The basicities of 2 H- and 4 H-1,3-benzothiazines (1, 2) were measured spectrophotometrically in buffer solutions. The rates of quaternization with methyl iodide were monitored in sealed ampoules by an oscillometric method. The stabilities were determined by measurement of the rate of acidic hydrolysis via changes in the uv/vis spectra. As a cyclic Schiff base, 4-aryl-2 H-1,3-benzothiazine (2) is the stronger base, and reacts about ten times faster with methyl iodide; its stability is greater than that of 2-aryl-4 H-1,3-benzothiazine (1). On the basis of these results and by means of the different water-solubilities of the hydrochlorides the isomers formed during the synthesis can be separated and purified.
- Rate of hydrolysis
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