Hydrolysis mechanisms for acylhydrazines in aqueous sulfuric acid, determined using the excess acidity method

1984 ◽  
Vol 62 (8) ◽  
pp. 1613-1617 ◽  
Author(s):  
Robin A. Cox ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Proton Transfer ◽  
Medium Composition ◽  
Hydrolysis Rate ◽  
Bond Rupture ◽  
Rate Data ◽  
Dilute Acid ◽  
Aqueous Sulfuric Acid ◽  
Rate Determining Step

The excess acidity method has been applied to hydrolysis rate data for some acyl- and benzoylhydrazines, obtained as a function of medium composition in aqueous sulfuric acid mixtures. Two hydrolysis mechanisms are indicated, both involving a second proton transfer to monoprotonated substrate. In the first mechanism this transfer is to oxygen, which is the rate-determining step in dilute acid, followed by attack of a water molecule in an A-2 hydrolysis, which is rate determining in more concentrated acid. Bisulfate ion becomes the nucleophile at high acidity. The second mechanism, found at higher acid concentrations, involves rate-determining nitrogen protonation, probably concerted with C—N bond rupture, to give an acylium ion, for those substrates capable of forming one.

Mechanistic studies in strong acids. VIII. Hydrolysis mechanisms for some thiobenzoic acids and esters in aqueous sulfuric acid, determined using the excess acidity method

1982 ◽  
Vol 60 (24) ◽  
pp. 3061-3070 ◽  
Author(s):  
Robin A. Cox ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Proton Transfer ◽  
Strong Acid ◽  
Weak Acid ◽  
Medium Composition ◽  
Hydrolysis Rate ◽  
Water Molecules ◽  
Aqueous Sulfuric Acid ◽  
Ion Formation ◽  
Rate Determining Step

The excess acidity method has been applied to hydrolysis rate data, obtained as a function of medium composition, for four thiobenzoic acids, thioacetic acid, eight ethyl thiolbenzoates, and eight ethyl thionbenzoates in aqueous sulfuric acid. The mechanistic behaviour thus revealed has both similarities to and differences from that of a typical ester like ethyl benzoate, which gives benzoic acid by an A-2 reaction involving two water molecules in weak acid, and by A-1 acylium ion formation in strong acid. The thioacids follow this behaviour, except that the A-2 process involves three water molecules, and that the mechanistic changeover occurs in 60% rather than 80% acid. The A-2 process for the ethyl thiolbenzoates is slow; the major hydrolysis mechanism is acylium ion formation, not in an A-1 reaction but by a concerted A-SE2 process involving both proton transfer to sulfur and carbon–sulfur bond breaking. The major proton transfer agent is the undissociated sulfuric acid molecule. The thionbenzoate esters, in contrast, undergo very fast A-2 hydrolysis; so fast, in fact, that the initial protonation of sulfur is the rate-determining step in acids more dilute than about 62% w/w. It appears that proton transfer to sulfur is a comparatively slow process.

THE WALLACH REARRANGEMENT: PART II. KINETICS AND MECHANISM OF THE ACID-CATALYZED REARRANGEMENT OF AZOXYBENZENE

1965 ◽  
Vol 43 (4) ◽  
pp. 862-875 ◽  
Author(s):  
E. Buncel ◽  
B. T. Lawton
Keyword(s):  
Sulfuric Acid ◽  
Proton Transfer ◽  
Acid Concentration ◽  
Entire Range ◽  
Kinetics And Mechanism ◽  
The Other ◽  
Rate Data ◽  
Spectrophotometric Methods ◽  
Aqueous Sulfuric Acid ◽  
Acid Catalyzed

The rate of rearrangement of azoxybenzene to p-hydroxyazobenzene has been measured in 75.3–96.4% sulfuric acid at 25° and in 65.0–90.4% sulfuric acid at 75.5° by spectrophotometric methods. The pKa of azoxybenzene in aqueous sulfuric acid has also been determined. It is found that although azoxybesssnzene is almost completely protonated over the entire range of acid concentration studied, the rate increases by more than 1 000-fold. A two-proton process is therefore indicated and mechanisms are proposed involving a dication (II) as the key intermediate. The rate data do not allow differentiation between two proposed mechanisms, one involving two equilibrium protonations, and the other a single equilibrium protonation followed by rate-determining proton transfer. Past mechanisms of the Wallach rearrangement are discussed.

