A-SE2 Mechanism Research Articles

Hydrolysis Mechanisms for the Organopalladium Complex [Pd(CNN)P(OMe)3]BF4 in Sulfuric Acid

The acid-catalyzed hydrolysis of the organopalladium complex [Pd(CNN)P(OMe)3]BF4 species was monitored spectrophotometrically at different sulfuric acid concentrations (3.9 and 11.0 M) in 10% v:v ethanol-water over the 25-45 degrees C temperature range and in 30% and 50% (v/v) ethanol-water at 25 degrees C. Two acidity regions (I and II) could be differentiated. In each of the two regions the kinetic data pairs yielded two different rate constants, k(1obs) and k(2obs), the former being faster. These constants were fitted by an Excess Acidity analysis to different hydrolyses mechanisms: A-1, A-2, and A-SE2. In region I ([H2SO4] < 7.0 M), the k(1obs) values remained constant k(1obs)(av) = 1.6 x 10(-3) s(-1) and the set of k(2obs) values nicely matched an A-SE2 mechanism, yielding a rate-determining constant k(0,ASE2) = 2.4 x 10(-7) M(-1) s(-1). In region II ([H2SO4] > 7.0 M), a switchover was observed from an A-1 mechanism (k(0,A1) = 1.3 x 10(-4) s(-1)) to an A-2 mechanism (k(0,A2) = 3.6 x 10(-3) M(-1) s(-1)). The temperature effect on the rate constants in 10% (v/v) ethanol-water yielded positive DeltaH and negative DeltaS values, except for the A-1 mechanism, where DeltaS adopted positive values throughout. The solvent permittivity effect, epsilonr, revealed that k(1obs)(av) and k(0,A2) dropped with a fall in epsilonr, whereas the k(0,ASE2) value remained unaffected. The set of results deduced is in line with the schemes put forward.

Ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes

α-Methylstyrene and nine ortho-substituted analogs have been synthesized and the kinetics of their acid-catalyzed hydration in aqueous solutions of sulfuric acid at 25 °C have been investigated. The kinetic acidity function HS has been constructed from the dependence of the observed rate constants kobs on the sulfuric acid concentration. The catalytic rate constants of the acid-catalyzed hydration kortho have been calculated as well. The identical shape of the kinetic acidity functions for ortho- and para-derivatives confirms what the consistent mechanism A-SE2 of the acid-catalyzed hydration has already proved for the corresponding para-derivatives. The A-SE2 mechanism involves a rate-determining proton transfer of the hydrated proton to the substrate. From the dependence of the catalytic rate constants of the ortho-derivatives on the catalytic rate constants of the para-derivatives, it is seen that the logarithm of the catalytic rate constant for hydrogen as a substituent is markedly out of the range of the other substituents and, simultaneously, that the ortho-derivatives react significantly slower than the corresponding para-derivatives. In correlation with the substitent constants σp+, a reaction constant of ρ+ = –1.45 have been found. The constant is, in absolute value, considerably smaller than that for para-derivatives (ρ+ = –3.07). In parallel, the steric effects are enforced more significantly for the monoatomic substituents (slope of the Charton’s constants 3.92) than for substituents including more atoms (slope of the Charton’s constants 2.09). A small value of the reaction constant ρ+ has been elucidated due to the lower conjugation between the reaction centre and the benzene ring as a consequence of the geometric twist of the reaction centre out of the main aromatic plane accompanied by fading mesomeric interaction between the reaction centre and the substituents attached to the benzene ring. The isopropyl group in the carbocation is twisted less out of the aromatic plane for the monoatomic substituents and, therefore, also a small difference in the bulk of substituents has considerable steric influence on the conjugation between the carbocation and the benzene ring bearing substituents. On the contrary, the isopropyl group in the carbocations with polyatomic substituents is twisted to such a degree that changes in the bulk of substituents affect the resonant stabilization negligibly. Similar conclusions were also deduced from the correlations of the substitution constants σI and σR+.

