Rosemary S. Satchell

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.

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.

Quantitative aspects of Lewis acidity. Part XVI. Acidity of covalent metal halides towards substituted anilines in dioxan. The validity of ΔH0 as a measure of Lewis acid strength

AlCl3, GaCl3, SnCl4. PhSnCl3, and ZnCl2 form 1 : 1 adducts with substituted anilines in dioxan. The equilibrium constant for adduct formation, K, has been determined for each acid with several anilines at, normally, several temperatures; ΔH0 and ΔS0 values have been derived. Whereas, for any given base. K25 values follow the sequence SnSl4 > AlCl3 > GaCl3 > PhSnCl3 > ZnCl2(MeSnCl3, Ph2SnCl2, HgCl2), the values of –ΔH0 follow the different sequence PhSnCl3 > SnCl4 > GaCl3 ZnCl2. The corresponding ΔS0 values vary irregularly from one acid to another. For most of the acids adduct formation probably involves the replacement, by an aniline molecule, of a solvent molecule co-ordinated to the metal atom, but for PhSnCl3 the reaction is probably an addition of a base molecule to the solvent–acid adduct. A comparison is made of the level of acidity exhibited by the various acids in dioxan with that exhibited in diethyl ether, and for each acid equations are given relating the pK in dioxan of unhindered anilines to the pKa values of the anilinium ions in water. The changes in pK found on moving from one substituted aniline to another, although always regularly related to changes in pKa, are not determined primarily by changes in either ΔH0 or ΔS0; both quantities are important. This fact is discussed.

Acceptor properties of metal halides. Part VII. Tin, zinc, and antimony halides as catalysts for the racemisation of α-methylbenzyl chloride in diethyl ether

The catalytic efficiencies of stannic chloride, stannic bromide, zinc chloride, zinc bromide, trichloro(phenyl)tin, and antimony trichloride in the racemisation of α-methylbenzyl chloride in ether have been compared. For all the catalysts the reaction is first order in the alkyl halide, but the order in the catalyst concentration depends upon the system. With stannic and zinc halides the order is a mixture of first and second, with trichloro(phenyl)tin a mixture of first and third, and with antimony trichloride fourth. The reaction mechanism is discussed. The first-order terms in catalyst concentration are interpreted as representing paths in which the transition state is solvated by ether molecules, and the higher order terms in catalyst concentration as representing paths in which the transition state is solvated by metal halide–ether molecules. The first-order catalytic rate constants are in the sequence SnCl4 > SnBr4 > ZnBr2 > ZnCl2 > PhSnCl3 > SbCl3. The enthalpy of activation and entropy of activation have been determined for both paths for the stannic halides and the zinc halides. The values generally support the suggested mechanism. The effect of water on the stannic chloride and on the zinc chloride catalysis has been studied. The racemisation rate is decreased: the reduction corresponds to the formation of 1SnCl4,2H2O and of 1ZnCl2,1H2O adducts. Addition of hydrogen chloride leads to an increase in the rate of the stannic chloride and of the antimony trichloride catalysed reactions. This increase probably results from transition state solvation by hydrogen chloride–ether molecules. Molecular weight determinations show that zinc bromide, zinc iodide, and stannic bromide are monomeric in ether, and that zinc chloride is monomeric in acetone.

Kinetics of aminolysis of some benzoyl fluorides and benzoic anhydrides in non-hydroxylic solvents

The kinetic form of the spontaneous aminolysis of benzoyl fluorides in non-hydroxylic solvents is unlike that reported for the other benzoyl halides but is similar to that found for the aminolysis of esters. Variations on the mechanisms currently advocated for ester aminolysis are suggested for the benzoyl fluoride reactions. Tetrahedral intermediates are likely, but their rate-determining breakdown to products may involve a simultaneous proton transfer to the leaving fluoride ion. The kinetic behaviour differs from that found for aqueous solutions. The kinetics of the spontaneous aminolysis of benzoic anhydrides by primary amines, and by morpholine, in dioxane solution are first-order in each reagent over a wide amine concentration range, but the aminolysis by imidazoles involves also an important kinetic term second-order in amine. The mechanistic implications are discussed. Again the observations differ from some of those reported for aqueous solutions. For aminolyses of a variety of acylating agents, kinetic observations using non-hydroxylic solvents show that the easier it is for the leaving group to depart, owing to the structure of the acylating agent and/or that of the attacking amine, the less important become paths involving two or more amine molecules, but that such paths are generally more important than they are in hydroxylic solvents.