The decomposition of aliphatic N-nitro amines in aqueous sulfuric acid. Bisulfate as a nucleophile

1996 ◽  
Vol 74 (10) ◽  
pp. 1774-1778 ◽  
Author(s):  
Robin A. Cox
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Rate Constants ◽  
Activation Parameters ◽  
Standard State ◽  
Aqueous Sulfuric Acid ◽  
Alkyl Groups ◽  
Different Temperatures ◽  
State Rate ◽  
Acid Catalyzed

In aqueous sulfuric acid, aliphatic N-nitro amines decompose to N2O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonated aci-nitro tautomer, the nucleophile being a water molecule at acidities below 82–85% H2SO4, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the σ* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly the m≠m* slopes also correlate with σ*, although the scatter is bad. Key words: N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.

Study of the Wallach rearrangement and related processes in the 100% sulfuric acid region

1970 ◽  
Vol 48 (2) ◽  
pp. 377-382 ◽  
Author(s):  
E. Buncel ◽  
W. M. J. Strachan
Keyword(s):  
Sulfuric Acid ◽  
Proton Transfer ◽  
Kinetic Method ◽  
Rate Data ◽  
Upper Limit ◽  
Transfer Step ◽  
Rearrangement Process ◽  
Equilibrium Process ◽  
Acid Catalyzed ◽  
Acid Region

Study of the acid-catalyzed Wallach rearrangement of azoxybenzene is extended into the 100% H2SO4 region. The rate of formation of 4-hydroxyazobenzene can be followed spectrally in a straight-forward manner until close to 99% H2SO4, but in higher acidities sulfonation of this product becomes kinetically important. The advent of second equilibrium protonations of 4-hydroxyazobenzene and of 4-hydroxyazobenzene-4′-slfonic acid further complicate the azoxybenzene rearrangement as followed spectrally. Above 100% H2SO4 a second sulfonation is also observed. A method is given for dissecting the rate data for the primary rearrangement process from the first of the sulfonation reactions.The rate of the azoxybenzene rearrangement is observed to increase continuously up to 99.99% H2SO4 (the upper limit of the present kinetic method). This suggests that the second proton transfer step to azoxybenzene is rate-determining and not an equilibrium process. These results permit a clarification of a previously proposed mechanism (1).

The hydrolyses of benzamides, methylbenzimidatium ions, and lactams in aqueous sulfuric acid. The excess acidity method in the determination of reaction mechanisms

1981 ◽  
Vol 59 (19) ◽  
pp. 2853-2863 ◽  
Author(s):  
Robin A. Cox ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Reaction Pathway ◽  
Sulfuric Acid Concentration ◽  
Strong Acid ◽  
Hydrolysis Rate ◽  
Reaction Conditions ◽  
Acid Media ◽  
Hydrolysis Of

The excess acidity method has been applied to hydrolysis rate data for a number of benzamides, methylbenzimidatium ions, and lactams, obtained as a function of sulfuric acid concentration and temperature. All of the substrates studied except β-propiolactam (8) and methyl-2,6-dimethylbenzimidatium ion (7) were found to follow the AOT2 mechanism at all acidities. The excess acidity method provided considerable mechanistic detail; in dilute acid the transition state contains O-protonated (or methylated) substrate and three water molecules (large negative ΔS≠), but in more concentrated solutions a one-water-molecule mechanism takes over (smaller negative ΔS≠). In strong acid bisulfate ion acts as the nucleophile (positive ΔS≠). N-protonated intermediates are not involved for "normal" substrates, being observed in this work only for 8, which follows the AND1 pathway. Observed differences between benzamide and methylbenzimidatium ion (4) hydrolyses are due to their differing activity coefficient behaviour, the mechanism being the same for both. The hydrolysis of 7 involves a one-water-molecule SN2 displacement at the O-methyl group. Comparison with 7 shows that this displacement is not likely to occur under the reaction conditions for 4; however, for the N-methyl and N,N-dimethyl derivatives studied it is probably an important reaction pathway. A comprehensive mechanistic framework for amide hydrolyses in strong acid media is given.

