Acetyl groups, estimation

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. 

Estimation of the Number of Acetyl Groups in an Acetyl Ester. 

It is obvious that this method serves for the estimation of acetyl groups present in esters of alcohols or phenols. It also serves for the estimation of acetyl groups present in substituted primary or secondary amines. 

By use of benzene sulphonic acid or phosphoric acid. When the ester yields on hydrolysis products which become coloured in presence of alkali and air, Method 1 is inapplicable. If the acid produced on hydrolysis is volatile in steam, benzene sulphonic or phosphoric acid may be used as hydrolytic agent, and the acid (from the ester) after separation by steam distillation is titrated with standard alkali. (See Estimation of Acetyl Group, p. 479.)... 

In the solid state, tri-O-acetyl-p-D-xylopyranosyl halides adopt the all-equatorial 4Ci conformation, -whereas tri-O-benzoyl-p-D-xylopyranosyl halides crystallize in the all-axial 1C4 conformation, except for tri-O-benzqyl-p-D-xylopyranosyl chloride, which adopts the (skew) conformation. The variations of bond lengths to the anomeric carbon atom are discussed with regard to the anomeric effect. By comparative studies of the conformational equilibria of acetylated and benzoylated 1,5-anhydropentitols on the one hand, and pentopyranoses exhibiting the anomeric effect on the other, data for the non-bonding interactions may be derived. Based on the principle of additivity of these factors, the conformational distribution of structurally related compound may be estimated. In connection with these results, it is noted that two benzoyl groups show a smaller 1,3-diaxial interaction than two acetyl groups the difference between these interactions is about... 

Acetylation, the addition of an acetyl group (CH3CO) to the amino group of the N-terminal residue, is the most common form of chemical modification, affecting an estimated 80 percent of all proteins ... 

Quantitative acid hydrolysis with 72% (w/w) H2SO4 [19] was used to characterize the BSG feedstock. The monosaccharides and acetic acid were determined by HPLC to estimate (after corrections for stoichiometry and sugar decomposition) the contents of glucan (cellulose) and hemicelluloses (xylan, arabinan, and acetyl groups) in the sample. The acid-insoluble residue after hydrolysis was recovered by filtration and considered as Klason lignin after correction for the acid-insoluble ash. Protein was determined by the KJeldahl method [20] using the Ax 6.25 conversion factor. 

In hardwood, the xylan, or glucuronoxylan, consists mainly of D-xylose residues, most of which are acetylated, and 4-0-methyl-D-glucuronic acid residues (Fig. 2). Minor amounts of other constituents, such as L-rhamnose and D-galacturonic acid, can also be found. The molar ratio of 4-0-methyl-a-D-glu-curonic acid, acetyl groups, and xylose has been estimated to be 1 7 10 [59]. Hardwood also contains small amounts (2-5% dry weight (DW)) of gluco-mannan, composed of D-glucose and o-mannose. 

Jute and kenaf, like most vegetable fibers, contain a proportion of acetyl groups that are readily hydrolyzed by dilute alkali to acetic acid. Estimation of the quantity of acetic acid produced per unit weight of fiber then provides an index of the acetyl content. 

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