A change in rate-determining step in the hydrolysis of cyclic ketals

1984 ◽  
Vol 62 (8) ◽  
pp. 1608-1612
Author(s):  
Robert A. McClelland ◽  
N. Esther Seaman
Keyword(s):  
Bond Cleavage ◽  
Structural Features ◽  
Product Formation ◽  
Low Ph ◽  
Alkoxy Group ◽  
Hydroxide Ion ◽  
Rate Determining Step ◽  
The Difference ◽  
Ortho Esters ◽  
Hydrolysis Of

A kinetic study is reported of the hydrolysis of 2-methoxy-2-phenyltetrahydrofuran and 2-ethoxy-2-phenyltetrahydrofuran. At pH > 6 the rate-determining step involves H+-catalyzed formation of the oxocarbocation, this reaction occurring with cleavage of the exocyclic alkoxy group to produce a cyclic cation. Between pH 5 and pH 6 a change-over occurs and at pH < 5, the rate-determining step in product formation is breakdown of the cyclic hemiketal intermediate, 2-hydroxy-2-phenyltetrahydrofuran. The changeover occurs because the H+-catalyzed breakdown of this intermediate is a slower process than the H+-catalyzed oxocarbocation-forming step. Hydroxide ion catalysis makes the hemiketal decomposition faster at higher pH. Analogous cyclic ortho esters (2-alkoxy-1,3-dioxolanes) show this same change in rate-determining step between high pH and low pH, while acyclic acetals, ketals, and ortho esters generally have the oxocarbocation-forming stage rate determining at all acidities. It is concluded that the structural features inherent in the cyclic systems are responsible for the difference. In particular, the oxocarbocation-forming stage involves exocyclic bond cleavage, giving it an entropic advantage over the hemiketal or hemiorthoester breakdown which is endocyclic.

Hydrolysis of trioxaadamantane ortho esters. II. Kinetic analysis and the nature of the rate-determining step

1984 ◽  
Vol 62 (6) ◽  
pp. 1074-1080 ◽  
Author(s):  
Robert A. McClelland ◽  
Patrick W. K. Lam
Keyword(s):  
Rate Constants ◽  
Product Formation ◽  
Low Ph ◽  
Acid Solutions ◽  
Ortho Ester ◽  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Cation Hydration ◽  
Ortho Esters ◽  
Hydrolysis Of

A detailed kinetic study of the hydrolysis of a series of 3-aryl-2,4,10-trioxaadamantanes is reported. These ortho esters equilibrate with the ring-opened dialkoxycarbocation, in a very rapid process which could be studied using temperature-jump spectroscopy for aryl = 2,4-dimethylphenyl. Relaxation rate constants are of the order of 104 s−1; these could be analyzed to provide the rate constants for both the ring opening and the ring closing. Product formation from this equilibrating mixture is much slower. In acid solutions (0.01 M H+ −50% H2SO4), first-order rate constants for product formation initially increase with increasing acidity, but a maximum is reached at 20–35% H2SO4 and the rate then falls. This behavior is explained by a counterbalancing of two factors. Increasing acidity increases the amount of the dialkoxycarbocation in the initial equilibrium, but, outside the pH region, it decreases the rate of hydrolysis of this cation through a medium effect. Rate constants over a range of pH have been measured for two trioxaadamantanes and for the cation DEt+ derived by treatment of the ortho ester with triethyloxonium tetraafluoroborate. The latter models the cation formed in the ortho ester hydrolysis but it cannot ring close. Rate–pH profiles obtained in these systems are more complex than expected on the basis of rate-determining cation hydration. An interpretation is proposed with a change in rate-determining step between high pH and low pH. Cation hydration is rate determining at high pH but at low pH hemiorthoester decomposition becomes rate determining. Under these conditions the hemiorthoester equilibrates with both the dialkoxycarbocation and with the trioxaadamantane. The change in rate-determining step occurs because acid-catalyzed reversion of the hemiorthoester to dialkoxycarbocation is a faster process than acid-catalyzed hemiorthoester decomposition. This makes the latter rate-determining in acid solutions. Additional pathways available to the decomposition, however, make it the faster process at higher pH. A kinetic analysis furnishes all of the rate and equilibrium constants for the system, and provides support for the mechanistic interpretation. A comparison of these numbers with those obtained for the three stages in the hydrolysis of a simple monocyclic ortho ester underlines the novelty of the trioxaadamantane system.

