Mechanisms of ester hydrolysis in aqueous sulfuric acids

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A detailed kinetic study of the hydrolysis of a series of 3-aryl-2,4, LO-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 lo4 s-'; 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% HzS04). first-order rate constants for product formation initially increase with increasing acidity, but a maximum is reached at 2 0~3 5 % H2S04 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 ratedetermining 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. la spectroscopie de saut de T lorsque I'unitC aryle = dimethyl-2,4 phtnyle. Les constantes de vitesse de relaxation sont de I'ordre de lo4 S-I; on peut les analyser pour obtenir les constantes de vitesse d'ouverture et de fermeture de cycle. La formation des produits a partir du mtlange ii I'Cquilibre est beaucoup plus lente. Dans des solutions acides (0,Ol M H+ -50% H2S04), la constante de vitesse d'ordre un de la formation du produit augmente au depart avec l'aciditt mais elle atteint un maximum ti 20-35% de H,SO,; ensuite, la vitesse diminue. On explique ce comportement par I'effet oppost de deux facteurs. L'augmentation de l'aciditt augmente la quantitC de dialkoxy...

Section: Equilibrationsupporting

confidence: 86%

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

McClelland¹,

Lam²

1984

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“…More importantly in our opinion it represents a requirement for an additional water molecule, functioning as a base to remove the OH proton as the ring closes. The requirement for this base is also seen in the addition of water to protonated esters (20) and dialkoxycarbocations (19). The rates of these reactions fall off considerably more rapidly with increased acid concentration than is expected on the basis of the decrease in water activity, slopes of log k versus log a~,~ being two and greater.…”

Section: Cation Identificationmentioning

confidence: 92%

“…The rates of these reactions fall off considerably more rapidly with increased acid concentration than is expected on the basis of the decrease in water activity, slopes of log k versus log a~,~ being two and greater. This is interpreted (19,20) in terms of a mechanism in which, in addition to the water molecule which acts as a nucleophile, a second water is present to remove the proton. …”

Section: Cation Identificationmentioning

confidence: 99%

Hydrolysis of trioxaadamantane ortho esters. I. Dialkoxycarbocation – ortho ester equilibrium and acidity function

McClelland¹,

Lam²

1984

Self Cite

. Can. J. Chem. 62, 1068Chem. 62, (1984. 3-Aryl-2,4,1O-trioxaadamantane ortho esters (T) undergo a rapid equilibration with a ring-opened dioxan-2-ylium ion (DH+) prior to hydrolysis to product (a 1,3,5-cyclohexanetrioI rnonobenzoate). The cation is stable in concentrated HZS04 solutions where it has been characterized by nrnr spectroscopy. It is observed using uv spectroscopy in dilute acids, and the ratio [DH']/[T] at equilibrium has been measured as a function of acidity. Reversibility of the ring opening is established by the pattern of plots of cation absorbance versus acid concentration and by the observation that solutions containing cation on neutralization or dilution yield ortho ester, not hydrolysis product. Equilibrium constants for the reaction DH+ e T + H+ have been measured by obtaining the acidity function HT for this system. The effects of the aromatic substituent and the steepness of the acidity function plot versus acid concentration are interpreted in terms of a strong intramolecular interaction in the cation between the cationic center and the hydroxyl oxygen. IAlthough the first reaction stage is in principle reversible, hydrolyses are usually carried out in a large excess of water, and under these conditions the probability is low that the cation once formed will undergo the back reaction with the small amount of alcohol being released. This situation could be different with cyclic systems, where a ring opens in the first stage.Here the reverse is an intramolecular ring closure, and this could in principle compete with water addition, even with water as solvent. Such a reversible ring opening has been suggested with cyclic acetals as the explanation behind their diminished hydrolytic reactivity (3). Evidence for reversibility has been obtained in two instances (4, 5 ) , although in both cases the rates of water addition and the ring closure were quite similar.OOCR ring opening since the departing hydroxyl group is forced to remain in close proximity to the cationic center. Lamaty, Moreau and co-workers (6, 7) were the first to suggest that pronounced reversibility does occur. Their suggestion was based on three kinetic observations: enormously retarded (lo6-lo9) rates of hydrolysis compared with those of acyclic analogs, negative entropies of activation, and a substituent effect, k(3-H) > k(3-CH3), opposite to that expected for ratelimiting cation formation. We have reported (8) in preliminary form verification of reversibility, in the actual observation of the 1,3-dioxan-2-ylium ion DHt in equilibrium with ortho ester. Although oxocarbocations can be isolated as stable salts or solutions (9) or observed as transient intermediates (2,(10)(11)(12), this represents the first example where equilibration with an acetal or ortho ester precursor is involved. We have now carried out a detailed study of the system. In this paper we discuss the quantitative aspects of the ring-opening equilibrium.

“…This number, interestingly enough, coincides with the number of repeat units of the compound (4), which is known from its synthesis. It may thus be concluded that conductometric titration in suitable non-aqueous solvents provides an excellent method for quantitative esti-In connection with our studies of equilibria (1) have found that the available data for the activity and rates (2) in concentrated mineral acids, we of water (3,4) were used for the interpolations in less than and more than 66% (wlw) H2S04 respectively.…”

mentioning

confidence: 87%

Determination of the activity of water in highly concentrated perchloric acid solutions

Wai¹,

Yates²

1969