The first and second pK, values for the title compounds and some nitro-substituted derivatives are reported. First protonation occurs in all cases on the ring nitrogen (pyridines) or at the N-oxide oxygen (N-oxides), and the pK, values are in line with predictions from the Hammett equation. The second protonation at the NMe, group follows the H,,"' acidity function, and thermodynamic second pK, values calculated using the slope m of [BH+]/[B] vs. Ho are also well correlated by the Hammett equation. of East Anglia, Norwich, NOR 88C pyridine 1-oxide.

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The nitration and hydrogen exchange in the 4-position of the title compounds and corresponding N-methyl cations are studied kinetically. Criteria previously developed to distinguish between reaction on the free-base or conjugate-acid forms of six-membered heterocyclic aromatic compounds are applied. Both hydrogen-exchange and nitration occur on the free-base form of 3.5-dimethylisoxazole. For 1,3,5-trimethylpyrazole and 3.5-dimethylisothiazole, nitration occurs on the conjugate acids, while hydrogen-exchange undergoes a changeover from free-base to conjugate-acid reaction as the acidity is increased.The kinetic rates are compared with those for related compounds and the influence of N, 0, S. and NMe atomic groupings discussed.

Two consecutive processes are observed in the reaction of methyl p-nitrophenyl sulphate in methanol containing lithium perchlorate, as followed by u.v.-visible spectroscopy at 25". In this solvolytic process, an initial conversion to p-nitrophenyl sulphate ion (carbon-oxygen scission) is followed by a further reaction to give p-nitrophenol (sulphur-oxygen scission). In reaction media containing perchloric acid the rate of the second stage is increased but the rate of the first stage remains unchanged. Therefore, there is no acid catalysis in the primary process yielding p-nitrophenyl sulphate ion. An apparent increase in the rate of this reaction which is observed in media containing hydrogen chloride can be ascribed to chloride ion catalysis. Similarly, nucleophilic catalysis is observed with bromide, fluoride, and azide ions. The reactivity order followed is typical of substitution a t aliphatic carbon. A preference of alkyl aryl sulphates for reaction with nucleophiles a t the alkyl carbon, rather than at aryl carbon or sulphonyl sulphur centres, is noted.

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