Base hydrolysis of trans-Halogenoamminebis(dimethylglyoximato) (III). Complexes

1969 ◽  
Vol 22 (12) ◽  
pp. 2569 ◽  
Author(s):  
SC Chan ◽  
PY Leung
Keyword(s):  
Equilibrium Constants ◽  
Base Hydrolysis ◽  
Hydroxide Ion ◽  
First Order ◽  
Rate Determining Step ◽  
Rapid Formation ◽  
Pseudo First Order ◽  
Conjugate Bases ◽  
Hydrolysis Of ◽  
Conjugate Base

The disappearance of trans-[Co(LH)2(NH3)X] (LH = dimethylglyoximate ion, X = chloride or bromide) has been studied in aqueous solutions over a range of alkali concentrations at various temperatures. The kinetics were done with excess of hydroxide ion at a constant ionic strength so that pseudo first-order rate constants were obtained in all the runs. The results were interpreted in terms of the rapid formation of a pre- equilibrium species which then reacts in a rate-determining step to give products. The relatively large equilibrium constants support a conjugate-base pre-equilibrium, in which the proton is lost from oxygen, while the relatively low reactivities of the conjugate-bases are consistent with the absence of electropositive electromeric effects. The similarity in the reactivities of the chloro and the bromo conjugate-bases suggests the possibility of an SN2CB mechanism.

ChemInform Abstract: HYDROLYSIS OF ORTHO ESTERS: FURTHER INVESTIGATION OF THE FACTORS THAT CONTROL THE RATE-DETERMINING STEP

1984 ◽  
Vol 15 (7) ◽  
Author(s):  
Y. CHIANG ◽  
A. J. KRESGE ◽  
M. O. LAHTI ◽  
D. P. WEEKS
Keyword(s):  
Rate Determining Step ◽  
Ortho Esters ◽  
Hydrolysis Of

ChemInform Abstract: DIOXOLENIUM ION TRAPPING IN THE HYDROLYSIS OF CYCLIC ORTHO ESTERS: THE RATE-DETERMINING STEP

1979 ◽  
Vol 10 (27) ◽  
Author(s):  
Y. CHIANG ◽  
A. J. KRESGE ◽  
C. I. YOUNG
Keyword(s):  
Ion Trapping ◽  
Rate Determining Step ◽  
Ortho Esters ◽  
Hydrolysis Of

Dioxolenium ion trapping in the hydrolysis of cyclic ortho esters: the rate-determining step

1979 ◽  
Vol 44 (4) ◽  
pp. 619-622 ◽  
Author(s):  
Y. Chiang ◽  
A. J. Kresge ◽  
C. I. Young
Keyword(s):  
Ion Trapping ◽  
Rate Determining Step ◽  
Ortho Esters ◽  
Hydrolysis Of

Hydrolysis of ortho esters: further investigation of the factors that control the rate-determining step

1983 ◽  
Vol 105 (23) ◽  
pp. 6852-6855 ◽  
Author(s):  
Y. Chiang ◽  
A. J. Kresge ◽  
M. O. Lahti ◽  
D. P. Weeks
Keyword(s):  
Rate Determining Step ◽  
Ortho Esters ◽  
Hydrolysis Of

scholarly journals Hydrolysis of Anilides. XI. Kinetic Evidence for Extremely Unfavoured Product Formation from a Bulky Anilide-Hydroxide Ion Intermediate.

1972 ◽  
Vol 26 ◽  
pp. 4186-4188 ◽  
Author(s):  
Urve Meresaar ◽  
Sven O. Eriksson ◽  
Arne F. Andresen
Keyword(s):  
Product Formation ◽  
Hydroxide Ion ◽  
Hydrolysis Of

Competitive endo- and exo-cyclic C–N fission in the hydrolysis of N-aroyl β-lactams

2005 ◽  
Vol 83 (9) ◽  
pp. 1432-1439 ◽  
Author(s):  
Wing Y Tsang ◽  
Naveed Ahmed ◽  
Karl Hemming ◽  
Michael I Page
Keyword(s):  
Ring Opening ◽  
Linear Free Energy Relationships ◽  
Aryl Substituent ◽  
Fission Reaction ◽  
Hydroxide Ion ◽  
Linear Free Energy ◽  
Ring Opening Reaction ◽  
Energy Relationships ◽  
The Difference ◽  
Hydrolysis Of

