Olefin concentration, dependence

In the homogeneous Dimersol process, the olefin conversion is highly dependent on the initial concentration of monomers in the feedstock, which limits the applicability of the process. The biphasic system is able to overcome this limitation and promotes the dimerization of feedstock poorly concentrated in olefinic monomer. 

The dependence of the epoxidation rate on the concentrations of olefin and hydroperoxide is described by the Michaelis-Menten equation... 

The dilution technique makes use of the different concentration dependence of a) S-T-intersystem crossing, and b) [1 -f-2]-cyclo-addition of a carbene to an olefin. The decay of the metastable singlet state is monomolecular, while the stereospecific addition is of the first order with respect to the concentration At high dilution with an inert solvent such as hexafluorobenzene or octafluoro-cyclobutane etc., the same cis-/trans-cyclopropane ratio should be obtained with cis- or trans-olefin as the starting compound. 

At lower temperatures (or in solution) and at high monomer concentration, a second chain termination process that could occur is direct j -hydrogen transfer to a second molecule of monomer. This kind of chain transfer step is now generally accepted for many transition-metal-catalyzed polymerizations, where direct /1-elimination would be too much uphill to explain the observed molecular weights, for olefin oligomerization at aluminium, a similar situation applies. Since insertion and j -hydrogen transfer have an identical concentration dependence, their ratio does not depend much on the reaction conditions (except temperature) and hence limits the molecular weight attainable in the Aufbau reaction. 

The derivation of a similarly atypical rate expression is required for the simulation of the electrochemical behavior encountered in electrohydrodimerization studies. In these studies, the variation of the bulk concentration of the olefin (e.g., ethyl cinnamate, diethyl furmarate) reveals that there is a concentration dependence to the reaction order associated with the dimerization of the electrogenerated radical ion [33]. This variation in apparent reaction order with concentration can only be attributed to a two-step mechanism [25] involving two independent rate or equilibrium processes. A mechanism that meets this criterion and appears to fit the electrochemical data is the preequilibrium mechanism [36] in which the electrogenerated radical ions first engage in an equilibrium dimerization before the rate-determining ring closure of the dimer takes place. Symbolically, this mechanism may be written ... 

Since the 1-olefin concentration-dependent hydroformylation in the presence of the above catalyst system has a slightly higher activation energy of about 22 kcal mol-1, it is proposed that the ratedetermining step of selective terminal 1-olefin hydroformylation may involve a transition state leading to the formation of a 1-alkyl bis-(trans-phosphine)rhodium carbonyl hydride complex rather than the dissociation of the trisphosphine complex. 

Thus it is found that the relative amounts of olefinic products which result from reactions of 2-bromobutane with MeOK/MeOH, EtOK/EtOH and t-BuOK/DMSO are insensitive to changes in the base concentration (Bartsch et al., 1973a). In contrast, for eliminations induced by t-BuOK/t-BuOH, both positional and geometrical orientations are base concentration dependent. As the concentration of t-BuOK increases, a relatively higher proportion of 1-butene is... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

DP 4 Find the species responsible for the respective rate-limiting steps In which way might their concentrations depend on olefin concentrations and (if applicable) on the type of anion present ... 

The difference in the apparent reaction orders using the two techniques is probably due to the fact that the concentration profiles of reactants and intermediates depend upon the time scale. Thus, differences can be expected for concentration dependent competing mechanisms. In general terms where A represents a diactivated olefin, the major mechanism of dimerization in solvents of low water content was proposed to be given by eqns (121)-(122)... 

The dilution technique makes use of the different concentration dependence of the S—T conversion (a) and the carbene addition to the olefin (/S). The decay of the metastable singlet state is unimolecular, while the stereospecific addition rate is first-order in olefin concentration 78). The dilution technique has not yielded a common ratio in the experiments with cis- or iraws-butene and 2c (see Table 11). Extrapolation of the data to infinite dilution gives a product ratio of 0.16, suggesting that... 

The analogous system containing Et2AlCl and the rhodium complex of PhP(CH2CH2CH2PCy2)2, where Cy = the cyclohexyl group, also catalyzes 1-octene at a rate comparable to that of RhCl(ttp) + Et2AlCl however, in this case, the rate depends on the concentration of olefin. 

The nature of the intermediates in the addition of tetracyanoethylene to electron-rich olefins such as enol ethers continues to arouse interest. Huisgen has studied the concentration dependence of the rate constant for the reaction of TCNE with dihydropyran in a variety of solvents." The logarithms of the second-order rate constants in the various solvents show a linear relationship with solvent polarity. The lack of deviation from linearity shows that the charge-transfer complex concentration does not affect the rate. Le Noble and Mukhtar reported a volume of... 

Photoionization detector, the photoionization detector ionizes analyte molecules with photons in the UV energy range, provides a concentration dependent signal. The photoionization detector is a selective detector that responds to aromatic compounds and olefins when operated in the 10.2 eV photon range, and it can respond to other materials with a more energetic light source. 

When specific interactions between penetrant and polymer become important, such as when hydrogen bonding is involved, the relationship among diffusivity, solubility and measured permeability is more complicated. In the presence of swelling liquids or vapours, such as water in ethyl cellulose and many hydrocarbons in olefinic polymers, diffusivity and solubility show concentration dependence. The system can progressively lose its compactness and, as a consequence, at higher concentrations the polymeric film can dissolve completely in the vapour. 

The extent of olefin formation depends on the position of the functional group, " on the degree of a-substitution and on the concentration of the hydride (or deuteride). Usually olefin formation can be largely suppressed by increasing the concentration of lithium aluminum deuteride. With certain tosylhydrazones, however, such as the C-17 derivative (103), olefin (104) is a major product irrespective of the quantity of the reagent used. ... 

In certain cases this reduction (with lithium aluminum hydride) takes a different course, and olefins are formed. The effect is dependent on both the reagent concentration and the steric environment of the hydrazone. Dilute reagent and hindered hydrazone favor olefins borohydride gives the saturated hydrocarbon. The hydrogen picked up in olefin formation comes from solvent, and in full reduction one comes from hydride and the other from solvent. This was shown by deuteriation experiments with the hydrazone (150) ... 

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