. 

Regioselective enzymatic acylation of large, insoluble polysaccharides is still a quite difficult task and therefore it is not surprising that only scant data have been reported up to now, most of them describing reaction outcomes which met with limited success. Nevertheless, enzymatic derivatization of polysaccharides has been performed in nonpolar organic solvents using insoluble polysaccharides with soluble [51] or suspended enzymes [52]. Chemically modified celluloses with either enhanced solubility or more readily accessible hydroxyl groups, like cellulose acetate or hydroxypropyl cellulose, were acylated by CalB, as reported by Sereti and coworkers [53]. However, the same authors failed to modify crystalline cellulose under the same reaction conditions. 

Deuteration of the accessible hydroxyl groups is accomplished with saturated deuterium oxide vapor at room temperature. The extent of deuteration and therefore accessibility can be estimated gravimetrically by infrared spectroscopy. Accessibility rather than crystallinity is measured because deuteration of the hydroxyl groups on crystallite surfaces can also occur. 

Polysaccharides. Polysaccharides possess readily accessible hydroxyl groups that may be derivatized for selective protein immobilization. We will consider the two most commonly used immobilization reactions, the triazine and the cyanogen bromide reactions. 

The sorption of formaldehyde from aqueous solution by cellulose depends on diffusion and, ultimately, on chemical substitution of accessible hydroxyl groups by hemiacetal (hydroxymethyl) groups. " Full acetal links are formed only when the pH of the aqueous formaldehyde is below 3, below which point, there is a linear relationship between the logarithm of the amount of chemically combined aldehyde and the pH. The reaction, which is slow at room temperature, is markedly affected by temperature, and the energies of activation (23 and 21 kcal./mole, for cotton and viscose rayon, respectively) are much higher than the value observed for the reversible sorption of formaldehyde (13 kcal./mole). 

A chemical agent from a simple reaction leaves a tag on accessible hydroxyl groups and the tag is identified and located through a series of chemical operations and gas-liquid chranatogra y (14-17). To be tagged the hydroxyl group must lie on a surface accessible to the reagent and it must be available for reaction,... 

Most important is the number and distribution of xanthate groups on the cellulose chains. When cellulose is xanthated, the reaction is heterogeneous and the most accessible hydroxyl groups react first, resulting in a very nonuniform distribution. Then, as the cellulose xanthate is dissolved, the conditions for the ripening reactions become homogeneous. Andersson and Samuelson [166] demonstrated the reversible nature of the xanthation reaction by bubbling... 

The plant cell walls of bast fibers consist mainly of cellulose, hemicellulose, and lignin. The uptake of water by a hygroscopic material as cellulose or hemicellulose is a hydration process involving accessible hydroxyl groups, or in this case -CH2OH groups of the host material. Water molecules absorbed by dry cellulose form a true cellulose hydrate, and the reaction is exothermic, which provides the driving force (Pizzi 1987). 

In the single-stage development, adding chemicals in two steps was found to be beneficial. Cotton gauze was first treated with alcoholic caustic soda for 15 minutes at 100°F [to swell cotton and increase accessible hydroxyl groups] before treating it in the same bath with alcoholic monochloroacetic acid for 4 to 5 hours at 155 +/- 3 F [refer to... 

When most of the accessible hydroxyl groups in CF6 are derivatized with aromatic groups, however, the general enantioselectivity for primary amines is almost completely lost (Fig. 9c), whereas enhanced separations of a great variety of other chiral compounds can be obtained (Fig. 9f). These observations are readily... 

Cellulose is esterified by reactions of accessible hydroxyl groups with a number of acids and anhydrides. The reactions may be either batch or continuous processes with cellulose, at least initially, in a solid fibrous form. For some uses, it is important that during esterification a minimum of cellulose degradation occurs. Inorganic, organic and mixed esters of cellulose have been produced, are soluble in a number of solvents, can be hydrolyzed and yield a number of useful products. ... 

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