The balance between endo- and exo-cyclic C–N fission in the hydrolysis of N-aroyl β-lactams shows that the difference in reactivity between strained β-lactams and their acyclic analogues is minimal. Attack of hydroxide ion occurs preferentially at the exocyclic acyl centre rather than that of the β-lactam during the hydrolysis of N-p-nitrobenzoyl β-lactam. In general, both endo- and exo-cyclic C–N bond fission occurs in the alkaline hydrolysis of N-aroyl β-lactams, the ratio of which varies with the aryl substituent. Hence, the Brønsted β-values differ for the two processes: –0.55 for the ring-opening reaction and –1.54 for the exocyclic C–N bond fission reaction. For the pH-independent and acid-catalysed hydrolysis of N-benzoyl β-lactam, less than 3% of products are derived from exocyclic C–N bond fission. Key words: β-lactams, hydrolysis, linear free energy relationships, strain.

The hydrolysis of coumarin diethyl acetal and the lactonization of coumarinic acid ethyl ester. The partitioning of tetrahedral intermediates generated from independent sources

1979 ◽  
Vol 57 (17) ◽  
pp. 2260-2267 ◽  
Author(s):  
R. A. McClelland ◽  
R. Somani ◽  
A. J. Kresge
Keyword(s):  
Ethyl Ester ◽  
Acid Ester ◽  
Acid Ethyl Ester ◽  
Major Product ◽  
Low Ph ◽  
Diethyl Acetal ◽  
Acidic Solutions ◽  
Rate Determining Step ◽  
Hydrolysis Of ◽  
Tetrahedral Intermediates

The hydrolysis of coumarin diethyl acetal to coumarin proceeds via two detectable intermediates. A short-lived transient is observed in strongly acidic solutions (pH < 2.5); this is the oxocarbonium ion intermediate of the hydrolysis. That this cation can be detected suggests unusual stability, a fact which can be explained in terms of its pyrilium ion nature. A long-lived intermediate is also observed; kinetic and spectral evidence suggest that this is coumarinic acid ethyl ester. The lactonization of this ester shows a change in rate-determining step as the pH is varied. A corresponding change in products is found in the acetal hydrolysis, the coumarinic acid ester being the major product at high pH, with coumarin the major product at low pH. Both observations can be explained in terms of different modes of partitioning of cationic and neutral tetrahedral intermediates. Analysis in quantitative terms shows that the same tetrahedral intermediate is generated in the two different cases.

scholarly journals Quantum mechanical investigations of base-catalyzed cycloaddition reaction between phenylacetylene and azidobenzene

2020 ◽  
pp. 174751982094625
Author(s):  
Mohammad Abd Al-Hakim Badawi ◽  
Sultan T Abu-Orabi
Keyword(s):  
Density Functional ◽  
Activation Barrier ◽  
Cycloaddition Reaction ◽  
Product Formation ◽  
Dipolar Cycloaddition ◽  
Kinetic Properties ◽  
Functional Theory ◽  
Rate Determining Step ◽  
Charged System ◽  
The Difference

In this study, the mechanism for both the Huisgen 1,3-dipolar cycloaddition and the base-catalyzed cycloaddition reactions between phenylacetylene and azidobenzene has been investigated with density functional theory, namely at the M06-2X/6-31G(d) computational level. Later, the reaction has been modeled with a representative simple alkyne and a simple azide to concentrate solely on how the difference bases affect the mechanism of the reaction between phenylacetylene and azidobenzene as charged components. In this study, another mechanism of this reaction with uncharged components has been proposed to compare the calculated thermodynamic and kinetic properties for charged and uncharged systems. The calculated activation barrier differences between the catalyzed and the uncatalyzed reactions are consistent with the faster and the regioselective synthesis of the triazole product in the presence of solvents. The calculated barrier of the rate-determining step in the base-catalyzed mechanism with the uncharged system is lower than that with charged systems. Finally, the reaction leading to final product formation in uncharged system is more spontaneous than that in the charged system, and the same applies to the total reaction in the presence of solvents.

Sign in / Sign up

Export Citation Format

Share Document