Roy B. Moodie

The kinetics of hydrolysis of methyl and phenyl lsocyanates

The hydrolysis of phenyl isocyanate is subject to general base catalysis by tertiary amines and the point for water falls on the Bronsted plot, which indicates that the uncatalysed reaction involves two molecules of water, one acting as a nucleophile and the other as a general base. The rather small solvent isotope effect, kwH2O/kwD2O= 1.65, and the proton inventory, are discussed. The hydrolysis of methyl isocyanate (unlike phenyl isocyanate) is acid- catalysed, probably proceeding with pre-equilibrium protonation. Methyl isocyanate reacts with hydrogenphosphate dianion and with hydrogensulphate ion, forming mixed anhydride species. The formal reaction with hydrogensulphate ion may proceed by pre-equilibrium protonation followed by nucleophilic attack by sulphate ion.

Kinetics of reaction of substituted pyridines and oxygen anions with methyl chloroformate in aqueous solution

The rate constants for nucleophilic reaction of nine substituted pyridines with methyl chloroformate gives a sharply curved Bronsted plot, the slope changing from β 0.93 to 0.15 with increasing reactivity. This contrasts with the straight line (β 0.93) observed with p-nitrophenyl acetate, and is shown to be quantitatively consistent with a change in rate-determining step. It is argued that this is from breakdown to formation of a zwitterionic tetrahedral intermediate which is only just stable enough to exist. The hydrolytic stabilities of some of the methoxycarbonylpyridinium ions produced by this reaction, and the nucleophilic reactivities towards methyl cloroformate of imidazole, of phenolate, p-nitrophenolate, and acetate anions, and of phosphate dianion are reported and discussed.

The nitrous acid-catalysed nitration of phenol

The reaction of phenol with nitrous acid (10–4M < [NIII] < 10–2M) and nitric acid (10–3M < [NV] < 0.3M) in aqueous sulphuric acid (19–45% H2SO4) at 25 °C gives rise to p-nitrophenol by a nitrosation–oxidation pathway, and concurrently to a 57 : 43 mixture of o- and p-nitrophenol by catalysed nitration. The latter reaction is the major one, and is first-order in phenol with a first-order rate coefficient given by [NIII][NV]/(x[NIII]+y[NV]). The x and y are constants for a given concentration of sulphuric acid. The dependence of x and y upon acidity, and comparison of the reactivity of phenol with that of hexadeuteriophenol and anisole, leads to a proposed mechanism for catalysed nitration. In this there is pre-equilibrium formation, from phenol and NIII, of a intermediate with the formula PhONO. This gives rise to a phenoxyl radical and nitric oxide, a step which is rate limiting when the rate is fully enhanced by [NV]. Nitric oxide is reversibly oxidised by NV to give NO2 and NIII. Reaction is completed by combination of the phenoxyl radical and NO2, in a step which is rate limiting when the rate is fully enhanced by [NIII].

Kinetics of hydrolysis and aminolysis of 1-methoxycarbonylpyridinium ions

Rate constants are reported for reactions of 1-methoxycarbonylpyridinium ions in aqueous solution with water, hydroxide ion, and primary and secondary amines. The water reaction is general base-catalysed; solvent isotope effects are reported. Both the water and hydroxide reactions proceed with rate-determining formation of a tetrahedral intermediate. With the primary and secondary amines, either formation or breakdown of a tetrahedral intermediate is rate-determining, depending upon the ring substituent. Structure–reactivity plots lead to estimates of relative leaving-group abilities, which are compared with others in the literature. Hydroxide-ion catalysed aminolysis is observed in some cases, and occurs by a concerted mechanism.

Electrophilic aromatic substitution. Part 21. Rate constants for formation of nitronium ion in aqueous sulphuric, perchloric, and methanesulphonic acids

Studies of the kinetics of nitration of anisole in 66–80% H2SO4, 64–70% HClO4, and 88–97% CH3SO3H, and of toluene in 74–79% H2SO4 are reported. Concentrations of the aromatic compound were high enough for the reaction to be less than first-order in them. Rate constants for formation of the nitronium ion are reported, and rate constants for the reverse reaction estimated. It is suggested that the reaction is a one-step process.

