Isotope effects, and solvent

The second step in acetal and ketal hydrolysis is conversion of the hemiacetal or hemiketal to the carbonyl compound. The mechanism of this step is similar to that of the first step. Usually, the second step is faster than the initial one. Hammett a p plots and solvent isotope effects both indicate that the transition state has less cationic character than... 

Hyland, L., Toma.szek, T, and Meek, T, 1991. Human immunodeficiency viru.s-1 protease 2 Use of pH rate. studies and solvent isotope effects to elucidate details of chemical mechanism. Biochemistry 30 8454-8463. 

Further substrate and solvent isotope effects were measured by Batts and Gold472 for the dedeuteration and detritiation of labelled 1,3,5-trimethoxy-benzene in aqueous protium- and deuterium-containing perchloric acid. Contrary to the observations above, they found the rate coefficients for dedeuteration to detritiation to be independent of the concentration of the catalysing acid (Table 125). Detritiation in the deuterium-containing aqueous perchloric acid media occurred 1.68 times faster than in the protium-containing media. 

RATE COEFFICIENTS AND SOLVENT ISOTOPE EFFECT FOR REACTION OF 4-ClC6H4SiMe3 WITH AQUEOUS TRIFLUOROACETIC ACID AT 25 "C471... 

Studies of relative rates, activation parameters, kinetic isotope, and solvent isotope effects, and correlation of rates with an acidity function, have elucidated the mechanisms of cyclization of diacetyl aromatics (23-26) promoted by tetramethyl-ammonium hydroxide in DMSO.32 Rate-determining base-catalysed enolate anion formation from (24-26) is followed by relatively rigid intramolecular nucleophilic attack and dehydration whereas the cyclization step is rate determining for (23). 

Traylor TG, Xu F (1990) Mechanisms of reactions of iron(III) porphyrins with hydrogen peroxide and hydroperoxides solvent and solvent isotope effects. J Am Chem Soc 112 178-186... 

The results obtained for the hydrolyses of ethyl orthoformate and ethyl orthobenzoate in purely aqueous solutions and in 20 % dioxane—water are in agreement with the A1 mechanism [28]. General catalysis cannot be detected, and solvent isotope effects (Table 14) are similar to those found in the hydrolyses of simple acetals. 

In the presence of picric acid, the reaction products were the same, but general acid catalysis could be observed. The solvent isotope effect was determined using a mixture of 82.5 % ethanol and 17.5 % water. If 38 % of the mobile (O-bonded) hydrogen of the solvent was replaced by deuterium the reaction rate was decreased by a factor of 1.45. Linear extrapolation to 100% deuteration led to an approximate result of kH/kD 2.2. Similar experiments referring to general catalysis and solvent isotope effects have been done for the ethanolysis of 9-diazo-fluorene, and similar results have been obtained [215]. 

Reference probes, usually employed as external standards, are VOCI3 (neat) in the case of NMR, and [Et4N][NbCl6] ([Et4N]Cl + freshly sublimed NbCls, saturated in ultrapure acetonitrile and sealed under N2 or Ar) in the case of Nb NMR. All data presented in this text are referenced against these standards. Since there are distinct temperature, counterion and solvent isotope effects upon shielding (cf. Section 2.1.3), these should be taken into account in order to provide reproducible data. 

In these reactions, retention can only be accommodated by formation of a phosphorane intermediate, followed by pseudorotation, before loss of the leaving group. Inversion could result either from a concerted reaction, or a stepwise process, if the phosphorane collapses before pseudorotation can occur. The stereochemical and LFER data indicate that both mechanisms occur. In a study of similar cyclic triesters (Fig. 12c and d) that combined LFER studies, stereochemical analysis, and solvent isotope effects, Rowell and Gorenstein found a trend in / g values as a function of attacking nucleophile.61 These, and the stereochemical data, are strong evidence for a mechanistic continuum from concerted to stepwise reactions of six-membered cyclic phosphate triesters.61... 

A review with 116 references was given. Isotope effects on chemical shifts, nA C(D), nA H(D), 1A N(D) and 1A C( 0), and solvent isotope effects in proteins are reviewed and references are provided to related cases. 

Differences in the rate of mutarotation of sugars in water and in deuterium oxide provide a valuable means for studying mutarotation reactions.135,224,233 237,238 The difference in rates arises from a combination of kinetic and solvent isotope-effects, and is usually expressed as a ratio,knlkD, called the isotope effect. Kinetic isotope-effects are caused by differences in the energy required for alteration of the normal and the isotopic bonds in the corresponding transition states solvent isotope-effects can exist when the isotopic compound is used both as a reactant and as a solvent. 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and solvent isotope effect are consistent with rate-limiting protonation of alkenes such as 2-methylpropene and 2,3-dimethyl-2-butene. 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data. ... 

Elcb(rev) based on obsen/ation of CH solvent exchange and solvent Isotope effect CH30-or... 

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