Hydroxyl groups, acetylation

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Cellulose Triacetate. Cellulose acetate having 92% or more of the hydroxyl groups acetylated is referred to as triacetate. This fiber is characteristically more resistant to alkaU than the usual acetate and may be scoured, generally, in openwidth, with aqueous solutions of a synthetic surfactant and soda ash. 

Stevens and coworkers used their c.d. data on the various D-glucans to assign, tentatively, the bands to specific chromophores. They found that derivatives of these polysaccharides that have all of their hydroxyl groups acetylated still exhibit the 177-nm band. They assigned this band (which occurs at somewhat shorter wavelengths for the helical polymers) to the ether of the acetal chromophore. This assignment is essentially consistent with the results obtained by Johnson and coworkers on unsubstituted monosaccharides. 

Cellulose Triacetate. Cellulose acetate having 92% or more of the hydroxyl groups acetylated is referred to as triacetate. This fiber is characteristically more resistant to alkali than the usual acetate and may be scoured, generally, in open-width, with aqueous solulions of a synthetic surfactant and soda ash. Triacetate is a hydrophobic liber, as compared to secondary acetate, and consequently does not dye rapidly. It is necessary to increase the rate of diffusion of the disperse dye into the fiber by increasing the dyeing temperature to 110— 130CC or using a dye accelerant or carrier, or both. 

The acetyl derivative (each hydroxyl group acetylated) is prepared and purified, and a weighed quantity of it hydrolysed with benzene sulphonic acid. The acetic acid liberated is separated by steam distillation and titrated with baryta solution. 

Cellulose (Figure 8.19) may be extracted from wood pulp, and is usually partially hydrolysed with acid to give microcrystalline cellulose. These materials are used as tablet diluents. Semi-synthetic derivatives of cellulose, e.g. methylcellulose, hydroxymethylcellulose, and carboxymethylcellulose, are used as emulsifying and suspending agents. Cellulose acetate phthalate is cellulose with about half the hydroxyl groups acetylated, and the remainder esterified with phthalic acid. It is used as an acid-resistant enteric coating for tablets and capsules. 

Cellulose acetates. Cellulose acetate is a well-known derivative of cellulose and has found many uses as a fiber. Acetate, diacetate, and triacetate are similar in chemical structure with acetate having about 83 percent of the hydroxyl groups acetylated, and not less than 92 percent hydroxyl groups are acetylated in triacetate. Triacetate and diacetate fibers are manufactured by the acetylation of refined wood pulp or purified cotton linters. The acetylation reaction is chemically quite simple and may be visuahzed in the following manner ... 

Acylation of aquatic humic substances with acetic anhydride in a variety of solvents was found to be unsatisfactory because of dehydration and 7r-bond formation. The mild acylating reagent, A-acetylimidazole, did not give complete acylations of hindered hydroxyl groups. Acetyl chloride was tested under a variety of conditions and gave complete derivatization of primary and secondary hydroxyls under the conditions shown in reaction (3) ... 

Brucinonic acid and dihydrobrucinonic acid are similarly constituted but the carbonyl group of the former is replaced by an alcoholic hydroxyl group (acetyl derivative (71) and oxidation to brucinonic acid (55)) in the latter. Contrary to expectation, brucinonic acid is not reduced (NaHg in acid medium (57, 69, 122) ) to dihydrobrucinonic acid but to the stereo-isomeric brucinolic acid. These changes may be schematically represented as follows ... 

Cellulose with about 97% of the hydroxyl groups acetylated is used to make acetate rayon. 

Alkoxyl group replaced by hydroxyL t Caibetho group hydrolysed, f Hydroxyl group acetylated during aslaotonisatioii. 

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