Acetic anhydride: acetylation with physical properties

To illustrate this strategy, we begin again with the two enantiomers of Ai-acetyl alanine, which were prepared by treating a racemic mixture of R)- and (S)-alanine with acetic anhydride (Section 28.3A). Enzymes called acylases hydrolyze amide bonds, such as those found in Ai-acetyl alanine, but only for amides of L-amino acids. Thus, when a racemic mixture of Ai-acetyl alanines is treated with an acylase, only the amide of L-alanine (the S stereoisomer) is hydrolyzed to generate L-alanine, whereas the amide of D-alanine (the R stereoisomer) is untouched. The reaction mixture now consists of one amino acid and one Ai-acetyl amino acid. Because they have different functional groups with different physical properties, they can be physically separated. 

As the boiling points of 0-, m-, and p-toluidine are, respectively, 199°, 200°, and 198°, it is impossible to identify these compounds by a determination of their boiling points. The identification can be effected, however, by a determination of the melting points of their acetyl derivatives, which are formed when the amines are treated with acetic anhydride. The acetyl derivatives of 0-, m-, and p-toluidine melt at 107°, 65°, and 147°, respectively. In order to identify a substance it is necessary to determine the physical properties of the substance and of at least one of its derivatives. The method used in the identification of the toluidines is an excellent example of this principle. 

A-Acetylchitosan gel and partially 0-acetylated TV-acetylchitosan gel have been prepared by reaction of chitosan with acetic anhydride in 10% acetic acid and in aqueous acetic acid-methanol at room temperature. Physical properties, including specific rotation, of the gels were investigated. 

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