Wasfy N. Wassef

The reaction of trihalogenomethyl anions with carbonyl compounds: competitive reactivity comparisons and applications to the synthesis of α-trihalogenomethyl alcohols

Trihalogenomethyl anions, generated by decarboxylation of trichloro- and tribromo-acetic acid in dimethyl sulphoxide solution, react with added aldehydes. In the presence of 1,3,5-trinitrobenzene, the reaction with aldehydes competes with the formation of the coloured Meisenheimer adduct. The reduction in absorbance from the value in the absence of aldehyde has been used to measure the reactivity of trihalogenomethyl anions towards a series of aldehydes relative to their reactivity towards trinitrobenzene. For 4-substituted benzaldehydes, the reactivities obey a linear ρσ– relationship. The most reactive aldehyde used is only two times less reactive towards CCl3– or CBr3– than hydrogen ions, and it is concluded that, in dimethyl sulphoxide solutions, the reaction between trichloromethyl anions and hydrogen ions is not encounter-controlled. The reactions with aldehydes have been used to prepare several new compounds of the formula RCH(OY)CX3 where R = aryl or pyridyl, X = Br or Cl, and Y = H or COCH3.

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.

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.

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.

Addition of cyanide ions to aromatic aldehydes: a competition method for the determination of equilibrium constants of nucleophilic addition reactions

The equilibrium concentration of the cyanide Meisenheimer adduct of 1,3,5-trinitrobenzene, formed from potassium cyanide–18-crown-6 and trinitrobenzene, in DMSO solution, is reduced in the presence of an aromatic aldehyde, with an accompanying decrease in the absorbance of the solution due to the intensely coloured Meisenheimer adduct. Equilibrium constants have accordingly been determined for the formation of the cyanide Meisenheimer adduct of trinitrobenzene and for the formation of members of a series of substituted benzaldehyde cyanohydrin anions in dimethyl sulphoxide solution.

Hydride transfer from cyclohexadienyl anions: the reaction between the hydride Meisenheimer adduct of 2,4-dinitroaniline and 1,3,5-trinitrobenzene

The hydride Meisenheimer adduct of 2,4-dinitroaniline transfers hydride to 1,3,5-trinitrobenzene to form its corresponding hydride Meisenheimer adduct. The reaction is practically irreversible. In dilute solution in dimethyl sulphoxide it exhibits second-order kinetics and the observation of several sharp isosbestic points shows that it occurs without formation of any detectable intermediate species.

The decomposition of trihalogenoacetic acids in dimethyl sulphoxide: a mild route from carbonyl compounds to trihalogenomethylethanol and trihalogenomethyl ketones

Solutions of trichloro- and tribromo-acetic acid in dimethyl sulphoxide in the presence of carbonyl compounds react at room temperature to give insoluble products corresponding to addition of H–CX3 across the carbonyl double bond.