THE KINETICS AND MECHANISM OF OXIDATION OF HYDROQUINONE AND CHLOROHYDROQUINONE IN THE PRESENCE OF NITROUS ACID IN AQUEOUS ACID SOLUTION

Hydroquinone is converted quantitatively into benzoquinone by nitrous acid in air-saturated aqueous acid solution, by a process which is kinetically first order, whether or not the nitrous acid is present in excess. Chlorohydroquinone behaves similarly and substituent effects are discussed. The inorganic and organic steps in the mechanism put forward are shown by numerical integrations to account for all the observations. MOPAC calculations suggest that 4-hydroxy-6-nitrosocyclohexa-2,4-dienone is the first-formed intermediate. Homolysis of this to give the 4-hydroxyphenoxyl radical and nitric oxide is rate determining in the organic sequence and takes place by uncatalysed and acid catalysed pathways. Comparison with previous work on 4-methoxyphenol, where the later step of combination of the 4-methoxyphenoxyl radical with nitrogen dioxide is rate determining, permits an estimate of the relative rate-constants for reaction of the substituted phenoxyl radical with NO and NO2.

Nitrosation and nitrous acid-catalysed nitration of anisole and 2,6-dimethylanisole

Rate–acidity profiles have been obtained for the nitrosation of anisole, 2,6-dimethylanisole (DMA), and 2,6-dimethylphenol (DMP) in aqueous sulphuric acid. The phenol is more reactive than the corresponding anisole, and DMA has a more shallow profile than anisole. A deuterium kinetic isotope effect (kH/kD) of 4.0 for [4-2H]anisole indicates that the product of nitrosation of anisole in 46.5% sulphuric acid (p-nitrosophenol) is formed by slow proton loss from the Wheland intermediate. Intense colours were associated with the nitrosation of these compounds when nitrous acid was in excess.The kinetics of nitrous acid-catalysed nitration of anisole were studied in 43.0 and 47.0% sulphuric acid; the product is p-nitrophenol. Nitrosation followed by oxidation by NV was the major pathway at these acidities. The other pathway has a kinetic form given by kC=k3[NIII][NV], consistent with a process where oxidation by NV is rate-limiting. Product studies show that p-nitrophenol is formed at lower acidities and o- and p-nitroanisole at higher acidities. A mechanism is suggested involving a radical cation species, which would predominate at higher acidities and account for the kC pathway at lower acidities.The nitrous acid-catalysed nitration of DMA gives 2,6-dimethyl-4-nitroanisole in higher yield as the acidity increases.

Electrophilic aromatic substitution. Part XV. The kinetics, mechanism, and products of nitrodebromination in sulphuric acid

The kinetics of nitration in sulphuric acid of o- and p-dibromobenzene, p-bromo- and p-chloro-toluene, 2-bromo-m-xylene, and p-bromochlorobenzene are reported. For these compounds and for bromobenzene and p-bromofluorobenzene the yields of products formed over a range of acidities have been determined. Nitrodebromination was not detected with o-dibromobenzene and 2-bromo-m-xylene, but was a major outcome of nitrating p-dibromobenzene, p-bromotoluene, and p-bromochlorobenzene. The degree of nitrodebromination increased with increasing dilution of the sulphuric acid and evidence is provided to show that Wheland intermediates formed at brominated carbon atoms (WiBr) are either debrominated or rearranged by nitro-group migration. There is no intramolecular migration of bromine and little or no nucleophilic capture of the Wheland intermediates.When methyl groups are present ipso-nitration at C(Me) occurs and is followed by nucleophilic capture by water and by nitro-group migration in proportions which varied with the acidity. Nitrodechlorination was not observed, but even if attack at C(Cl) is assumed to occur a choice between subsequent decomposition of the Wheland intermediate to its components and nitro-group migration cannot be made. In p-dibromobenzene the C(Br) positions are at least as reactive as the C(H) positions.

Kinetics of hydrolysis and aminolysis of methyl chloroformate in aqueous solution

The behaviour in the title reaction of ten primary amines, with a basic pKa range of 11 units, has been studied. Uncatalysed nucleophilic attack of free base amine and competing uncatalysed hydrolysis are the only reactions found except in the case of hydrazine, where an additional term is attributed to nucleophilic attack of the hydrazinium cation. The Bronsted plot shows that the sensitivity of methyl chloroformate to the basicity of the nucleophile decreases with increase in the latter, and the significance of this is discussed.

Nitrosation of m-xylene, anisole, 4-nitrophenyl phenyl ether and toluene in trifluoroacetic acid or in acetic–sulfuric acid mixtures under nitric oxide

Nitrosation in trifluoroacetic acid or in acetic–sulfuric acid mixtures is regioselective and accompanying non-selective nitrous acid catalysed nitration can be avoided by purging with nitric oxide.