Dehydration kinetic isotope effect

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.]...

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.

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. ... 

Abeles and co-workers extended the work of Buckel on the properties and mechanism of the reductive dehydration of lactate to propionate in C. propionicum (234, 235). Studies by Abeles concluded that acrylate is indeed an intermediate in lactate dehydration, based on the observation of a primary kinetic isotope effect of 1.8 on the conversion of [3- H3]lactate to propionate, indicating that -hydrogen abstraction is at least partially rate limiting. The true substrates for the reaction were determined to be the CoA thioesters of lactate and acrylate, hence the enzyme has been aptly named lactyl-CoA dehydratase (234). The reaction was found to be mediated by two proteins, El and E2, and like the (R)-2-hydroxyglutarate-dehydrating enzyme from A. fermentans, both El and E2 from... 

The concerted E2 process in Figure 10.44 seems not to apply to benzylic or 3° (alkyl) alcohols, which can react by an El pathway, but the situation for 2° alcohols in aqueous solution is less certain. Lomas studied the acid-catalyzed dehydration of l,l -diadamantylethanol (25) to l,l-bis(l-adamantyl)ethene (26) in anhydrous acetic acid solution. ° Dehydration of the trideuterio-methyl analog produced a deuterium kinetic isotope effect, with ku/ku equal to 1.32. This result is not consistent with a mechanism in which the /S-proton is lost in the rate-limiting step, but it is consistent with rate-limiting formation of the carbocation intermediate (with the h/ d ratio reflecting a secondary kinetic isotope effect), as shown in equation 10.54. 

For many alcohols, exchange of the hydroxyl group with the solvent is faster than dehydration. This indicates reversible formation of the carbonium ion. The same conclusion can be drawn from the appearance of a kinetic isotope effect when the jS-position is substituted by deuterium ... 

Differential scanning calorimetry experiments revealed the presence of at least four types of lactose besides the known a- and 3-modifications, some of which are dehydrated forms. Attempts have been made to identify the rate-determining step in the dehydration of fructose to S-(hydroxymethyl)furfural by measuring kinetic isotope effects." A spin-probe EPR study at elevated temperatures of some sugars in connection with the desiccation tolerance of biologiGal materials indicated that trehalose and sucrose-raffinose mixtures are distinguished by stability of thdr phase state and their low effective activation energy for rotational molecular motions. ... 

Mechanistic insights into the rhenium-catalysed alcohol-to-olefin dehydration reaction have been reported. The experimental studies showed the dependence on water and the intolerance to base, and the involvement of proton transfer in the catalytic cycle. A car-benium ion intermediate has been suggested. Kinetic isotope effect studies, furthermore, ruled out proton loss from the / -position of the alcohol as the rate-determining step. The DFT calculations indicated that the lowest energy pathway most likely proceeds through coordination of the alcohol to the rhenium centre with a subsequent carbon-oxygen breakage that yields a carbenium ion. Proton transfer from the carbenium ion to water. 

Knozinger and Schengllia studied the kinetic isotope effect of the dehydration of t-butanol, rsr-butanol and isobutyl alcohol over alumina and found that the deuteration of the hydroxyl group does not give rise to an isotope effect, whereas substitution of 8-proton by deuterium produces an appreciable effect. From the dependence of the isotope effects on substrate structure and temperature, it was concluded that at temperatures below 573 K alcohols are dehydrated via 2-like reaction intermediates over alumina. With increasing temperature, the contribution of the ionic structure increased so that at elevated temperatures — depending on the reactant structure — the reaction may proceed via an l-mechanism. 

Intramolecular Cyclizations.—Base-induced 1,3-bonding occurs in the conversion of (24) into (25) even though the reaction is a 1,7-elimination. A spiro-product is also obtained in the dehydration of (26) with dicyclohexyl-carbodi-imide (DCQ. The mechanism of the 1,3-elimination from 3-phenyl-propyltrimethylammonium iodide with potassium amide in liquid ammonia has been investigated. The reaction is concurrent with 1,2-elimmation and shows a nitrogen kinetic isotope effect = 1.022 + 0.001). This and deuterium-... 

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,... 

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

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). 

Equilibrium isotopic fractions rebect the combined, unidirectional kinetic isotopic fractionations. In considering the one-way buxes as in Equation (12), estimates of the one-way kinetic fracbonations are needed. Knowledge of kinetic effects is obtained from controlled experiment with pure CO2 and water or with salt solutions, but for the chemically complex system that is ocean-water, empirical values are adopted. Eor example, laboratory experiments show that the hydration of aqueous CO2 to bicarbonate involves fractionation of 13%c and the dehydration reaction fractionate by 22%c (O Leary et al., 1992). The difference between these two kinetic fractionations, 9%c, corresponds to the equilibrium fractionation depicted above (Marlier and O Leary, 1984 O Leary et al., 1992). In practice, it was estimated... 

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... 

Van Eldick R (1987) High pressure studies of inorganic reactions. In Van Eldick R, Jonas J (eds) High Pressure Chemistry and Biochemistry. Reidel, Dordrecht, The Netherlands, p 333-356 Van Eldiek R, Palmer DA (1982) Effects of pressure on the kinetics of the dehydration of carbonic acid and the hydrolysis of CO2 in aqueous solution. J Solution Chem 11 339-346 Van Hook WA (1972) Vapor pressure isotope effect in aqueous systems. III. The vapor pressure of HOD (-60 to 200°C). J Phys Chem 76 3040-3043... 

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