Ikenna Onyido

Kinetics of the reactions of aniline and 2,2,2-trifluoroethylamine with 1,3,5-trinitrobenzene in dimethyl sulphoxide and acetonitrile, and of 2,2,2-trifluoroethylamine with 1-fluoro- and 1-chloro-2,4-dinitrobenzenes in these solvents

Meisenheimer complex formation between 1,3,5-trinitrobenzene (TNB) and the primary amines aniline and 2,2,2-trifluoroethylamine (TFE) has been studied in dimethyl sulphoxide and acetonitrile. The reaction with aniline is catalysed in both solvents by 1,4-diazabicyclo[2.2.2]octane (DABCO). With TFE, catalysis by both TFE and DABCO was observed in acetonitrile, but not in dimethyl sulphoxide. The reactions of TFE with 1-fluoro- and 1-chloro-2,4-dinitrobenzenes are not base catalysed in dimethyl sulphoxide or acetonitrile.

Catalysis of the aromatic nucleophilic substitution reactions of anilines in aprotic solvents

The reaction of 1-chloro-2,4-dinitrobenzene with p-anisidine in benzene is catalysed by the nucleophile and by tetra-n-butylammonium chloride. When the nucleophile is N-methyl-p-anisidine the reaction is not catalysed by the nucleophile, tetra-n-butylammonium chloride, DABCO, or pyridine. The reactions of both p-anisidine and N-methyl-p-anisidine with 1-fluoro-2,4-dinitrobenzene have been shown to be catalysed by the nucleophile, tetra-n-butylammonium chloride, and (in the case of N-methyl-p-anisidine) by DABCO and pyridine. Mechanisms are proposed to rationalize the results.

Base catalysis of aromatic nucleophilic substitution reactions in aprotic and dipolar aprotic solvents

For base-catalysed aromatic nucleophilic substitution reactions in benzene, catalysis by added base is observed irrespective of whether the catalyst is a stronger or a weaker base than the nucleophile. In acetonitrile, catalysis is only observed if the catalyst has either approximately the same strength or is a stronger base than the nucleophile. These observations are shown to indicate a difference in the mechanism of catalysis in the two solvents.

Reaction of 2,4-dinitrophenyl phenyl ether with morpholine in dimethyl sulphoxide, acetonitrile, tetrahydrofuran, and ethyl acetate and of 1-chloro-2,4-dinitrobenzene with morpholine in ethyl acetate

The reaction of morpholine with 2,4-dinitrophenyl phenyl ether is base-catalysed in all the solvents investigated. The values of k3/k2 provide information about the mechanism of the uncatalysed decomposition of the intermediate formed in nucleophilic aromatic substitution in solvents of high dielectric constant.The reaction of morpholine with 1-chloro-2,4-dinitrobenzene in ethyl acetate is not base-catalysed. Comments are made on the mechanism of base-catalysed nucleophilic aromatic substitution in solvents of low dielectric constant and basicity.

Heteroaromatic azo-activated substitutions. Part 4. Kinetics and mechanism of the hydrolysis of 3-(4-methoxyphenylazo)-5-methylisoxazole in aqueous sulphuric acid media

The kinetics of the hydrolysis of the title compound, (3), have been investigated in moderately concentrated aqueous sulphuric acid media at 30 °C. Activation parameters have also been determined. Rate correlation by the Cox–Yates excess acidity method shows that hydrolysis occurs from the monoprotonated substrate by the A-SE2 mechanism of the SNAr type. Nucleophilic attack by water at the aryl carbon and a subsequent proton-transfer equilibrium are fast processes which precede the electrophilically catalysed separation of the leaving group, in which the functional catalysts are all general acids in solution. An abnormal value of 1.4 is obtained for the slope parameter m‡(Kresges αA); this is discussed in terms of differential solvation of the initial and transition states. The Bunnett–Olsen slope parameter (φ‡–φe) of –2.0 indicates that the transition state of the reaction is substantially less solvated than its initial state. The values of ΔH‡ and ΔS‡ vary to compensate each other and the decreasing ΔS‡ values accord with the ordered transition state proposed.

Catalysis by amine salts of some aromatic nucleophilic substitution reactions

The reaction of 1-chloro-2,4-dinitrobenzene with aniline in acetonitrile is catalysed by R4NY where Y is Cl, Br, I, or toluene-p-sulphonate, but not by 1,4-diazabicyclo [2.2.2.] octane (DABCO). When the nucleophile is changed to n-butylamine or morpholine, addition of tetraethylammonium chloride has only a small effect; the reactions of all three nucleophiles are not catalysed by tetraethylammonium perchlorate. The reaction of 1-fluoro-2,4-dinitrobenzene with aniline is strongly catalysed by tetraethylammonium chloride, to a lesser extent by the bromide and toluene-p-sulphonate and also by trimethylamine hydrochloride, but not by tetraethylammonium perchlorate. The reactions of morpholine with 1-fluoro-2,4-dinitrobenzene and piperidine with 2,4-dinitrophenyl phenyl ether are not catalysed by amine salts. The results are consistent with the formation and stabilisation of the intermediate formed in aromatic nucleophilic substitution reactions by the anions of the salts, when the nucleophile is aniline.

