Kinetics of reactions in heterocycles. Part V. Replacement of the methylsulphinyl and methylthio-groups in substituted six-membered nitrogen heterocycles by methoxide ion.
Kinetic studies of the reactions of methylsulphinyl derivatives of pyridine, pyrazine (I), pyridazine (II), quinoline, isoquinoline, quinoxaline (III), cinnoline (IV), and phthalazine (V) with sodium methoxide revealed a high reactivity, comparable with that of the methylsulphonyl analogues. The methylthio-heterocycles were very much less reactive (between 5 × 103 and 3·7 × 105 fold) than their methylsulphonyl analogues, at 30°. These methylthio-compounds were from 1·2 × 102 to 3·4 × 103 times less reactive than the chloro-analogues.
Although the reactions of the methylsulphinyl and methylthio-compounds with sodium methoxide were bimolecular, regeneration of methoxide ion was such that the rate coefficients were calculated with modified forms of the normal second-order rate equation. For the reactions of methylthio-compounds, the methoxide ion concentration remained constant throughout the reaction, whereas for the methylsulphinyl compounds, approximately 0·5 ion equivalent of methoxide ion was consumed for each mole of sulphoxide.
4-Methylsulphinyl-pyridazine and -cinnoline with sodium methoxide were anomalous and gave 6 and 20% of methylthio-compound respectively. The preparations of the methylsulphinyl compounds are described. Ionization constants and spectra are recorded and discussed.