Acid-catalyzed decomposition of stable 1-(2,1-benzisothiazol-3-yl)-3-phenyltriazenes

Abstract The acid-catalyzed decomposition of unusually stable 1-(2,1-benzisothiazol-3-yl)-3-phenyltriazenes in either aqueous perchloric acid or an aqueous mixture of perchloric and acetic acid was studied under pseudo-first order reaction conditions at 25 °C. Different products were obtained according to substitution on nitrogen N-3. For a triazene carrying hydrogen, the corresponding 3-amino-2,1-benzisothiazole and benzenediazonium salts were formed whereas in the case of substitution by an alkyl group (methyl and n-butyl) the 2,1-benzisothiazole-3-diazonium salt and N-alkylaniline were obtained. The observed rate constant (kobs) of the acid-catalyzed decomposition increased, initially, nonlinearly with increasing concentration of acid. Subsequently, kobs decreased slightly and at high acid concentration, increased steeply once again. An A-SE2 mechanism in which protonation of the triazene nitrogen proceeds simultaneously with cleavage of the N–N bond is proposed. Tautomerism of 1-(2,1-benzisothiazol-3-yl)-3-phenyltriazene was investigated using 1H NMR spectroscopy.

Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines.

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.

Styrene hydration and stilbene isomerization in strong acid media. An excess acidity analysis

Rate constants obtained for the hydration of some ring-substituted styrenes, YC6H4CR=CH2 (R = H, CH3, CF3) (1-3), and for the isomerization of some cis-stilbenes substituted in both rings, YC6H4CH=CHC6H4Z (4), in aqueous H2SO4, D2SO4, and HClO4 media at 25°C and, in some cases, at other temperatures, have been subjected to an excess acidity analysis. Data obtained by different groups and in different media for the same substrates agree very well. The m‡ values obtained show that all the substrates react via the A-SE2 mechanism, rate-determining proton transfer to carbon; the transition state is late, with the proton transfer being about three-quarters complete. The reactivities of 1-3 in the aqueous standard state are obtained by extrapolation; they are 1:103:10-7, respectively. The stilbenes are relatively unreactive; log k0 values for compounds with substituents in the ring adjacent to the developing positive charge show a good correlation with σ+ with a large negative ρ+ value, but those for compounds with substituents in the other ring correlate with σ with a small negative ρ. Extrapolated log k0 values for 1 and 3 show a good correlation with σ+, but those for 2 correlate with neither σ+ nor σ, because the α-CH3 group twists the molecule and reduces the resonance interaction of ring substituents with the developing positive charge. On the other hand, this effect is not seen in 3, when the strongly deactivating α-CF3 group is present, meaning that the transition state is forced to be planar in this case. The activation parameters, solvent isotope effects, and excess acidity slope parameters m‡m* obtained in the analysis are tabulated and discussed. The parameters obtained for the parent styrenes 1 are very similar to those previously obtained for the phenylacetylenes YC6H4Ctriple bondCH, meaning that reactions with vinyl cation intermediates and reactions with benzyl cation intermediates can be quite similar. Key words: styrenes, stilbenes, hydration, excess acidity, LFER, mechanism.

Acid-catalyzed decomposition of diazodiphenylmethane in dimethylsulfoxide

Acid-catalyzed decomposition of diazodiphenylmethane (DDM) has been studied in DMSO containing varying amounts of water. The reaction was found to be first-order in DDM and first-order in acid. The Brønsted plot for a series of carboxylic acids is curved with the H+ point falling below the curve defined by the carboxylic acids. In near-anhydrous DMSO, kinetic hydrogen isotope effects (KIEs) are 1.7 and 1.6 for acetic acid and chloroacetic acid, respectively. The chloroacetic acid KIE increases with increasing water concentration, rising to 2.9 at 3.6 M water. As a function of [H2O], kHA for chloroacetic acid shows a sharply defined minimum occurring at 1 M water. This behavior and the water effect on KIE suggest that the carboxylic acid-catalyzed reactions in near-anhydrous DMSO do not use the ASE-2 mechanism attributed to the reaction of DDM in hydroxylic solvents. A mechanism, leading to an azoalkane, is suggested. Either directly or indirectly, this may lead to diphenylcarbene, which would account for the observed products: benzophenone, benzhydrol, and benzhydryl esters. For the H+ catalyzed decomposition of DDM in near-anhydrous DMSO, benzophenone is not found among the products, and we suggest that this reaction does not undergo a change in mechanism and continues to use the ASE-2 mechanism.Key words: decomposition mechanism, kinetics, solvent effect, reaction with carboxylic acids.

