Nitrosation by alkyl nitrites. Part 7. Comparison with thionitrites: reactions with phenols

Abstract

Both isopentyl nitrite (lPN) and S-nitroso-N-acetylpenicillamine (SNAP) react with phenol in the pH range 2–9 to give overwhelmingly 4-nitrosophenol. For IPN the reaction is first order in both IPN and phenol and the measured rate constant is much reduced by the addition of isopentyl alcohol, suggesting that reaction occurs by prior hydrolysis of IPN to nitrous acid. When allowance is made for the nitrous acid ionisation we find no acid catalysis in the pH range 3.62–5.25 but acid catalysis at higher acidities. The corrected rate constant also increases with pH at pH values greater than 6. There is no bromide ion catalysis at pH 4 but a substantial kinetic primary hydrogen isotope effect (using [2H6] phenol) of 4.0 at pH 2.55 which decreases towards 1 as the pH is increased. All of these results are consistent with rapid and reversible hydrolysis of IPN, and phenol nitrosation results from the nitrous acid produced. All of the experimental features parallel those found in the nitrous acid nitrosation of phenol. The kinetic pattern for the reaction of SNAP with phenol is quite different, showing autocatalytic features. We have obtained an EPR spectrum from the photolysis of SNAP, characteristic of RS˙, and for the phenoxy radical derived from the reaction mixture of SNAP with 2,6-di-tert-butyl-4-methylphenol at pH 7. We propose a radical mechanism for the reaction of SNAP with phenol involving hydrogen-atom abstraction by the RS˙ radical and subsequent reaction of the phenoxy radical with nitric oxide formed by homolysis of SNAP.