Pyrazoles, methyl-, hydrogen exchange

IR spectra, 5, 201 Pyrazole, 1-methyl-anodic oxidation, 5, 247 hydrogen exchange acid-catalyzed, 5, 239 2-oxide... 

Electrophilic Reactions. Further studies aimed at the correlation of the reactivity of five-membered heteroaromatic rings towards electrophiles have been reported. Acid-catalysed hydrogen exchange rates were measured (n.m.r.) for isoxazole, isothiazole, and their 3- and 5-methyl derivatives. Extrapolated values for the rate constants at 100 °C and pH 0 were obtained and compared with those for other ring systems. Quantitative effects of the methyl groups reflect the relative degree of bond fixation, and indicate that the aromaticity increases in the series isoxazole, pyrazole, and isothiazole. ... 

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