PRESSURE EFFECT AND MECHANISM IN ACID CATALYSIS: VII. HYDROLYSIS OF METHYL, ETHYL, AND t-BUTYL ACETATES

1961 ◽  
Vol 39 (5) ◽  
pp. 1094-1100 ◽  
Author(s):  
A. R. Osborn ◽  
E. Whalley
Keyword(s):  
Hydrochloric Acid ◽  
Water Molecule ◽  
Butyl Acetate ◽  
Carbonyl Oxygen ◽  
Methyl Ethyl ◽  
Dilute Acid ◽  
Aqueous Acid ◽  
Ether Oxygen ◽  
Acid Catalyzed ◽  
Hydrolysis Of

The effect of pressures up to 3 kbar on the rate of the acid-catalyzed hydrolysis of methyl, ethyl, and t-butyl acetates in dilute aqueous acid and of ethyl acetate in concentrated hydrochloric acid has been measured. The volume of activation for t-butyl acetate is zero within experimental error, showing that the mechanism is unimolecular. Those for methyl and ethyl acetates are near –9 cm3mole−1 in both dilute and concentrated acid. We deduce from this that the mechanism is the same in 9.2-M hydrochloric acid as in dilute acid, that the transition state is not highly polar, and that if the proton in the reactive protonated ester is on the carbonyl oxygen then the attacking water molecule adds, and if the proton is on the ether oxygen then the attacking water molecule substitutes.

PRESSURE EFFECT AND MECHANISM IN ACID CATALYSIS: VIII. HYDROLYSIS OF ACETAMIDE AND BENZAMIDE

1961 ◽  
Vol 39 (5) ◽  
pp. 1101-1108 ◽  
Author(s):  
A. R. Osborn ◽  
T. C-W. Mak ◽  
E. Whalley
Keyword(s):  
Water Molecule ◽  
Transition State ◽  
Perchloric Acid ◽  
Acid Catalysis ◽  
Pressure Effect ◽  
Protonated Form ◽  
Dilute Acid ◽  
Additional Information ◽  
Acid Catalyzed ◽  
Hydrolysis Of

The effect of pressures up to 3 kbar on the rate of the acid-catalyzed hydrolysis of acetamide and benzamide in both dilute and concentrated perchloric acid has been measured. The volumes of activation in dilute acid are consistent with a transition state that is not highly polar. It follows from this that if the attacking water molecule adds to the amidium ion then the reactive amidium ion is the O-protonated form, and if the attacking water molecule substitutes then the reactive amidium ion is the N-protonated form.The volume of activation for acetamide in concentrated acid provides no additional information about the mechanism. That for benzamide in concentrated acid is tentatively interpreted as favoring the O-protonated benzamidium ion as the reactive ion.

scholarly journals The mechanism of the hydrolysis of acylimidazoles in aqueous mineral acids. The excess acidity method for reactions that are not acid catalyzed

1997 ◽  
Vol 75 (8) ◽  
pp. 1093-1098 ◽  
Author(s):  
Robin A. Cox
Keyword(s):  
Imidazole Ring ◽  
Carbonyl Oxygen ◽  
Pseudo First Order Reaction ◽  
Acid Media ◽  
Tetrahedral Intermediate ◽  
Reaction Rate Constants ◽  
Rate Determining Step ◽  
Reversible Addition ◽  
Acid Catalyzed ◽  
Hydrolysis Of

The mechanism of the hydrolysis of acetylimidazole in aqueous perchloric, sulfuric, and hydrochloric acid mixtures has been determined. Benzoylimidazole was also studied in the latter two acids. The method of analyzing the available data, pseudo-first-order reaction rate constants as a function of acid concentration and, in one case, temperature, is the excess acidity method, here applied to the same reaction in the three different acid media, allowing their comparison. The reaction is not acid catalyzed; the rates decrease with increasing acidity. The substrate reacts in the form that is monoprotonated on the imidazole ring; it is 100% protonated at acidities much lower than those used here. Acetylimidazole is shown to become diprotonated at high acidity [Formula: see text], protonating on the carbonyl oxygen, but the diprotonated form is not reactive. The hydrolysis involves the reversible addition of one water molecule to the substrate to give a tetrahedral intermediate; at low acidities the decomposition of this hydrate is the rate-determining step, but as the acidity increases and the water activity decreases its formation becomes rate limiting. Hydroxide catalysis was also observed in dilute perchloric acid, but this is swamped by nucleophilic catalysis by the acid anion in HCl and H2SO4. Keywords: acylimidazoles, excess acidity, hydrolysis, protonation, tetrahedral intermediate.

