Rate coefficient epoxides

If the activation energies for the epoxidation and combustion reactions on silver oxide equal E, then the rate coefficients k in equations (14) can be expressed as... 

In a study by Corma and coworkers, the rate of epoxidation of 1-hexene on Ti,Al-P matched, for a homogeneous series of solvents, the trend of adsorption. However, it was twice as fast in acetonitrile than in methanol, in contrast to partition coefficients which are ordered in the reverse direction [77, 167]. The relationship for cyclohexanol oxidation was more complex, the rate increasing with the polarity of aprotic solvents and decreasing with polarity increase in protic ones [77]. 

In principle, the solvent can alTect catalysis in a multiplicity of modes, for example through the adsorption of the reagents, the stabilization of Ti—OOH species or interaction with oxidation intermediates [25]. It is worth noting that the first two phenomena produce effects in the same direction, and their individual contributions are not easily distinguished. For instance, the rate of epoxidation in Fbutanol may be slower than in methanol owing to smaller partition coefficients of the olefin, to more sterically hindered active species and to less electrophilic peroxidic oxygen. 

Rate coefficients of ring-opening reactions of epoxides... 

Rate coefficients of hydrolysis and other nucleophilic reactions of epoxides have been measured by various authors (49, 150—154]. The data are reviewed insofar as they are of interest with respect to acid—base catalysis. Measurements have been done mainly by the dilatometric method or by continuous titration of the base formed in the reaction. Table 9 contains rate coefficients, referring to rate eqn. (44), and... 

Rate coefficients and Arrhenius parameters of ring-opening reactions of epoxides in aqueous solution at 25 °C (coefficients in eqn. (44))a... 

Rate coefficients of acid catalyzed hydrolysis of various epoxides in 0.69 M aqueous HC104 (tf0 = 0) at 0 °C [153]... 

Arrhenius parameters (as far as available) for the ring-opening reactions of ethylene oxide, propylene oxide, and isobutylene oxide. Overall values of ftCi and fcHcl obtained for propylene oxide have been split into rate coefficients for attack at primary carbon and attack at secondary carbon, utilizing gas chromatographic product analysis data [152]. (It is interesting to note that the results for attack at primary carbon are of the same order of magnitude as the corresponding values for ethylene oxide.) First-order rate coefficients at a constant acid concentration for the acid catalyzed hydrolysis of various epoxides [153] are collected in Table 10. Rate coefficients of the uncatalyzed and acid catalyzed reactions of ethylene oxide with various nucleophiles [151, 154] can be found in Table 11. 

The substituent effect of a methyl group on the rate coefficient of the basic hydrolysis of ethylene oxide is small. The kOH values for ethylene oxide, propylene oxide, and isobutylene oxide are almost the same (Table 9). It has been demonstrated by Long and Pritchard [150] with the aid of experiments in oxygen-18 labeled water that hydroxide ion attacks propylene oxide and isobutylene oxide predominantly at the primary carbon. Consequently, the base catalyzed hydrolysis of epoxides is a simple bimolecular nucleophilic substitution. 

Similar SN 2 mechanisms occur in the uncatalyzed ring-opening reactions of ethylene oxide and other epoxides with nucleophiles such as halide ions or amines [151, 154]. The dependence of the rate coefficient kx on the attacking nucleophile is the same as in SN 2 reactions of alkyl halides. It follows the Swain—Scott [155] and Edwards [156] relationships. 

The similar magnitudes of the rate coefficients k0 (Table 9) for three epoxides are in agreement with the simple SN 2 mechanism. Similar magnitudes of rate coefficients have been found also for the uncatalyzed reaction of ethylene oxide and the primary carbon in propylene oxide with chloride ion (Table 9). In the case of the two-step mechanism, methyl substitution would increase the basicity of the oxygen in the ring, favoring formation of the protonated intermediate, and exert a small influence on the Sn 2 reactivity of the primary carbon. The overall effect would be a rate increase. However, the experimental data do not agree with this expectation and, consequently, the two-step mechanism may be ruled out. 

By studying several epoxides Gee et al. [15] were able to show the large steric effect of alkyl substitution on the rate of reaction. The second order rate coefficient for attack at a —CHj— (in EO or in PO), k2, was... 

In a companion paper Price and Akkapeddi [22] report the kinetics of base initiated polymerization of epoxides in DMSO and hexamethyl-phosphoramide (HMPT). The initiator is potassium t-butoxide. Second order rate coefficients for (R,S)—PO were about double those for (+)—(R) or (—)—(S) monomer. They conclude that the steric factor favouring alternation of isotactic and syndiotactic placement of the t-BuEO also influences PO. Chain transfer to solvent (DMSO) was also studied. For PO polymerization in DMSO they obtain k = 1.5 x 10 exp(—17,200/RT). However, due to some erratic results they are not very confident about the accuracy. In HMPT rates are about three fold faster than in DMSO k = 7.3 X 10 exp(—16,300/RT). Three other epoxides were also studied in HMPT EO, k = 2.75 x 10 exp(-13,300/RT) t-BuEO, fe = 2.0 x 10 exp(-17,100/RT) phenylglycidyl ether (PGE), fc = 5.4 x 10" ... 

By the example of various catalytic reactions (ethylene epoxidation over Ag, deNOx with methane on Co-ZSM-5, Fischer-Tropsch synthesis over Co-based systems), we show how the reaction mechanism was revealed and the concentrations of key intermediates and reaction rate coefficients were estimated using different isotope labels ( 0, etc.). A single instance... 

The concentration 0 was estimated from equilibrium constant of the 7i-complex formation and recalculated for the given reaction conditions T = 180°C, ethylene = 0.022 atm). Substitution of the values for [OJ, [Oe], and 0 in Equation 51.10 indicates that rate coefficients ks and ke have an order of 10 -10 /s. It means that both ethylene epoxidation and deep oxidation proceed very fast while the processes of oxygen species formation are rather slow. It is remarkable that despite of the low time resolution of the experimental setup (ca. 1 s), the SSITKA allows quantitative evaluation of the reactions that occur in the microsecond range. 

The hydroxylation of n-hexane on TS-1, in contrast to the epoxidation of propene, reached its maximum rate in the least polar solvent, t-butanol (Table 18.13). Acetonitrile behaved quite similarly to methanol and water [24, 25, 169]. On the assumption that t-butanol was comparable to i-propanol for the effects on adsorption, a clear relationship between rates and partition coefficients was lacking. Considering that hydroxylation and epoxidation involve different active species and mechanisms, a diverse role of the solvent in the two active species could contribute to the differences, whereas the partition coefficients alone could not... 

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