The kinetics and mechanism of the thallium(III) ion-promoted hydrolysis of thiolurethanes in aqueous solution. A metal ion-promoted elimination

The hydrolysis of thiolurethanes R1C6H4NHCOSC6H4R2(1), in dilute aqueous perchloric acid, under conditions where the spontaneous hydrolysis is negligible, is promoted by Tl3+ ions. The organic products are the corresponding anilinium ion and the thallium salt of the thiophenol. The effects of substituent changes (R1,R2) of changes in [H3O+], temperature, ionic strength, and of replacement of the NH proton by Me, are all compatible with hydrolysis occurring by elimination–addition mechanisms via the isocyanate as a reactive intermediate; thallium ion-promoted E1 cb and E2 routes are implicated. In effect the elimination–addition type of mechanism which is important for these esters at higher pH has been made available at low pH by complexation with Tl3+ ions. With the thiolurethanes RC6H4NHCOSEt, (2) which are less susceptible to the sponataneous and base-catalysed elimination–addition mechanisms of hydrolysis than are thiolurethanes (1), the presence of Tl3+ ions can also lead to promoted hydrolysis via elimination, but an AAC1-like route (with Tl3+ taking the role of H+) seems to be available to the N-Me derivatives.

The kinetics and mechanism of aminolysis of isothiocyanates

A study of the kinetics of aminolysis of p-nitrophenyl isothiocyanate by n-butylamine, benzylamine, dibenzylamine, p-anisidine, m-toluidine, N-methylaniline, and p-chloroaniline in diethyl ether and iso-octane as solvents has shown that many of the reactions involve kinetic terms that are second order in amine. The detailed behaviour reveals that aminolysis occurs via an intermediate of amine–isothiocyanate (1:1) stoicheiometry which undergoes subsequent prototropic rearrangement catalysed by a second amine molecule. Added carboxylic acids inhibit aminolysis by strong bases by their removal as inactive amine–acid (1 : 2) complexes, but with bases with which they form only a negligible amount of complex, acids can catalyse the aminolysis, probably by their effect on the prototropic rearrangement. Added thioureas (products) have negligible catalytic effects in the presence of an excess of amine. Our results and conclusions are similar to findings for the aminolysis of isocyanates in water.

Deuterium Isotope Effects on the Silver Ion-Promoted Hydrolyses of S,S-Diethvlacetals of p-Substituted Benzaldehydes

Solvent deuterium isotope effects support the view that the p-N02 and p-Me derivatives of benzaldehyde S,S-diethylacetal undergo silver ion-promoted hydrolysis in water by different mechanisms.

The kinetics and mechanism of the spontaneous hydrolysis of 4-chlorophenyl isocyanate in diethyl ether solution

The kinetics of hydrolysis of 4-chlorophenyl isocyanate have been studied at low concentrations in diethyl ether solution at three temperatures. When the ratio [H2O] : [RNCO] is sufficiently large the final organic product is 4-chloroaniline only, which arises from the loss of carbon dioxide from the initially formed carbamic acid, RNHCO2H. In the presence of a sufficient excess of water, the overall process comprises two consecutive first-order reactions [equation (4)], whose pseudo-first-order rate constants k1 and k2 both depend on the third power of the stoicheiometric water concentration. For the first reaction ΔH‡≃ 22 kJ mol–1 and ΔS‡≃–202 J mol–1 K–1; for the second reaction ΔH‡= 22 ± 1 kJ mol–1 and ΔS‡=–216 ± 3 J mol–1 K–1. We interpret our results in terms of cyclic transition states involving trimeric water. Ours is the first kinetic investigation to detect the two-stage process of aniline formation.

Quantitative aspects of Lewis acidity. Part XIII. Basicity of substituted benzamides towards metal halides. The validity of nuclear magnetic resonance chemical shifts as measures of Lewis acid strength

ZnCl2, ZnBr2, BF3, GaCl3, GaBr3, PhSnCl3, SnCl4, AlCl3, and AsCl3 from 1:1-adducts with substituted benzamides in diethyl ether or in tetrahydrofuran. The values of K, the equilibrium constant for adduct formation in diethyl ether, follow the sequence AlCl3 > SnCl4 > GaCl3 [graphic omitted] GaBr3 > ZnCl2 [graphic omitted] ZnBr2 BF3 > PhSnCl3 > SbCl3 > AsCl3. The K values are larger in diethyl ether than in tetrahydrofuran. It is shown that the carbonyl oxygen atom of the benzamide is the donor atom. It is also demonstrated that neither standard heats of formation of the adducts nor the resulting 1H n.m.r. chemical shifts of the N–H protons are reliable guides to the relative acidities of the covalent halides. The relative basicity of benzamides and anilines towards metal halides is very different to their relative basicity towards the proton in water. The pKa value of 2-methylbenzamide is –1·88 and molecular weight determinations show that gallium chloride is monomeric in ether.