Reducing endgroup

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

The reducing endgroup of cellulose is hemiacetal in nature and is partially converted to the open-chain aldehyde function in solution. The anomeric hydroxyl groups being the most acidic in cellulose [82,83] can be selectively etherified [84]. 

They also behave practically like an aldehyde and can be reduced or oxidized to a glucitol or gluconic acid moiety. Reduction with sodium borohydride is often used for a quantitative estimation of the reducing endgroup content [69]. Oxidation may be achieved by a variety of agents [85], but the yield is generally... 

Reducing endgroups play a significant role in the chemical process of cellulose conducted in alkaline media by being able to undergo the so-called peeling or endwise depolymeriztion reaction. 

Accessibility. Many reports [98-103] indicated that when hydrocellulose was treated with an alkali the stable residues still contained noticeable amounts of reducing endgroups [94,101-103]. This phenomenon was ascribed to a physical stopping process [100] when a degrading end reached a crystalline region inaccessible to the alkali. 

Overall Process. Figure 7 illustrates the alkaline degradation of cellulose at elevated temperatures [102,106]. The existing reducing endgroups will... 

Though the major alkaline reactions of cellulose have been relatively well defined, the role of cellulose physical structure in those reactions has not been clearly established. Cellulose molecules have been reported to undergo physical stopping of the peeling reaction when a molecule is peeled back to a crystalline region in the cellulose structure, with the result that the reducing endgroup... 

In all cases, kp decreased with reaction time. Thus, the accessible reducing endgroups in both hydrocelluloses were more reactive initially, apparently due to their location in less ordered regions of the respective physical structures. As the less ordered material was removed, the accessible reducing endgroups occupied increasingly ordered regions of the structures and were therefore less reactive. 

In the case of chemical stopping, the rate of formation of carboxylic acid endgroups is also proportional to the number of accessible reducing endgroups. The pseudo-first-order rate expression is given by ... 

The rate coefficients for chemical stopping decreased with time for both substrates in a pattern similar to that for peeling. Thus, as the accessible reducing endgroups occupied progressively more ordered regions of the structures, their reactivity toward chemical stopping also decreased. 

In both the 60 and 80 C reactions, the fibrous hydrocellulose exhibited higher kpg values than the amorphous hydrocellulose (Table V). This appears to be due to the involvement of more molecules in crystalline domains of the fibrous substrate. The greater inhibition of chemical stopping by cellulose I than cellulose II domains may also have contributed to this effect by allowing more molecules in the fibrous hydrocellulose to peel to a point where the reducing endgroup would be inaccessible. 

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