Derek P. N. Satchell

Kinetics of hydrolysis of 2-aryl-2-phenyl-1,3-dithianes in 10%(v/v) dioxane–water, containing perchloric acid. Acidity functions in this solvent and the reactivity of α-thio carbocations

The acid-catalysed hydrolysis of 2-aryl-2-phenyl-1,3-dithianes in 10%(v/v) dioxane–water occurs at a convenient rate at 25 °C in the presence of 5–8 mol dm–3 perchloric acid. The effect on the rate of changes in substituents, acidity and temperature, and of the use of a deuteriated solvent are described. Measurements of the H0and X acidity functions for the solvent are reported; their values are very similar to those found for pure water over most of the acidity range. The mechanism of hydrolysis is believed to change from an ASE2 scheme for the most reactive dithianes to an A2-like scheme for the least reactive. In very concentrated acid solutions the dithianes (and other suitable S,S-acetals) lead to stoichiometric amounts of the α-thio carbocations often postulated as low-concentration intermediates in S,S-acetal hydrolysis. The kinetics of the reaction of these ions with water to give the benzophenone (or corresponding carbonyl compound) are described, and compared with findings for similar α-oxo carbocations. Previous views on the mechanism of this reaction are criticised.

The Brønsted acid-catalysed hydrolysis of acyl fluorides in aqueous media; evidence for two mechanisms

In dioxane–water mixtures rich in dioxane the hydrolysis of benzoyl fluoride is shown to exhibit two mechanisms of acid catalysis: one at low, and one at high values of [H3O+]; only the second mechanism (A1) is detectable in pure water.

The kinetics of aminolysis of acyl halides

The morphinolysis of benzoyl fluoride in various non-hydroxylic solvents exhibits a kinetic form containing both first- and second-order terms in morpholine, a result suggesting that general-base catalysis in the aminolysis of acyl halides may be more common than previously assumed.

The brønsted acid-catalysed hydrolysis of benzoic anhydride in aqueous solutions. Evidence for two mechanisms and the effect of dioxane on their detectability

The hydrolysis of benzoic anhydride enjoys two mechanisms of acid catalysis in 60%(v/v) dioxane–water as solvent, one at low and another at high values of [H3O+]; the former becomes effectively undetectable in a purely aqueous solvent. Implications are discussed.

Mechanism of the Brønsted acid catalysed hydrolysis of 2-aryl-2-methyl-1,3-dithianes in aqueous perchloric acid

The acid catalysed hydrolysis of five para-substituted 2-aryl-2-methyl-1,3-dithianes in aqueous perchloric acid (3–9 mol dm–3) has been studied kinetically. The effect of changes in substituents (ρ=–3.1), temperature (ΔS‡=–38–60 J K–1 mol–1) and acidity (m‡= 0.54–0.74), and the solvent isotope effects (kobsD2O/kobsH2O < 1), are all compatible with an essentially ASE2 mechanism for the MeO, Me, H and Cl derivatives. For the p-NO2 compound the acidity dependence, and the values of ΔH‡ and ΔS‡(ca.–200 J K–1 mol–1) suggest a change to an A2-like mechanism. Our results are discussed in the light of previous work on acetal hydrolysis.

The Brønsted acid-catalysed hydrolysis of acyl fluorides in aqueous media

In dioxane-water mixtures rich in dioxane, the hydrolysis of benzoyl fluoride is catalysed by hydrogen ions by two mechanisms, one dominant at low, and the other at high, values of [H3O +]. In purely aqueous solutions, and in water-rich dioxane–water mixtures, the only catalysis observed is that at high acid concentrations. The effect of temperature, and of p-substituents suggests this later catalysis involves an A1 mechanism. We tentatively assign an ABAC3 mechanism to the catalysis found at low acid concentrations in dioxane-rich media. The hydrolysis of phenylacetyl fluoride in dioxane-water exhibits a behaviour pattern similar to that found for benzoyl fluoride.