The kinetics of the reactions of aniline with 1-fluoro- and 1-chloro-2,4-dinitrobenzenes in dimethyl sulphoxide, dimethylformamide, acetonitrile, and nitromethane

The reaction of aniline with 1-fluoro-2,4-dinitrobenzene is base catalysed in the solvents dimethylformamide, acetonitrile, and nitromethane, but not in dimethyl sulphoxide. The reaction with 1-chloro-2,4-dinitrobenzene is not base catalysed in any of these solvents. The results provide information about the mechanism of the uncatalysed decomposition of the intermediate formed in nucleophilic aromatic substitution in solvents of high dielectric constant.

Mechanism of aromatic nucleophilic substitution in aprotic solvents

The effect of various hydrogen-bond acceptors on the reactions of 1-chloro- and 1-fluoro-2,4-dinitrobenzenes with morpholine in benzene at 30°C has been investigated. The reaction of the chloro substrate is not base-catalysed and the additives produced no acceleration in the concentrations employed. The reaction of the fluoro substrate is base-catalysed. The addition of hydrogen-bond acceptors frequently leads to accelerations which vary linearly with the concentration of the acceptor. An approximately linear relationship exists between the logarithm of the slope of these correlations and the hydrogen-bonding parameter pkHB. These effects are interpreted as electrophilic catalysis, by the heteroconjugates of the acceptors with the conjugate acid of morpholine, of the ratedetermining decomposition of the intermediate in these reactions.

Heteroaromatic azo-activated substitutions. Part 3. Catalysis in the reaction of 4-(4-methoxyphenylazo)pyridinium methiodide with amines in dipolar aprotic solvents. Kinetic form and the mechanism of the uncatalysed reaction in nucleophilic aromatic substitution

The kinetics of the reaction of the title compound with piperidine, n-butylamine, morpholine, and benzylamine have been studied in dimethyl sulphoxide and, for piperidine and n-butylamine only, in acetonitrile. The predominant reaction in all cases was nucleophilic displacement at the aryl carbon (SNAr reaction), accompanied by an SN2 attack at the alkyl carbon when secondary amines were the nucleophiles. The SN2 rate constants were independent of amine concentration. The SNAr reactions were all base-catalysed independent of whether the nucleophile was a primary or secondary amine, indicating that decomposition of the first formed intermediate was rate limiting in all cases. The activating influence of the azopyridinium function is discussed. The hitherto observed dichotomy in the reactions of the two amine types is considered. It is shown that in the absence of an ortho-nitro group in the substrate, as in the present case, both the catalysed and uncatalysed reactions occur by the specific base-general acid (SB–GA) mechanism.

Mechanism of the uncatalysed path of aromatic nucleophilic substitution in dipolar aprotic solvents when primary and secodary amines are the nucleophiles; a search for electrophilic catalysis of these reactions

The kinetics of the reactions of 1-chloro- and 1-fluoro-2,4-dinitrobenzene with morpholine have been studied in dimethyl sulphoxide, dimethylformamide, and nitromethane. The results confirm that the decomposition of the intermediate to products by the uncatalysed path takes place by a unimolecular mechanism. The kinetics of the reactions of 2,4-dinitroanisole with n-butylamine and piperidine in dipolar aprotic solvents have been determined. They show that when primary amines are the nucleophiles, reaction by the uncatalysed path does not occur by the unimolecular mechanism, but by the specific base-general acid (SB–GA) route. The reactions in dimethyl sulphoxide give another example of secondary amines reacting by a base catalysis mechanism whereas the corresponding reaction of a primary amine of the same basicity is not base-catalysed.In a search for electrophilic catalysis the effects of lithium, trialkylammonium, and tetraalkylammonium ions on the reactions of piperdine with 2,4-dinitroanisole and of morpholine with 2,4-dinitrophenyl phenyl ether in dimethyl sulphoxide were investigated. No catalysis was found and a tentative reason is given. When aniline reacts with 2,4,6-trinitrophenyl methyl ether in dimethyl sulphoxide, 82% of the reaction occurs at the methyl carbon atom, and when the substrate is 2,4,6-trinitrophenyl phenyl ether the reaction is not base-catalysed. The first two observations of the change from base-catalysed to uncatalysed aromatic nucleophilic substitution reactions brought about by a change from protic to dipolar aprotic solvent are recorded.