Olefins equilibrium

In 55 to 80% aqueous H2SO4 the simple alcohol olefin equilibrium (found in 0-55% H2SO4) is complicated by a polymerization process which produces predominantly dimer, trimer, and some traces of higher boiling olefins. This behaviour was first noticed by Butlerow (1879). Two fundamental papers by Whitmore (1932, 1934) first presented the now generally accepted mechanistic pathway with carbonium ion... 

The selectivity towards alcohols increased from 81 % to 97% at 85°C. This enhanced selectivity can be ascribed to either a kinetic suppression of an irreversible dehydration or to a change in the alcohol/olefin equilibrium due to the higher amount of water present. In the case of a shift in equilibrium, it should be possible to oxidise olefins with ketones in the presence of water via in situ formed alcohols. In an attempt to oxidise cyclohexene with acetone in the presence of a water, no conversion to cyclohexanone was observed between 85 and 400 °C. It is therefore concluded that under the experimental conditions used, 4-methylcyclohexene is formed irreversibly from the cis-alcohol (Fig. 7) and the increased selectivity towards alcohols should therefore be ascribed to a deactivation of the dehydration sites in the presence of water. 

Higher temperature favours hydride transfer reactions, but also shifts the olefin equilibrium towards lighter olefins, away from the heavier sorbed olefins which can lead to cyclized hydrocarbons. Thus the combination of high temperature (500°C) and low Al/Si ratio (10" ) 1n the zeolite favours exclusive olefin formation (ref. 28). 

MOGD olefin product distribution is determined by thermodynamic, kinetic, and shape-selective limitations. The equilibrium calculation was greatly simplified by assuming the isomers for a given carbon number to be at equilibrium (ref. 19). At low pressure and high temperature, olefin equilibrium is reached, while at higher pressure kinetic limits prevent equilibrations at commercially feasible space velocities. Isomerization reactions are fast at all carbon numbers, and isomer equilibrium is achieved for low carbon numbers. Shape selectivity determines isomer equilibrium for higher carbon... 

Results for the separation of olefins are given in Table 6.2 [77]. It can be seen that the use of this rhodium complex permits a substantial increase in the selectivity of olefin separation compared with squalane. Also, the olefin equilibrium constants with the rhodium complex are usually much higher than those of the corresponding complexes with silver [62]. In fact, the difference between these complexes should be even greater, as the data for the complexes with rhodium were obtained at a temperature 10°C higher than those for the complexes with silver. Attention should be paid to the relationship between the olefin structure and the equilibrium constant of the complexes with rhodium. 

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

Anhydrous ammonia adds tofluorooleftns to produce nitriles Tins phenomenon IS used to characterize chemically the terminal difluoromethylene olefin that IS claimed to be m equilibrium with the internal isomer [4] (equation 2) Thus isomerization to the terminally unsaturated isomer prior to attack by ammonia yields the cyanoenamine... 

Alk = Et), a-methylstyrene (156, Ar = Ph), and allyl-benzene (160, Ar = Ph) are representatives of the four types of monosubstituted propenes (olefins 157 and 158, etc. written in Table I as equilibrium pairs undergo equilibration during the course of the reaction and yield the same pyrylium salt on diacylation). Disubstituted propenes which gave pyrylium salts on diacylation are 2-pentene (163, Alk = Me) and 1,2-diphenylpropene (164, Ar = ph),305 3-ethyl-2-pentene represents a trisubstituted propene... 

Olefin metatheses are equilibrium reactions among the two-reactant and two-product olefin molecules. If chemists design the reaction so that one product is ethylene, for example, they can shift the equilibrium by removing it from the reaction medium. Because of the statistical nature of the metathesis reaction, the equilibrium is essentially a function of the ratio of the reactants and the temperature. For an equimolar mixture of ethylene and 2-butene at 350°C, the maximum conversion to propylene is 63%. Higher conversions require recycling unreacted butenes after fractionation. This reaction was first used to produce 2-butene and ethylene from propylene (Chapter 8). The reverse reaction is used to prepare polymer-grade propylene form 2-butene and ethylene ... 