Acid-catalyzed decomposition of diazodiphenylmethane in dimethylsulfoxide

Acid-catalyzed decomposition of diazodiphenylmethane (DDM) has been studied in DMSO containing varying amounts of water. The reaction was found to be first-order in DDM and first-order in acid. The Bronsted plot for a series of carboxylic acids is curved with the H + point falling below the curve defined by the carboxylic acids. In near-anhydrous DMSO, kinetic hydrogen isotope effects (KIEs) are 1.7 and 1.6 for acetic acid and chloroacetic acid, respectively. The chloroacetic acid KIE increases with increasing water concentration, rising to 2.9 at 3.6 M water. As a function of (H2O), kHA for chloroacetic acid shows a sharply defined minimum occurring at 1 M water. This behavior and the water effect on KIE suggest that the carboxylic acid-catalyzed reactions in near-anhydrous DMSO do not use the ASE-2 mechanism attributed to the reaction of DDM in hydroxylic solvents. A mechanism, leading to an azoalkane, is suggested. Either directly or indirectly, this may lead to diphenylcarbene, which would account for the observed products: benzophenone, benzhydrol, and benzhydryl esters. For the H + catalyzed decomposition of DDM in near- anhydrous DMSO, benzophenone is not found among the products, and we suggest that this reaction does not undergo a change in mechanism and continues to use the ASE-2 mechanism.

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.

Identification of an ASE2 mechanism in the hydrolysis of cyclic thioacetals

2-Phenyl-2-methyl-1,3-dithiane and its p-methoxy derivative undergo hydrolysis in concentrated aqueous perchloric acid via the ASE2 mechanism rather than via the A1 mechanism.

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‡(Kresge's 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.

Intramolecular acid-catalyzed reactions of o-carbomethoxy-?-diazoacetophenones

1. A variety of new o-carbomethoxy-ω-diazoacetophenones and isochromane-1,4-diones were synthesized. 2. The decomposition of o-carbomethoxy-ω-diazoacetophenones in methanol-H2SO4, in constrast to reactions of their o-unsubstituted analogs, takes place via the A-SE2 mechanism. The intramolecular nucleophilic assistance of the o-CO2CH3 group increases the decomposition rate of ω-diazoacetophenones by 13–25 times. 3. A correlation was observed between the acid-catalyzed reaction rates of o-carbomethoxy-ω-diazoacetophenones and Hammet's σ constants for benzene ring substituents.

Kinetics and mechanism of acid catalyzed decomposition of substituted 1,3-diphenyltriazenes

The kinetics of acid catalyzed decomposition of 3-substituted and 1,3-disubstituted 1,3-diphenyl-3-methyltriazenes have been studied in 40% aqueous ethanolic buffers at 25 °C. The reaction constant for substitution of the phenyl ring at the 3 position of the triazene chain has been determined and the reaction constants of the protonation and bond splitting between the nitrogen atoms 2 and 3 have been estimated. By the covariance analysis it has been found that the standard constants of the Hammett relation are better than the constants determined in the media of organic solvents. Also estimated were the values of the slope β1g of the linear relation between the rate constants and the equilibrium between the diazonium salt and diazotate. The A-SE2 mechanism is suggested as probable, the bond between nitrogen atoms of triazene chain being more than half split in the transition state and the nitrogen-proton bond formation being more advanced than the nitrogen-nitrogen bond splitting.

