Although sulpyrine (I) can be quantitatively determined by iodometric titration in cold strong acidic solution, decrease of titration value is observed by allowing to stand it as aqueous solution. The rate of this decrease is more rapid in acidic solution than in neutral or alkaline solution. From the studies of proton magnetic resonance spectra of I in deuteroxide and D
2SO
4, the cause of this decrease was found to be based on the dissociation of I to 4-(methylamino) antipyrine (III) and sodium hydroxymethanesulfonate (IV), which does not react with iodine. In this connection, it has also been found that N-methanesulfonic acid is reactive to iodine, whereas O-methanesulfonic acid is not. pH dependency of dissociation and formation rate of I were studied from the curve of this decrease. In the reaction of sulpyrine (I) &rlarr^^^(k
1)___(k
2) 4-(methylamino) antipyrine (III)+HOCH
2SO
3Na (IV) dissociation and formation rate constants of I are calculated as follows : k
1=2.30×10
-1h
-1, in aqueous solution, 1.11×10
-1h
-1 in 1N hydrochloric acid, k
2=1.19×10
2M
-1·h
-1 in aqueous solution, and 6.35×10
-1M
-1·h
-1 in 1N hydrochloric acid. In view of these results, pH dependency of dissociation rate is found to be small, while that the formation rate is much affected by pH. It is supposed that formation of 4-(methylamino) antipyrine cation in acidic medium prevents the condensation of III with IV and therefore, formation rate of I becomes slow. This might be the causue of pH dependency of titration value decrease. Proton magnetic resonance spectra of I in D
2SO
4 and the solution obtained from the reaction of I with iodine have shown that sodium hydrogen sulfite, formaldehyde, and/or 4-(N-hydroxymethyl-N-methyl) aminoantipyrine (II) were not formed in the former solution, but that II and/or formaldehyde was formed in the latter solution immediately after the addition of iodine. In consideration of these facts, mechanism of this reaction might be concluded that iodine attacks I directly and quantitatively oxidized it to yield sulturic acid, hydrogen iodide, and II, which dissociates rapidly to formaldehyde and III.
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