No abstract

Both isopentyl nitrite (IPN) 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 [*H,]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.

N-Methyl-N-nitrosotoluene-p-sulphonamide (MNTS) reacts readily with the thiolate anion of cysteine and some of its derivatives, in aqueous ethanol, to give the corresponding S-nitroso species. The measured second order rate constant k, increases with pH, and the experimental points lie close to the S-shaped curve calculated for reaction via the thiolate anion. The pK, values for SH ionisation were calculated from the experimental results, and gave figures generally in good agreement with the literature. There is no evidence of reaction via the thiol RSH, or of any reaction, under these conditions, with less powerful nucleophiles including amines, alcohols and an enol. There was no evidence of S-nitrosation of cysteine using dimethylnitrosamine, nitrosoproline or nitrososarcosine. The results are consistent with a direct transfer of the nitroso group to the sulphur atom of the thiolate anion. The results are compared with similar reactions of alkyl nitrites.Many nitroso compounds (both inorganic and organic) can themselves act as nitrosating agents, i.e., they can transfer the nitroso group in an electrophilic sense to a suitable nucleophilic site.' This usually occurs intermolecularly but can, under certain circumstances, take place within the same molecule. In most cases the transfer occurs indirectly; another nucleophile (sometimes the solvent or for example a halide ion) first reacts with the nitroso compound to give a species which is the true nitrosating agent. An example is provided by the nitrosation reactions brought about by alkyl nitrites in aqueous acid. Here, it has been shown' that hydrolysis of the alkyl nitrite occurs first (rapidly and reversibly) liberating nitrous acid (i.e. an 0nitrosation of water) which generates the true nitrosating agent NO+ or H,NO,+. In this case 0-N bond fission is greatly assisted by protonation of the oxygen atom (bound to the alkyl group) in the alkyl nitrite. Nitrosamines behave similarly. However in basic and neutral solution (where hydrolysis is much slower, and in any case yields nitrite ion which is not normally a nitrosating agent) alkyl nitrites act directly as nitrosating agents towards a number of substrates including amines; secondary amines yield nitro~amines.~ This reaction is greatly facilitated by the presence of electron-withdrawing substituents within the alkyl nitrite (see Scheme l), as expected R+O-N<:NHR; 0 No -RO-+ O N~H R ; 3. 1 ROH ONNR; Scheme 1.for an electrophilic It is not known with certainty whether corresponding N-nitroso compounds can act in a similar way, i.e. in a direct reaction, but there are many examples in the literature5 of the indirect reaction where hydrolysis (or reaction with a nucleophile such as halide ion) of nitrosamines in acid solution releases a reactive nitrosating species. There are two reports 6,7 of a somewhat unusual reaction which occurs at high acidity, where the protonated forms of some aromatic nitrosamines react with the protonated form of some aniline derivatives.In this paper we are seeking to establish whet...

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