Acid-catalyzed hydrolyses of 2-alkoxytetrahydropyrans: evidence for the changeover from an A-1 to an ASE2 mechanism

Acid-catalyzed hydrolyses of 2-alkoxytetrahydropyrans: evidence for the changeover from an A-1 to an ASE2 mechanism

ChemInform Abstract: MECHANISMS AND CATALYSIS IN VINYL ESTER HYDROLYSIS

Abstract A critical survey of literature concerning the catalysis, mechanisms, and kinetics in vinyl ester hydrolysis is presented together with new results. The relatively fast alkaline and acid-catalysed hydrolyses usually take place with acyl-oxygen fission. General base and nucleophilic catalyses are known. Appreciable neutral ester hydrolysis by general base catalysis of water occurs generally. The unsymmetrically acid-catalysed partition of the tetrahedral intermediate formed in the neutral hydrolysis has been found to lead to acid catalysis if the ester has electronegative substituents. Vinyl esters differ from other esters by the possible electrophilic addition to the double bond. Thus mercury(II) and thallium(III) ions catalyse the reaction, and acid catalysis takes place by ASE2 mechanism at high acidities or when the formed carbenium ion is structurally stabilized.

Mechanisms and catalysis in vinyl ester hydrolysis

Abstract

Complete kinetic analysis of the A-SE2 mechanism of acid-catalyzed addition of methanol to an olefinic double bond

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTComplete kinetic analysis of the A-SE2 mechanism of acid-catalyzed addition of methanol to an olefinic double bondClaude F. Bernasconi and William J. Boyle Jr.Cite this: J. Am. Chem. Soc. 1974, 96, 19, 6070–6077Publication Date (Print):September 1, 1974Publication History Published online1 May 2002Published inissue 1 September 1974https://doi.org/10.1021/ja00826a019RIGHTS & PERMISSIONSArticle Views135Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (924 KB) Get e-Alerts Get e-Alerts

The kinetics and mechanism of the hydrolysis of 2,3-(phenylmethylenedioxy)benzene (Oo′-benzylidenecatechol) and benzaldehyde diphenyl acetal

General-acid catalysis has been detected in the hydrolysis of OO′-benzylidenecatechol with a Bronsted α-value of 0·47 for catalysis by carboxylic acids. There is also a relatively fast spontaneous hydrolysis with k(H2O)/(D2O)= 1·61 and ΔS‡=–21·1 cal deg–1 mol–1 at 75°. The entropy of activation for the hydronium-ion catalysed reaction is –8·78 cal deg–1 mol–1 and the isotope effect k(H3O+)/k(D3O+)= 0·92 at 75°. These reactions are thought to proceed via A-SE2 mechanisms.The hydrolysis of benzaldehyde diphenyl acetal is much faster than that of benzylidenecatechol and in 20%(w/w) dioxan–water mixtures is also general-acid catalysed with α approximately 1 and k(H3O+)/k(D3O+)= 0·67 at 15°. An A-SE2 mechanism was also favoured for this reaction.

Struktur und reaktivität aliphatischer diazoverbindungen—XVIII

The rate of the acid-catalysed hydrolysis of p-nitrophenyl-diazomethane has been measured in dioxan-water/HClO4. As indicated by the solvent deuterium isotope effect kH/kD = 3·76 and the general acid catalysis in acetic acid-sodium acetate buffer solutions, the decomposition proceeds by rate-controlling protonation (A-SE2 mechanism). These results are not in accordance with the recent findings of the A-2 hydrolyses of primary and of the A-SE2 hydrolyses of secondary diazo alkanes and α-diazocarbonyl compounds. The stability toward acids is increased by substitution at the diazo C atom of IIIa. In the reaction series IIIa–g the rate of hydrolysis at 20° decreases from k2 = 82.10−1 1.mol−1.s−1 for IIIa to k2 = 11·3.10−5 1.mol−1.s−1 in the case of IIIg. A variation of the mechanism of hydrolysis was not observed. The plot of the logarithms of the rate constants k2 for the acid-catalysed hydrolysis of IIIb–g vs σ*, the Taft substituent constant, has been found to be linear with a slope ϱ* = − 1·869. With decreasing reactivity also the solvent isotope effect kH/kD decreases from a value of 3·76 in the case of IIIa to a range between 1·55 and 1·95 for IIId–g. An approximately linear relationship is indicated between the magnitude of kH/kD and the reactivity lg k2 (Fig 1). This correlation shows the theoretically expected substituent effects on transition-state symmetry for rate-determining proton-transfer reactions.