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.

ChemInform Abstract: An ab initio Calculation of the Acid-Catalyzed Hydrolysis of N-Nitrosoamines. A Hypothesis on the Rate-Determining Step.

ChemInform ◽  
1987 ◽  
Vol 18 (28) ◽  
Author(s):  
NGUYEN MINH THO NGUYEN MINH THO ◽  
A. F. HEGARTY
Keyword(s):  
Ab Initio ◽  
Ab Initio Calculation ◽  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Hydrolysis Of

A comparison of the mechanisms of hydrolysis of benzimidates, esters, and amides in sulfuric acid media

2005 ◽  
Vol 83 (9) ◽  
pp. 1391-1399 ◽  
Author(s):  
Robin A Cox
Keyword(s):  
Sulfuric Acid ◽  
Isotope Effects ◽  
Ester Hydrolysis ◽  
Water Molecules ◽  
Acid Media ◽  
Tetrahedral Intermediate ◽  
Amide Hydrolysis ◽  
Solvent Isotope ◽  
Reaction Media ◽  
Hydrolysis Of

The mechanisms given in textbooks for both ester and amide hydrolysis in acid media are in need of revision. To illustrate this, benzimidates were chosen as model compounds for oxygen protonated benzamides. In aqueous sulfuric acid media they hydrolyze either by a mechanism involving attack of two water molecules at the carbonyl carbon to give a neutral tetrahedral intermediate directly, as in ester hydrolysis, or by an SN2 attack of two water molecules at the alkyl group of the alkoxy oxygen to form the corresponding amide, or by both mechanisms, depending on the structure of the benzimidate. The major line of evidence leading to these conclusions is the behavior of the excess acidity plots resulting from the rate constants obtained for the hydrolyses as functions of acid concentration and temperature. The first of these mechanisms is in fact very similar to one found for the hydrolysis of benzamides, as inferred from: (1) similar excess acidity plot behaviour; and (2) the observed solvent isotope effects for amide hydrolysis, which are fully consistent with the involvement of two water molecules, but not with one or with three (or more). This mechanism starts out as essentially the same one as that found for ester hydrolysis under the same conditions. Differences arise because the neutral tetrahedral intermediate, formed directly as a result of the protonated substrate being attacked by two water molecules (not one), possesses an easily protonated nitrogen in the amide and benzimidate cases, explaining both the lack of 18O exchange observed for amide hydrolysis and the irreversibility of the reaction. Protonated tetrahedral intermediates are too unstable to exist in the reaction media; in fact, protonation of an sp3 hybridized oxygen to put a full positive charge on it is extremely difficult. (This means that individual protonated alcohol or ether species are unlikely to exist in these media either.) Thus, the reaction of the intermediate going to product or exchanged reactant is a general-acid-catalyzed process for esters. For amide hydrolysis, the situation is complicated by the fact that another, different, mechanism takes over in more strongly acidic media, according to the excess acidity plots. Some possibilities for this are given.Key words: esters, amides, benzimidates, hydrolysis, excess acidity, mechanism, acid media.

Acid-catalyzed hydrolysis of 4-diazo-isothiochroman-3-one. Comparison with the acyclic analog and the corresponding oxygen system

1997 ◽  
Vol 75 (1) ◽  
pp. 56-59 ◽  
Author(s):  
E.A. Jefferson ◽  
A.J. Kresge ◽  
S.W. Paine
Keyword(s):  
Carbon Atom ◽  
Negative Charge ◽  
Molecular Orbital Calculations ◽  
Am1 Calculations ◽  
Charge Delocalization ◽  
Rate Determining Step ◽  
Hydronium Ion ◽  
Acid Catalyzed ◽  
Group Migration ◽  
Hydrolysis Of

The acid-catalyzed hydrolysis of the cyclic diazothiolactone, 4-diazoisochroman-3-one (3) was found to occur with the hydronium-ion isotope effect, [Formula: see text] and to give the ring-contracted product, 1,3-dihydrobenzo[c]thiophene-1-carboxylic acid (4). This shows that protonation of the diazo carbon atom occurs in the rate-determining step and that the reaction also involves migration of the thio group. The hydronium-ion catalytic coefficient for this reaction, [Formula: see text], is 45 times less than that for hydrolysis of its acyclic thio ester analog, S-methyl phenyldiazothioacetate (5). Semiempirical AM1 molecular orbital calculations support the idea that this difference in reactivity is the result of increased delocalization of negative charge into the aromatic ring in the case of the cyclic substrate, which reduces the negative charge on the diazo carbon atom and makes it less susceptible to protonation. Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations, thio group migration.

