Acid-catalyzed rearrangement of K-region arene oxides: observation of ketone intermediates and a sterically induced change in rate-determining step

1993 ◽  
Vol 115 (5) ◽  
pp. 1723-1730 ◽  
Author(s):  
Nashaat T. Nashed ◽  
Jane M. Sayer ◽  
Donald M. Jerina
Keyword(s):  
Rate Determining Step ◽  
Acid Catalyzed ◽  
Sterically Induced

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.

Sterically induced methoxyl migration on acid-catalyzed dehydration of K-region trans-dihydrodiol monomethyl ethers

1993 ◽  
Vol 58 (23) ◽  
pp. 6344-6348 ◽  
Author(s):  
Nashaat T. Nashed ◽  
Tata Venkata S. Rao ◽  
Donald M. Jerina
Keyword(s):  
Acid Catalyzed ◽  
Sterically Induced

ISOTOPE RATE EFFECTS IN THE ENOLIZATION OF OXALACETIC ACID

1962 ◽  
Vol 40 (7) ◽  
pp. 1280-1284 ◽  
Author(s):  
George W. Kosicki
Keyword(s):  
Direct Evidence ◽  
Aqueous System ◽  
Enol Form ◽  
Rate Effect ◽  
Keto Form ◽  
Oxalacetic Acid ◽  
Rate Determining Step ◽  
Rate Effects ◽  
Acid Catalyzed ◽  
Catalyzed Reactions

Acid-catalyzed enolization of oxalacetic acid in H2O and D2O gives rise to a constant isotope rate effect [Formula: see text] of 2.4 over the pH (pD) range of 5.5 to 7.5. Base-catalyzed enolization of oxalacetic acid in H2O and D2O gives rise to a constant isotope rate effect [Formula: see text] of 4.5 over the pH (pD) range of 7.0 to 8.0. The percentage of enol form of oxalacetic acid was calculated to be 15.3% at pH 8.0, using the absorption of the enol form in ether at 255 mμ and the absorption of the keto form at pH 0.5 in an aqueous system. The spectrophotometric measurement of the isotope rate effect involved in the enolization of oxalacetic acid gives direct evidence for the rate-determining step in both the acid- and the base-catalyzed reactions without subsequent reactions.

Kinetics and Mechanism of Acid Catalyzed Decomposition of 1-Phenyl-3,3-dialkyltriazenes

1994 ◽  
Vol 59 (2) ◽  
pp. 401-411 ◽  
Author(s):  
Miroslav Ludwig ◽  
Pavla Valášková ◽  
Oldřich Pytela
Keyword(s):  
Rate Constants ◽  
Steric Effect ◽  
Aqueous Ethanol ◽  
Pivalic Acid ◽  
Decomposition Kinetics ◽  
Methyl Ethyl ◽  
Rate Determining Step ◽  
Alkyl Groups ◽  
Acid Catalyzed ◽  
Catalytic Rate

Five model 1-phenyl-3,3-dialkyltriazenes (methyl, ethyl, 2-propyl, butyl, cyclohexyl) have been synthesized and their acid-catalyzed decomposition kinetics have been investigated spectrophotometrically in aqueous ethanol (40 vol.%) with pivalic acid as the catalyst. The results show that the rate-determining step is catalyzed by the proton. The decrease in the observed rate constant at higher concentrations of pivalic acid is explained by the formation of an unreactive complex of the nondissociated acid and respective triazene. The steric effect of alkyl groups on the catalytic rate constants is discussed.

EXPLORING THE POTENTIAL ENERGY SURFACE OF THE CATALYZED ISOMERIZATION OF NITROSOMETHANE TO FORMALDOXIME

2007 ◽  
Vol 06 (01) ◽  
pp. 187-195 ◽  
Author(s):  
GUO-MING LIANG ◽  
YI REN ◽  
SAN-YAN CHU ◽  
NING-BEW WONG
Keyword(s):  
Activation Energy ◽  
Formic Acid ◽  
Energy Surface ◽  
Reaction Pathway ◽  
Hydroxyl Group ◽  
Rate Determining Step ◽  
Reaction Channels ◽  
Acid Catalyzed ◽  
Favorable Reaction ◽  
The One

The mechanism of the isomerization of nitrosomethane to formaldoxime catalyzed by neutral molecule ( H 2 O and HCOOH ) has been investigated at the level of B3LYP/6-311+G**. Calculated results indicate that the rearrangement from nitrosomethane to more stable trans-formaldoxime can proceed via two different reaction channels, but the favorable reaction pathway catalyzed by water and formic acid is different from the one in the catalyst-free reaction. It is more favorable that the tautomeric reaction involves the formation of cis-formaldoxime and a subsequent rotation about the N – O bond to form trans-formaldoxime in the catalyzed reaction. The activation energy of rate-determining step was reduced from 197.9 kJ/mol to 138.7 kJ/mol in the water-catalyzed reaction and 79.6 kJ/mol in the formic acid-catalyzed reaction, respectively, due to the catalysis of hydroxylic groups, but the catalysis of more acidic hydroxyl group in the latter system has been shown to be more efficient.

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.

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