THE ACID HYDROLYSIS OF GLYCOSIDES: II. EFFECT OF SUBSTITUENTS AT C-5

1965 ◽  
Vol 43 (8) ◽  
pp. 2296-2305 ◽  
Author(s):  
T. E. Timell ◽  
W. Enterman ◽  
F. Spencer ◽  
E. J. Soltes
Keyword(s):  
Ester Group ◽  
Rate Coefficients ◽  
Dilute Acid ◽  
First Order ◽  
Conformational Effect ◽  
Acid Catalyzed ◽  
The Difference ◽  
The Stability ◽  
First Time ◽  
Hydrolysis Of

First-order rate coefficients at three temperatures, and energies and entropies of activation, have been determined for the acid-catalyzed hydrolysis of methyl glucopyranosides containing various substituents at C-5 and for glycopyranosiduronic acids with different aglycones. Substitution at C-5 increased the stability towards acids of methyl α- and β-D-glucopyranosides, but there was no correlation between either the polarity or the size of the substituent and the rates of hydrolysis. The operation of either an inductive or a conformational effect alone was accordingly deemed unlikely.Methyl α- and β-D-glucopyranosiduronic acids and methyl α-D-galactopyranosiduronic acid were only slightly more stable towards acids than the glycoside analogs, while benzyl β-D-glucopyranosiduronic acid was three times more stable. The presence of a methyl ester group at the carboxyl function increased the stability of the glycuronide bond. Isopropyl, n-butyl, isobutyl, and neopentyl β-D-glucopyranosiduronic acids were hydrolyzed approximately twice and cyclohexy β-D-glucopyranosiduronic acid five times as fast as the corresponding glucosides. This appears to be the first time that glycuronides have been found to be hydrolyzed by dilute acid at a higher rate than their glycoside analogs.The energies and, especially, the entropies of activation were, throughout, lower for the glycuronides than for the glycosides. The difference in entropy suggests that the two classes of compounds are hydrolyzed by different mechanisms.

The acid-catalyzed hydrolysis of methyl 2-chloro-2-deoxy-β-D-glucopyranoside

1967 ◽  
Vol 45 (5) ◽  
pp. 515-519 ◽  
Author(s):  
E. Buncel ◽  
P. R. Bradley
Keyword(s):  
Hydrochloric Acid ◽  
Acid Solutions ◽  
Hydrochloric Acid Solutions ◽  
Mechanism Of Hydrolysis ◽  
Entropy Of Activation ◽  
Acid Catalyzed ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the hydrolysis of methyl 2-chloro-2-deoxy-β-D-glucopyranoside have been determined in hydrochloric acid solutions over a range of acid concentrations and temperatures. Chloro substitution reduces the rate by a factor of 35 compared with the hydroxy analogue. Application of the Hammett criterion indicates a unimolecular (A-1) mechanism of hydrolysis, as does application of the Bunnett criterion. The entropy of activation, however, is considerably smaller than that observed for the hydrolysis of methyl β-d-glucopyranoside. This is interpreted as being indicative of partial A-2 character.

Revised mechanism for the hydrolysis of ethers in aqueous acid

2012 ◽  
Vol 90 (10) ◽  
pp. 811-818 ◽  
Author(s):  
Robin A. Cox
Keyword(s):  
Positive Charge ◽  
Bond Cleavage ◽  
Aqueous Media ◽  
Nucleophilic Attack ◽  
Subsequent Paper ◽  
Reaction Intermediates ◽  
Aqueous Acid ◽  
Acid Catalyzed ◽  
Protonated Species ◽  
Hydrolysis Of

It has been shown recently that many supposed reaction intermediates in aqueous media do not have lifetimes long enough for them to serve this purpose. Among these are oxygen-protonated species where the positive charge is not delocalized, primary and secondary carbocations, and the commonly written species H3O+ and HO–. This means that the mechanisms for many of the organic reactions that take place in aqueous media are in need of revision. This paper concerns the acid hydrolysis of simple ethers, many of which cannot form carbocations stable enough to exist in water. Rather than an A1 process in which an oxygen-protonated species dissociates into an alcohol and a carbocation, which is then quenched by water, or an A2 process in which a water molecule or another nucleophilic species assists in this, the mechanism for most ethers is a general-acid-catalyzed process in which proton transfer to oxygen is concerted with C–O bond cleavage in cases where a stable carbocation can exist, or additionally concerted with nucleophilic attack for those cases in which stable carbocation formation is not possible. All of the cases for which rate constant data could be found in the literature are analyzed and discussed in this paper, with the exception of the hydrolyses of several azoethers, where additional hydrolysis mechanisms are possible. These will be discussed in a subsequent paper.

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