Dehydration, isotope effects

The only kinetic isotope effects so far reported for these reactions are those given by Pocker (1960), without experimental detail. He reports closely similar values for the rates of solvent-catalysed hydration of the species CHg. CHO, CD3. CHO, CH3. CDO and CD3. CDO in water at 0° C the replacement of CH3 by OD3 increases the velocity by about 7%. The same effect is reported for solutions in deuterium oxide at 0° C, presumably super-cooled. A comparison was also made of rates of hydration in HjO and DgO at 0°C, giving the following values for k(H.z0)lk(T>20) in presence of different catalysts H+/D+, 1 -3 AcOH/AcOD, 2 5 AcO , 2-3 H2O/D2O, 3-6. Almost exactly the same ratios were obtained by measuring rates of dehydration at 25° C in dioxan containing 10% of H2O or D2O and various catalysts. The presence of a considerable solvent isotope effect is consistent with the mechanism given in Section IV,B, and it would not be expected that substitution of deuterium on carbon would have an appreciable effect on the rate. 

Fig. 3. Correlation of the slopes p for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning (41), and deuterium kinetic isotope effects (68). [Reprinted with permission from Berdnek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]...

Yapp Cl (1983) Stable hydrogen isotopes in iron oxides - isotope effects associated with the dehydration of a natural goethite. Geochim Cosmochim Acta 47 1277-1287... 

Dehydration of the hydrate gives uracil depleted in tritium, as is to be expected, but the small amount of the depletion indicates a large isotope effect in the dehydration. Further indication of large isotope effects is the observation80 that the tritium content of uracil irradiated in tri-tiated water contains only 27% of the expected amount of tritium. Dehydration of tritiated hydrate in tritiated water also gives uracil depleted in tritium but less so than the product obtained in ordinary... 

For isoenzymes I and II, the CO2 hydration rates are independent of buffer at high buffer concentrations, indicating thereby that a reaction step other than the buffer-dependent step becomes rate limiting. Studies of both hydration and dehydration reactions at high concentrations of buffers in H20 and DoO indicated that the kinetic parameter, kCSLt, for isoenzyme II has large isotope effect (k jkV) 3-4) (45b). This is consistent with involvement of H+ transfer in the rate-limiting step. The H+ transfer half-reaction is composed of at least two steps,... 

Kinetic isotope effects of deuterium ( h/ d) n the dehydration of 2-propanol on various catalysts at 300°C [123]... 

Pines and Manassen [7] suggested that tertiary alcohols are dehydrated by the El mechanism involving the formation of more or less free car-bonium ions, secondary alcohols by a mechanism lying somewhere between El and E2 (i.e. synchronous with a ionic contribution) and primary alcohols by a concerted E2 mechanism. However, the large kinetic isotope effect for the dehydration of fully deuterated tert-butanol on alumina [122] indicates that, even in this case, some synchrony must exist. Alumina strongly prefers the concerted process and with other catalysts the situation may differ. 

Fig. 5. Correlation of the Taft reaction parameter for the dehydration of secondary alcohols (see Table 4) on four different oxide catalysts with the heat of adsorption, A//ads> °f water and diethylether, with the sensitivity of the rate to pyridine poisoning 7> [55] and with the value of the deuterium kinetic isotope effect [123] for the same catalysts.

Base-catalysed cyclization of proximate diacetyl aromatics [e.g. o-diaccty I benzene (36)] gives the corresponding enone (37). Relative rates, activation parameters, and isotope effects are reported for (36), and also for 1,8-diacetylnaphthalene, 4,5-diacetylphenanthrene, and 2,2/-diacetylbiphenyl, in aqueous DMSO.61 Reaction proceeds via enolate formation (rate determining for the latter three substrates), followed by intramolecular nucleophilic attack [rate determining for (36)], and finally dehydration. 

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). 

The extent of retention of tritium is 79 + 2 % in the former case and 21 2 % in the latter 155) complete retention (or complete loss) of tritium is, of course, not to be expected, since the isotope effect for extrusion of deuterium rather than protium in the condensation shown in Fig. 68 is not infinite. The percent tritium retention in the process of dehydration and water exchange (fig. 58) is now generally called the F-value 1491 thus enantiomerically pure CHDTC02H gives an F-value of either 79 (R configuration) or 21 (S configuration) values below 79 or above 21, i.e. closer to 50, indicate that the acid is not enantiomerically pure. The F-value determination... 

Figure 9 Oxygen isotopic compositions of metamorphosed CM- and Cl-like materials, identified by their reflectance spectra, which show dehydration of phyllosilicates (Akai, 1990). The upper group may have had Cl-like precursors, and the lower group may have had CM-like precursors, but the isotopic effects of metamorphism are uncertain (source Clayton and Mayeda, 1999).

The dehydration reaction shows a normal kinetic deuterium isotope effect indicating loss C2 proton as the rate-determining step. This fits with fast protonation at oxygen followed c formation of the same cation with the rest of the mechanism the same. The dehydration is I than the isomerization so loss of water is faster than protonation of the alkene. We have... 

The racemizalion doesn t include that last step, rate-determining in the dehydration. Now w. an inverse solvent deuterium isotope effect indicating specific acid catalysis. The loss of wa -. form the same cation is now Ure slow step. The large Hammett p value also suggests the fonr.i the racem/zat/on mechanism... 

Mechanistic Studies of Alcohol Dehydration on Zeolites. - Gentry and Rudham and Jacobs et al have proposed mechanisms for the dehydration of propan-2-ol and butan-2-ol on X-zeolites. Both groups of workers are in basic agreement about the mechanism, which involves the formation of oxonium and carbonium ions. The formation of olefins from the above alcohols appeared to occur via an El -like mechanism and this was supported very strongly by the behaviour of butan-2-ol, which gave a primary isotope effect, but an absence of one for C 3- H, rules out the E2 mechanism. 

The use of isotopes has found surprisingly few applications in the determination of mechanisms of dehydrations. Below the temperature of appreciable water evolution, DjO/HjO exchange could, in principal be used to measure, in partially decomposed salt, e quantities of water held at surface sites of different reactivities, i.e. water physically adsorbed, chemisorbed, within amorphous phases and at the reaction interface. Similarly, kinetic isotope effects in dehydration reactions have not been extensively investigated. If corresponds to the enthalpy of dissociation, the substitution of a deuterium bond for a hydrogen bond could result in a change of the magnitude of E. ... 

A recent study has employed deuterium labeling to show that the mechanism for the oxidative N-demethylation of nicotine may involve two modes of breakdown for a proposed carbinolamine intermediate, dealkylation with formaldehyde formation and dehydration to an iminium ion.72 The formation of such an sp2-hybrid intermediate may help to explain why both a primary and substantial / -secondary deuterium isotope effect were observed for the N-deethylation of the antiarrhythmic agent, lidocaine.73 In contrast, only a primary isotope effect was observed on the rate of oxidative O-deethylation of deuterated analogs of the analgesic, phenacetin. 77 These results indicate differences in the mechanism of oxidative 0- and N-dealkylation. A final example of the use of secondary deuterium isotope effects in studying enzymes involved in drug metabolism revealed an SN-2-like transition state for the transfer of a methyl group catalyzed by catechol-O-methyl transferase.73... 

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