Concerted rate-limiting proton transfer to sulfur with nucleophilic attack at phosphorus — A new proposed mechanism for hydrolytic decomposition of the P=S pesticide, Diazinon, in moderately acidic sulfuric acid media

2007 ◽  
Vol 85 (6) ◽  
pp. 421-431 ◽  
Author(s):  
Doreen Churchill ◽  
Julian M Dust ◽  
Erwin Buncel
Keyword(s):  
Proton Transfer ◽  
Organophosphorus Pesticide ◽  
Nucleophilic Attack ◽  
Acid Media ◽  
Hydronium Ion ◽  
Neutral Aqueous Solution ◽  
Region Product ◽  
Rate Limiting ◽  
Hydrolysis Of ◽  
Acid Region

We report herein the first kinetic study of a P=S containing organophosphorus pesticide, Diazinon (1), in the moderately concentrated acid region. Product analyses (31P NMR) show that reaction occurs only at the P centre. The rate-acidity profile (kobs vs. molarity of H2SO4) appears as a curve in which the initial slight downward trace (molarity = 1 to ca. 5) is followed by sharper upward curve (molarity ca. 5 to 14). Using treatments involving the excess acidity (X) method, the A-1 and A-2 mechanistic possibilities were found to be inoperative over the full acidity range. A novel mechanism is proposed for the higher acidity (X ca. 2–6) region. This mechanism involves proton transfer to P=S from hydronium ion with concomitant proton transfer from water, which effectively delivers hydroxide to the P centre in a variant of the A-SE2 process. A putative A-2 mechanism in this region is supplanted by the proposed A-SE2 variant where the cyclic array results in proton transfer being efficiently coupled with nucleophilic attack involving water. This constitutes the first report of rate-limiting proton transfer at the P=S functionality in acid hydrolysis of this class of organophosphorus neutroxins. A 600 000-fold acceleration in the decomposition of Diazinon is associated with the change of medium from neutral aqueous solution to the most acidic medium studied (X ca. 6). Key words: phosphorothioate ester hydrolysis, acid catalysis, rate-limiting proton transfer at P=S, excess acidity analysis, new A-SE2 variant mechanism.

scholarly journals Study of Kinetics, Equilibrium and the Influence of Steric Effects on Proton-Transfer in the Reactions of 2, 2, 4- and 2, 6- Substituted Anilines with 2-Phenoxy-3,5-dinitropyridine in DMSO

2010 ◽  
Vol 7 (1) ◽  
pp. 253-259 ◽  
Author(s):  
Basim H. Asghar
Keyword(s):  
Proton Transfer ◽  
Steric Effect ◽  
Nucleophilic Attack ◽  
Steric Effects ◽  
Substituted Anilines ◽  
Phenoxy Group ◽  
Kinetics Studies ◽  
Zwitterionic Intermediate ◽  
Rate Limiting ◽  
Step Mechanism

Kinetic and equilibrium results for the reactions of 2-phenoxy-3,5-dinitropyridine (1), with a series of 2, 2, 4- and 2, 6- substituted anilines (2a-f), in the presence of DABCO in DMSO are reported. The reactions yield the 2-anilino derivatives (5), without the accumulation of intermediates. Kinetics studies are compatible with a two-step mechanism involving initial nucleophilic attack by amine at the ring carbon substituted by phenoxy group followed by either base-catalyzed or uncatalyzed conversion to the product. The base-catalyzed pathway is likely to involve rate-limiting proton-transfer from the zwitterionic intermediate to base. This work indicates a steric effect to proton transfer in reactions involving 2, 6-disubstituted anilines. The results were compared with those for reactions of 1, 3, 5-trinitrobenzene with anilines.

