Activation Energies of the Hydrolysis of Esters and Amides Involving Carbonyl Oxygen Exchange 1

Kinetic evidence is presented for the establishment of an equilibrium, in dilute aqueous alkaline solutions, between alkyl dithioesters and a species I. The species I is formed cither by the extraction of a proton from, or by the addition of an hydroxide ion to, the dithioester. In the former case I would be an anion (probably an enethiolate) formed by the removal of a proton from the α-carbon atom, and in the latter case I would be a stabilized tetrahedral intermediate formed by the addition of an hydroxide ion to the thiocarbonyl carbon atom. It has not been possible, using kinetic evidence alone, to distinguish conclusively between these two possibilities. However, using the available evidence, arguments are presented in favor of I being an enethiolate anion.

Section: Introductionmentioning

confidence: 64%

Section: Tetrclhedral Inter-nzediatementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

An investigation of the biphasic kinetics observed during the hydroxide ion-catalyzed hydrolysis of alkyl dithioesters

Storer¹

1983

“…The exchange of I 8 0 between hydrolyziilg ethyl benzoate and the solvent (5,6) show that water can add reversibly to the ester, and though it is not unlikely that the adduct is an intermediate in the hydrolysis, this…”

Section: Location Of the Protonmentioning

confidence: 96%

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

Osborn¹,

Whalley²

1961

The effect of pressures up to 3 lcbar 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 lnechanism is unimolecular. 'Those for methyl and ethyl acetates are near -9 cm3 mole-I in both dilute and concentrated acid. We deduce from this that the mechanism is the same in 9.2-114 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 the11 the attaclcing water molecule adds, and if the proton is on the ether oxygen then the attacking \\rater molecule substitutes. INTRODUCTIOSI n spite of a large amount of experimental work, the hydrogen-ion-catalyzed hydrolysis of simple aliphatic esters is not well understood. There is little doubt, because, for example, of the relative rates in water and heavy water (1-3), that a pre-equilibrium proton transfer occurs, but the location of the proton in the reactive conjugate acid is not known. There is little doubt, mainly because of substituent effects (4, p. 770), that hydrolysis occurs by an attack of a water nlolecule on the conjugate acid, but it is not known whether the attack is a n addition to a water molecule to form a stable intermediate or whether it is a simple substitution. I t is, however, known that when ethyl benzoate is hydrolyzed by acid catalysis, the carbonyl oxygen of the unhydrolyzed ester exchanges with the oxygen of the solvent water (5, G), but i t is not lcnown whether the hydrolysis and exchange follow similar or unrelated mechanisms. There are differing opinions (e.g. refs. 7-11) about whether the transition states that determilie the rates have high dipole ~nonlents or not.The effect of pressure on the rates of acid-catalyzed hydrolyses can provide useful information about the mechanisms (11-lG), and a preliminary discussioll of the hydrolysis of methyl acetate based on a few results reported by Bogojawlenslty and Talnnlann (17) has already been given (11). In this paper we report measurelnents of the effect of pressures up to 3 kbar on the rates of hydrolysis of methyl and ethyl acetates in dilute hydrochloric acid.Tal'vik and Pal'm (18) have argued that in collcelltrated acids ethyl acetate becomes essentially conlpletely protonated, and that if allowance is made for the protonation, then the rate becomes nearly proportional to EIarnmett's acidity function ho rather than to the concentration of hydrogen ion. By using the Zuclter-EIammett hypothesis (19, 20) they deducecl that the slow step is An attempt to verify this mechanism in concentrated acids has been tilade by measuring the volume of activation in concentrated hydrochloric acid.

“…There is, to date, only one report in the literature (8a) which gives direct support to intermediate formation in a displacement reaction on an acyclic phosphorus ester (phos- phonate). The existence of intermediates is, however, well documented both in the displacement reactions on cyclic phosphorus esters (8b) as well as in the reactions of carboxylic esters (9).…”

Section: Introductionmentioning

confidence: 99%

The acid-catalyzed hydrolysis of phosphinates. IV. Pentacoordinate intermediate formation during hydrolysis of a phosphinothionate

Cook¹,

Metni²

1985

. Can. J. Chem. 63, 3 155 (1985).The hydrolysis of methyl dimethylphosphinothionate in D2S04-D20 solution gives as products the phosphinic acid and methyl dimethylphosphinate. The bell-shaped pH-rate profiles for both reactions as well as the values of Bunnett's w and w* (3.3 1 and -0.21 respectively), Bunnett-Olsen's c$ (0.82), and Yates-McClelland's r (6.56) support an A-2 pathway. The results are interpreted in terms of the presence of a common pentacoordinate intermediate along the reaction pathway. The pK, of the above ester as well as that of methyl dimethylphosphinodithioate are reported (-5.53 and -5.38 Introduction From our previous studies on the acid-catalyzed hydrolysis of phosphinates we reported that methyl dialkylphosphinates hydrolyze by an A-2 process and, based on relative rate data for the esters l a -d of 11 : 2.5: 1.2: 1, we suggested that water attacks the carbon of the protonated ester with subsequent C-0 bond cleavage (2). On the other hand, McClelland has shown by 0 -1 8 studies that compounds 2a-c undergo acidcatalyzed hydrolysis with approximately 90% P-0 bond cleavage (3). In the alkaline hydrolysis of phosphinates, P-0 bond cleavage is the norm (4) except in the case when phosphorus is substituted with large alkyl groups like iC,H7 where Haake and Rahil (5) have shown that there is mixed cleavage (approximately 25% C-0 cleavage). In other phosphorus esters P-0 cleavage predominates in base and C-0 cleavage in acid (6).