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 4-diazo-3-isochromanone: the effect of coplanarity on the carbon protonation of α-phenyl-α-carbonyldiazo compounds

1996 ◽  
Vol 74 (7) ◽  
pp. 1369-1372 ◽  
Author(s):  
E.A. Jefferson ◽  
A.J. Kresge ◽  
S.W. Paine
Keyword(s):  
Carbon Atom ◽  
Negative Charge ◽  
Diazo Compound ◽  
Cyclic Compound ◽  
Phenyl Group ◽  
Molecular Orbital Calculations ◽  
Am1 Calculations ◽  
Charge Delocalization ◽  
Acyclic Analog ◽  
Hydrolysis Of

Hydrolysis of the cyclic α-phenyl-α-carbonyl-diazo compound, 4-diazo-3-isochromanone, in dilute aqueous perchloric acid solutions was found to give the hydronium ion isotope effect [Formula: see text] which shows that this reaction occurs by rate-determining hydronation of the substrate on the carbon atom α to its diazo group. Comparison of the rate constant obtained, [Formula: see text] with that for the corresponding acyclic analog, methyl phenyldiazoacetate, indicates that the cyclic compound is 57 times less reactive. Semi-empirical AM1 molecular orbital calculations suggest that this difference in reactivity is caused by enforced near-coplanarity of the diazo and phenyl groups in the cyclic substrate, as opposed to a staggered arrangement of these groups in the acyclic analog; this coplanarity then enhances delocalization of negative charge from the diazo α-carbon atom into the phenyl group, which reduces the negative charge density on the α-carbon atom and slows the rate of reaction. Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations.

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

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.

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

Vinyl ether hydrolysis XXVIII. The mechanism of reaction of methyl α-(2,6-dimethoxyphenyl)vinyl ether

1993 ◽  
Vol 71 (1) ◽  
pp. 38-41 ◽  
Author(s):  
J. Jones ◽  
A. J. Kresge
Keyword(s):  
Vinyl Ether ◽  
Nucleophilic Attack ◽  
Tracer Study ◽  
Solvent Isotope Effect ◽  
Methyl Group ◽  
Hydronium Ion ◽  
Mechanism Of Reaction ◽  
Acid Catalyzed ◽  
Solvent Isotope ◽  
Hydrolysis Of

The acid-catalyzed hydrolysis of methyl α-(2,6-dimethoxyphenyl)vinyl ether in aqueous solution at 25 °C occurs with the hydronium ion catalytic coefficient [Formula: see text] and gives the solvent isotope effect [Formula: see text] this indicates that reaction occurs by rate-determining proton transfer from the catalyst to the substrate to generate an alkoxycarbocation intermediate. An oxygen-18 tracer study shows further that, despite the steric hindrance provided by its two ortho substituents, this cation then reacts by addition of water to the cationic carbon atom to generate a hemiacetal, and not by nucleophilic attack of water on the methyl group remote from the carbocationic center:[Formula: see text]

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.

Acid-catalyzed hydrolysis of tetramethoxyethene in aqueous solution. Initial state stabilization by the methoxy groups

1990 ◽  
Vol 68 (10) ◽  
pp. 1786-1790 ◽  
Author(s):  
A. J. Kresge ◽  
M. Leibovitch ◽  
K. R. Kopecky
Keyword(s):  
Aqueous Solution ◽  
Dimethyl Acetal ◽  
Initial State ◽  
Olefinic Carbon ◽  
Hydronium Ion ◽  
Methoxy Groups ◽  
Rate Retardation ◽  
Acid Catalyzed ◽  
Solvent Isotope ◽  
Hydrolysis Of

The acid-catalyzed hydrolysis of tetramethoxyethene to methyl dimethoxyacetate in aqueous solution at 25 °C was found to occur with the hydronium-ion catalytic coefficient [Formula: see text], to give the solvent isotope effect [Formula: see text], and to provide a Brønsted relation based upon six carboxylic acids with the exponent α = 0.42. These data indicate that the reaction proceeds via rate-determining proton transfer from the catalyzing acid to an olefinic carbon atom of the substrate. They also show tetramethoxyethene to be 1.0 × 106 times less reactive than 1,1-dimethoxyethene (ketene dimethyl acetal), a rate retardation 600 times greater than that expected from initial state stabilization by the two additional methoxy groups in tetramethoxyethene; possible causes of this disparity are discussed. Keywords: tetramethoxyethene, carbon–carbon double bond reactivity, ketene acetal, vinyl ether hydrolysis.

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