Vinyl ether hydrolysis. XXIV. 1-Methoxycyclooctene and the question of reversibility of the carbon protonation step

1991 ◽  
Vol 69 (1) ◽  
pp. 84-87 ◽  
Author(s):  
A. J. Kresge ◽  
Y. Yin
Keyword(s):  
Reaction Mechanism ◽  
Proton Transfer ◽  
Vinyl Ether ◽  
Isotope Effects ◽  
Hydrolysis Rate ◽  
Hydronium Ion ◽  
Mineral Acids ◽  
Dilute Aqueous Solutions ◽  
Solvent Isotope ◽  
Hydrolysis Of

An argument is presented which suggests that hydrolysis of the vinyl ether group of 1-methoxycyclooctene may occur by reversible proton transfer from a catalyzing acid to the β-carbon atom of the substrate, instead of by the conventional reaction mechanism in which this proton transfer is rate determining and not reversible. Hydrolysis of this substrate is then examined by measuring rates of reaction in dilute aqueous solutions of strong mineral acids (perchloric and hydrochloric) as well as in buffer solutions of seven carboxylic acids, biphosphate ion, and 1,1,1,3,3,3-hexafluoro-2-propanol. General acid catalysis is observed and a Brønsted relation with the exponent α = 0.73 is constructed. That, plus the isotope effects kH/kD = 2.9 and 6.0 for catalysis by hydronium ion and acetic acid respectively, as well as the lack of deuterium incorporation into the substrate when the reaction is carried out in D2O with D2PO4−/DPO42− buffer at pD = 8, show that carbon protonation of the substrate is not reversible and that the conventional reaction mechanism is operative. Key words: 1-methoxycyclooctene, vinyl ether hydrolysis, rate-determining proton transfer, Brønsted relation, solvent isotope effect.

Studies of azo and azoxy dyestuffs. Part 18. Kinetics and mechanism of the hydrolysis of 3- and 4-(p′-methoxyphenylazo)pyridines in aqueous sulfuric acid media

1986 ◽  
Vol 64 (11) ◽  
pp. 2115-2126 ◽  
Author(s):  
Erwin Buncel ◽  
Ikenna Onyido
Keyword(s):  
Sulfuric Acid ◽  
Transition States ◽  
Activation Parameters ◽  
Nucleophilic Attack ◽  
Acid Solutions ◽  
Acid Media ◽  
Charge Delocalization ◽  
Leaving Group ◽  
The Difference ◽  
Hydrolysis Of

The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine, 1, and its 3-isomer, 2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated, 1 reacted faster than 2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants, kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4 and 47% H2SO4 for 1 and 2 respectively. Activation parameters have also been determined. The pKa values for the second protonation equilibria of 1 and 2 have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for 1 both the pyridinium and protonated azonium functions are involved whereas for 2 only the azonium function participates in charge delocalization.

Vinyl ether hydrolysis XXVIII. The mechanism of reaction of methyl α-(2,6-dimethoxyphenyl)vinyl ether

1993 ◽  
Vol 71 (1) ◽  
pp. 38-41 ◽  
Author(s):  
J. Jones ◽  
A. J. Kresge
Keyword(s):  
Vinyl Ether ◽  
Nucleophilic Attack ◽  
Tracer Study ◽  
Solvent Isotope Effect ◽  
Methyl Group ◽  
Hydronium Ion ◽  
Mechanism Of Reaction ◽  
Acid Catalyzed ◽  
Solvent Isotope ◽  
Hydrolysis Of

The acid-catalyzed hydrolysis of methyl α-(2,6-dimethoxyphenyl)vinyl ether in aqueous solution at 25 °C occurs with the hydronium ion catalytic coefficient [Formula: see text] and gives the solvent isotope effect [Formula: see text] this indicates that reaction occurs by rate-determining proton transfer from the catalyst to the substrate to generate an alkoxycarbocation intermediate. An oxygen-18 tracer study shows further that, despite the steric hindrance provided by its two ortho substituents, this cation then reacts by addition of water to the cationic carbon atom to generate a hemiacetal, and not by nucleophilic attack of water on the methyl group remote from the carbocationic center:[Formula: see text]

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