Reactivity of phenolic nucleophiles towards nitroso compounds. Part 2.¹ Reaction with alkyl nitrites (O-nitroso compounds)

Abstract

Second-order rate constants, k, have been measured for the reaction of substituted phenolate ions with alkyl nitrites in aqueous solution at 25 °C. The final products of this reaction were identified as aromatic C-nitroso compounds and/or NO₂^–. Nitrosation of the phenolate ion yields ca. 90% of the p-nitroso product. However, methylation of the activated position of the ring (as in 4-methylphenol) does not result in the decrease in the overall reaction rate that would be expected. The reactivity of all the phenolate ions studied correlates well with their basicity, with the Hammett σ^– constant and with their reactivity in other reactions in which they are known to act as O-nucleophiles. All these facts suggest that the reaction of aryloxide ions with alkyl nitrites always occurs through the oxygen atom to yield an unstable O-nitroso compound. This intermediate is likely to undergo an internal rearrangement of the NO group to give the corresponding C-nitroso product, competing with homolysis of the O–NO bond to yield nitric oxide. Oxidation of NO in the presence of O₂ gives N₂O₃, which could act as a nitrosating agent towards ArO^–. Hydrolysis of N₂O₃ in basic media accounts for the presence of the nitrite ion as one of the products.

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