Anhydroglucose unit hydroxyl groups

In theory, for regular rayon manufacture only one of the hydroxyl groups on each pair of anhydroglucose units needs to be replaced by a xanthate group, ie, the target degree of substitution (DS) is 0.5, which if achievable without waste would need 23% CS2 on ceUulose. 

Many cellulose derivatives have been prepared of which the esters and ethers are important. In these materials the hydroxyl groups are replaced by other substituent groups. The degree of substitution is the term given to the average number of hydroxyl groups per anhydroglucose unit that have been replaced. 

How many hydroxyl groups are present on each anhydroglucose unit in cellulose ... 

Wetting and swelling. As stated above, the presence of hydroxyl groups in cellulose enables it to be esterified up to a maximum of three ester groups to every anhydroglucose unit. Further, the high content of hydroxyl groups endows cellulose with hydrophilic properties. Nevertheless, the amount of water absorbed by cellu-... 

Nitrocellulose of 12.65% N, according to this nomenclature, is called 2i-nitrate, etc. The degree of nitration can also be designated by the degree of substitution (D.S.) which is the average number of nitrated hydroxyl groups in one anhydroglucose unit. 

It has been shown (10) that if all three hydroxyl groups on every anhydroglucose unit are substituted, no enzymic attack is possible. A material with DS = 3 is very difficult to prepare because of steric hindrance and the effect of substitution on other physical properties. Wirick (11) has calculated the relationship between MS and DS, based... 

Due to the 3 hydroxyl groups available for oxidation within one anhydroglucose unit and due to the polymeric character of the cellulose a great variety of structural modifications and combinations is possible. As with other types of chemical changes at the cellulose molecule also in this case the oxidation can affect different structural levels differently. Depending on the oxidative stress imposed on the cellulose, the individual hydroxyls within the AGU and within the polymer chain are involved to varying extent and may respond to further treatment and reactions in a specific way. Despite their low concentration in the imol/g range, oxidative functionalities are one of the prime factors to determine macroscopic properties and chemical behavior of cellulosic materials (Fig. 1). 

Degree of substitution (DS is the average number of functional groups per anhydroglucose repeating unit arriving at a level of three after complete functionalisation of all hydroxyl groups)... 

Primary hydroxyl groups in carbon atoms 2 and 3 of the anhydroglucose unit do not cause reversion, even when this oxycellulose is hydrolyzed with HC1. 

FIGURE 6.1 Ordinary reactions of chemical agents with cellulose are almost exclusively with the 2,3 and 6 hydroxyl groups that are not involved in formation of the linear polymer consisting of D-anhydroglucose units joined via (3-1,4-linkages. 

Because it is a polyhydroxyl alcohol, cellulose can be esterified in strong acid mediums, anhydrides, and catalysts, and requires the absence of water for completion because it is a reversible reaction. Normally, the reaction proceeds rapidly and is permitted to continue until the three hydroxyl groups on each anhydroglucose unit have been replaced with the acyl group of the organic acid or mixture of acids. 

The preceding analysis suggests that water, indeed, penetrates throughout the amorphous regions of these materials and undergoes a specific interaction with available sorption sites, most likely the available hydroxyl groups on the anhydroglucose units. Differential heat of sorption results for various starches ... 

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