In the third sequence, the diastereomer with a /i-epoxide at the C2-C3 site was targeted (compound 1, Scheme 6). As we have seen, intermediate 11 is not a viable starting substrate to achieve this objective because it rests comfortably in a conformation that enforces a peripheral attack by an oxidant to give the undesired C2-C3 epoxide (Scheme 4). If, on the other hand, the exocyclic methylene at C-5 was to be introduced before the oxidation reaction, then given the known preference for an s-trans diene conformation, conformer 18a (Scheme 6) would be more populated at equilibrium. The A2 3 olefin diastereoface that is interior and hindered in the context of 18b is exterior and accessible in 18a. Subjection of intermediate 11 to the established three-step olefination sequence gives intermediate 18 in 54% overall yield. On the basis of the rationale put forth above, 18 should exist mainly in conformation 18a. Selective epoxidation of the C2-C3 enone double bond with potassium tm-butylperoxide furnishes a 4 1 mixture of diastereomeric epoxides favoring the desired isomer 19 19 arises from a peripheral attack on the enone double bond by er/-butylper-oxide, and it is easily purified by crystallization. A second peripheral attack on the ketone function of 19 by dimethylsulfonium methylide gives intermediate 20 exclusively, in a yield of 69%. 

The three steps 32-34 have been suggested77 to be equilibria, and the overall equilibrium must lie far to the left because no adduct 23 is found in the reaction mixture when the reaction of sulfonyl chloride with olefin is carried out in the absence of a tertiary amine. A second possible mechanism involving oxidative addition of the arenesulfonyl halide to form a ruthenium(IV) complex and subsequent reductive elimination of the ruthenium complex hydrochloride, [HRulvCl], was considered to be much less likely. 

Olefin metathesis is the transition-metal-catalyzed inter- or intramolecular exchange of alkylidene units of alkenes. The metathesis of propene is the most simple example in the presence of a suitable catalyst, an equilibrium mixture of ethene, 2-butene, and unreacted propene is obtained (Eq. 1). This example illustrates one of the most important features of olefin metathesis its reversibility. The metathesis of propene was the first technical process exploiting the olefin metathesis reaction. It is known as the Phillips triolefin process and was run from 1966 till 1972 for the production of 2-butene (feedstock propene) and from 1985 for the production of propene (feedstock ethene and 2-butene, which is nowadays obtained by dimerization of ethene). Typical catalysts are oxides of tungsten, molybdenum or rhenium supported on silica or alumina [ 1 ]. 

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

When the reaction conditions approach the thermodynamic equilibrium, isomerization follows. The distribution of the double bond is statistical. The molecular formation in the disproportionation stage is also statistical. Normally a run will produce 10-15% by weight of product, which is then suitable for LAB synthesis after distillation. The physical data of these internal olefins are shown in Table 4 [41]. 

We passed then to a particular olefin, adamantylideneadamantane, whose reaction with Br2 had been shown to stop at the stage of bromonium ion formation because of steric hindrance to backside nucleophilic attack. An UV-Vis spectrophotometric study (ref. 10) has shown that the complicated equilibrium reported in Scheme 4 is immediately established on mixing the olefin and Br2 in DCE. Equilibrium (1) could be isolated by working at low Br2 and ten to hundred fold higher olefin concentrations. A Scott plot followed by a NLLSQ refinement of the data gave a Kf = 2.89 x 10 (4.0) M-l. It is worth noting that conductimetric measurements showed the non-ionic nature of the 1 1 adduct, consistent with a CTC intermediate, but not with a bromonium-bromide species. 

Quantitative information about the equilibrium between olefin and Br2 on the one hand and CTC s and bromonium ion species on the other (Scheme 4) has been obtained by the already mentioned UV-Vis spectrophotometric study of the adamantylideneadamantane Br2 system (ref. 10). The spectrophotometric UV-Vis data of a large set of solutions of different reagents concentrations have been used to dissect, using a program based on NLLSQ fitting procedures, the complex spectra in those of the single species present at the equilibrium, as shown in Figure 5. 

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