Acid-catalyzed hydrolysis of 1,1-bis(methylthio)ethene. Buffer- and thiol-dependent changes in the rate-determining step

1983 ◽  
Vol 105 (10) ◽  
pp. 3220-3226 ◽  
Author(s):  
Tadashi Okuyama ◽  
Shoji Kawao ◽  
Takayuki Fueno
Keyword(s):  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Hydrolysis Of

The tetrahedral intermediate from the hydration of N-methylformanilide

1993 ◽  
Vol 71 (12) ◽  
pp. 2109-2122 ◽  
Author(s):  
J. Peter Guthrie ◽  
Jonathan Barker ◽  
Patricia A. Cullimore ◽  
Jinqiao Lu ◽  
David C. Pike
Keyword(s):  
Aqueous Solution ◽  
Free Energy ◽  
Dimethyl Acetal ◽  
Heat Of Formation ◽  
Basic Solution ◽  
Tetrahedral Intermediate ◽  
Rate Determining Step ◽  
Free Energy Of Formation ◽  
Hydrolysis Of ◽  
Energy Of Formation

Heats of hydrolysis of N-methylformanilide dimethyl acetal have been measured in basic solution. The heat of formation of N-methylformanilide was obtained by determining the equilibrium constant in aqueous solution for its formation from formic acid and N-methylaniline as a function of temperature:[Formula: see text]. These data permit the calculation of the heat of formation of N-methylformanilide dimethyl acetal, [Formula: see text]. The free energy of formation of the tetrahedral intermediate in the hydrolysis of N-methylformanilide was calculated by methods we have previously reported. Consideration of the energetics of the intermediates and the known rates of reaction leads to the conclusion that the rate-determining step for alkaline hydrolysis is cleavage of the C—N bond.

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.

The Wallach rearrangement. Part V. Acid catalyzed hydrolysis of azobenzene-4-hydrogen sulfate

1969 ◽  
Vol 47 (6) ◽  
pp. 911-917 ◽  
Author(s):  
E. Buncel ◽  
W. M. J. Strachan
Keyword(s):  
Potassium Salt ◽  
Reactive Species ◽  
Acidity Function ◽  
Hydrogen Sulfate ◽  
Sulfate Salt ◽  
Acid Media ◽  
Phenolic Oxygen ◽  
Acidic Conditions ◽  
Acid Catalyzed ◽  
Hydrolysis Of

In relation to the intermediacy of azobenzene-4-hydrogen sulfate in the Wallach rearrangement of azoxybenzene, the potassium salt has been characterized and subjected to examination under acidic conditions. A pKa value of −2.14 has been obtained, for N-protonation. The sulfate salt hydrolyzes to p-hydroxyazobenzene in acid media and the rate of the reaction has been measured over the region 22–42% H2SO4 at 25° spectrophotometrically. A correlation between rate and acidity function indicates that a two-proton process is involved; a reactive species is proposed having a nitrogen and the phenolic oxygen protonated. The relevance of these findings to Wallach rearrangement studies is discussed.

Benzamide hydrolysis in strong acids — The last word

2008 ◽  
Vol 86 (4) ◽  
pp. 290-297 ◽  
Author(s):  
Robin A Cox
Keyword(s):  
Rate Constants ◽  
Strong Acid ◽  
Acid Media ◽  
Acid Product ◽  
Reaction Rate Constants ◽  
Rate Determining Step ◽  
Amide Hydrolysis ◽  
The One ◽  
Strong Acids ◽  
Aqueous Hclo

Recently it has become apparent that the mechanism of amide hydrolysis in relatively dilute strong acid media is the same as the one observed for ester and benzimidate hydrolysis, two water molecules reacting with the O-protonated amide in the rate-determining step. This is not the whole story, however, at least for benzamide, N-methylbenzamide, and N,N-dimethylbenzamide, since the observed rate constants for these substrates deviate upwards from the observed excess acidity correlation lines at acidities higher than about 60% H2SO4, meaning that another, faster, reaction with a different mechanism is taking over at higher acidities. It has never been clear what this latter mechanism was until the work reported in this paper. An exhaustive excess acidity analysis of all the available measured reaction rate constants for the three substrates in three different acidic media, aqueous H2SO4, aqueous HClO4, and aqueous HCl, shows that this second mechanism involves a second rate-determining proton transfer to the O-protonated benzamide, followed by (or possibly concerted with) irreversible loss of +NH4 to give an acylium ion. Subsequent reaction of this with water (or bisulfate, etc.) eventually gives the observed carboxylic acid product. This latter reaction mechanism has never been previously considered for amide hydrolysis, but it may not be uncommon; at least one other reaction with a similar mechanism is known, and another possible case is suggested.Key words: amides, benzamides, hydrolysis, excess acidity, mechanism, acid media.

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