Hydrolysis of aryl and alkyl isothiocyanates in aqueous perchloric acid

The slow hydrolysis of aromatic and aliphatic isothiocyanates in water is promoted by added perchloric acid. The hydrolysis leads first to the thiocarbamic acid, but this species decomposes rapidly to the (protonated) amine, and is not normally detected. Convenient rates of hydrolysis are obtained at 50 °C when [HClO4]≳ 6.0 mol dm–3. The effects of substituents, temperature, and acid concentration on the observed rate constant have been studied. Aliphatic isothiocyanates are somewaht more reactive than aromatic derivatives, but the effect of substituent changes is generally small, with electron release favouring reaction. Substituents close to the nitrogen atom hinder reaction. The value of ΔS‡ is typically –120 to –220 J K–1 mol–1, and analysis of the acidity dependence by the excess acidity approach shows m‡≃ 0.8. Addition of water to the isothiocyanate NC double bond via a mechanism invloving simultaneous proton transfer to nitrogen and nucleophilic attack by water at carbon with a cyclic transition state is proposed. The carbamic acids formed by the aliphatic isothiocyanates are sufficiently basic for them to be increasingly trapped as their protonated forms when [HClO4] > 9.0 mol dm–3.

Kinetics and mechanism of hydrolysis of open-chain thioacetals derived from benzophenone and the reactivity of α-thiophenyl carbocations

In a 40%(v/v) dioxane–water solvent, in the presence of 0.3–4.0 mol dm–3 perchloric acid, the rates of hydrolysis of diethyl and diphenyl thioacetals derived from substituted benzophenones exhibit substituent effects, acidity dependencies, activation parameters and solvent isotope effects which all suggest that the hydrolyses follow the A1 mechanism. The diethyl acetals are ca. 104-fold less reactive than their O,O-analogues and ca. 104-fold more reactive than the corresponding dithanes, for both of which classes of acetal the ASE2 mechanism of hydrolysis has been suggested. In concentrated aqueous perchloric acid the diaryl thioacetals are, like the diethyl compounds, rapidly and quantitatively converted into the corresponding α-thin carbocations, which then undergo slow hydrolysis to the benzophenone. Kinetic measurements show that the α-thiophenyl carbocation Ph2C+–SPh is ca. 20-fold more reactive towards hydrolysis than is Ph2C+–SEt, but that substituents in the thiophenyl group have little effect on reactivity (ρ≃ 0.6). The detailed kinetic results are compatible with our previous suggestions about the mechanism of hydrolysis of α-thin carbocations.

The kinetics and mechanism of the mineral acid-catalysed hydrolysis of carboxylic acid anhydrides in water and in dioxane–water mixtures. Application of the excess acidity function to these systems

We report a kinetic study of the mechanism of the mineral acid-catalysed hydrolysis of some benzoic anhydrides, and of pivalic anhydride, in water and in dioxane–water mixtures. Employing a wider range of acid concentrations than used hitherto, we have considered the effects of acid concentration on the rate of hydrolysis via the excess acidity approach, and have examined the effects of substituents and of temperature. Contrary to previous suggestions, we find that benzoic and p-toluic anhydrides hydrolyse in water predominantly by the A1 mechanism. For these anhydrides, and for the p-methoxy and p-chloro derivatives, in 60%(v/v) dioxane–water as solvent, a change in mechanism (A2 to A1) is suggested by all the criteria used, but only the A1 mechanism is detectable for mesitoic anhydride. Pivalic anhydride, contrary to previous conclusions, exhibits the A2 and the A1 mechanisms in both water and dioxane–water. The advantage of dioxane–water mixtures over pure water as solvent for detecting mechanisms of acid catalysis, and the value of the excess acidity method for detecting changes in mechanism in the former type of solvent, are both illustrated.

The Kinetics and Mechanism of Addition of Water and Alcohols to p-Nitrophenyl Isothiocyanate. The Effects of Added Dimethyl Sulphoxide

The second order-rate constants for the addition of water and ethanol to p-nitrophenyl isothiocyanate are larger in dimethyl sulphoxide solution than in pure water or ethanol. The detailed behaviour over a wide composition range suggests that H-bonding by the hydroxylic reactant to the solvent favours reaction, whereas H-bonding to this reactant retards reaction. The behaviour and relative reactivities of isocyanates and isothiocyanates suggest that protontransfer concurrent with nucleophilic attack at carbon, is less important in additions of hydroxylic compounds to isothiocyanates than to isocyanates. Branched-chain alcohols react more slowly with isothiocyanates than do primary alcohols. An excess of ethoxide ions reacts relatively rapidly with p-nitrophenyl isothiocyanate in ethanol to give the ionized thiourethane. The kinetics of this process, and the equilibrium constant for proton transfer between thiourethane and ethoxide ions, have been determined.