The cyclization of 2-substituted imidazolines in sulfuric acid

1994 ◽  
Vol 72 (9) ◽  
pp. 1910-1914 ◽  
Author(s):  
Robin A. Cox ◽  
David B. Moore ◽  
Robert S. McDonald
Keyword(s):  
Sulfuric Acid ◽  
Water Molecule ◽  
Kinetic Analysis ◽  
Ring Closure ◽  
Base Catalyst ◽  
Aqueous Sulfuric Acid ◽  
Medium Acidity

A study of the rates of the cyclizations 1 → 3 and 2 → 4 as a function of medium acidity and temperature in aqueous sulfuric acid has been performed. The latter reaction is five times faster at all acidities. An excess acidity kinetic analysis reveals the probable involvement of a water molecule in the reaction in both cases. Mechanistic possibilities suggested by the observations are discussed; it is proposed that the water molecule acts as a base catalyst during the rate-determining ring closure.

Mechanistic variation in the reactions of substituted acetamides in aqueous sulfuric acid

1984 ◽  
Vol 62 (11) ◽  
pp. 2401-2414 ◽  
Author(s):  
Linda M. Druet ◽  
Keith Yates
Keyword(s):  
Sulfuric Acid ◽  
Water Molecules ◽  
Acid Solutions ◽  
Reaction Products ◽  
Aqueous Sulfuric Acid ◽  
Rate Determining Step ◽  
Wide Range ◽  
Acid Catalyzed ◽  
Catalyzed Reactions ◽  
Sulfuric Acid Solutions

The acid-catalyzed reactions of acetamide 1, N-tert-butylacetamide 2, and several p-substituted N-benzylacetamides (3 = H, 4 = CH3, 5 = OCH3, 6 = Cl, 7 = NO2) have been studied as a function of acidity and temperature over a wide range of aqueous sulfuric acid solutions (0–91%). Analysis of the reaction products and rate–acidity profiles revealed that four different mechanisms are operative over different acidity regions depending on the structure of the substrate. These are: two A-2 hydrolysis mechanisms with N-acyl fission of the substrate (with participation of one or several water molecules in the rate-determining step); A-1 hydrolysis with N-alkyl fission; and sulfonation, followed by hydrolysis. These conclusions are supported by three complementary and detailed kinetic treatments based on the hydration parameter, transition state activity coefficient, and excess acidity approaches. The acidity domains of each mechanism have been established for each substrate. The mechanistic conclusions are fully supported by the different values of ΔH‡ and ΔS‡ obtained in different regions of acidity.

Kinetic studies on the mechanism of the nitraminopyridine rearrangement

1982 ◽  
Vol 35 (10) ◽  
pp. 2035 ◽  
Author(s):  
LW Deady ◽  
OL Korytsky
Keyword(s):  
Sulfuric Acid ◽  
Rate Constants ◽  
Kinetic Studies ◽  
Current Evidence ◽  
Rate Data ◽  
Rate Determining Step

Rate data are reported for the rearrangement, in 92% sulfuric acid at 30�, of a series of 4-X-2-nitra-minopyridines (X = H, Me, Br, CI, MeO, CO2H) and of 4-methyl-2-nitramino(3-D)pyridine. Values of pKa for second protonation of the corresponding pyridin-2-amines were also measured and rate constants for nitration of the monoprotonated pyridinamines were thereby calculated. The results suggest that the rate-determining step occurs prior to formation of the appropriate 3-and 5-nitro σ complexes. The nature of this step is not clear, however, and a key role for the nitramine itself is not proven by the current evidence.

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