Isomeric reactions equilibrium ratio

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

Having established the effect of substitution on the rates of both monomer isomerization and polymerization, we addressed the question of polymer structure. Specifically, are norbornenyl imide units incorporated into the fully cured polymer with their norbornyl rings intact If so, does the polymer also reflect the equilibrium ratio of exo and endo ring fused monomers For our parent monomers, PN and PX, this question has been unanswerable. We have not found any direct probe that allows an unambiguous assessment of specific substructures within the cured polymer. We do, however, have some evidence bearing on this question for the phenyl substituted monomer. This evidence is attributable in part to our discovery of an unexpected side-reaction in the cure of the phenyl substituted monomer, and in part to the presence of a unique NMR diagnostic for phenyl substituted, endo norbornyl N-phenyl imides. Both of these results are detailed below. 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

The process for isomerization of EB requires that some fraction of the feed be maintained in a saturated state, as described in Sections 14.4.1.1 and 14.4.1.2. The ability to isomerize the EB is affected by the naphthene concentration and constrained by the equilibrium ratio of EB/xylenes at the reaction temperature. Use of a pore-restricted molecular sieve can be used to eliminate more sterically demanding species from the reaction network and can effectively remove these from consideration. In this maimer, one can achieve higher levels of a desired species, such as PX from EB, than could ordinarily be obtained by isomerization over a larger-pore zeolite. 

In the isomerization reactions of cis- and trans-[Co en2 Cy+ in dimethylformamide and dimethylacetamide, the equilibrium isomer ratios depend upon the concentration of chloride. This has been interpreted in terms of ion association, and the equilibrium constants have been computed (31). The dependence of the isomerization rate upon the chloride concentration was interpreted in terms of the different reactivities of the free ion and the ion pair. Preliminary chloride exchange experi-... 

The reactions using the three NO-containing complexes all showed equilibrium conversion to 2-butene and 3-hexene in 1 hr. The cis/trans ratios for all olefins were also at their equilibrium values (the initial 2-pentene was 48% trans, 52% cis). With the complex Mo(CO)4(bipy) there was observable disproportionation although the conversion was quite small. Some double-bond isomerization was observed with this system (1.2% 1-pentene present). The last complex of Table III also gave a trace of disproportionation, some double-bond isomerization (1.6% 1-pentene), and cis/trans isomerization (equilibrium ratio of cis/trans 2-pentene). 

The hardening effect of fatty oil isomerization processes is limited by the fact that cis-trans conversion of the double bond of mono-unsaturated fatty acids is an equilibrium reaction in the case of oleic and elaidic acid esters the equilibrium mixture consists of 67% elaidic acid and 33% oleic acid this equilibrium ratio is practically independent of the isomerization temperature77. 

It is interesting that the position of equilibrium depends on the type of catalyst in ACF- catalyzed isomerization of F-heptene-1 the equilibrium ratio of olefin-2/olefin-3 is different (5 95 vs 25 75 for SbF5-catalyzed process). It should be noted that this difference could also be a reflection of difference in reaction temperature, since isomerization catalyzed by more active ACF catalyst proceeds even at 25 °C [23]. When H, Cl, or F-alkyl group is located inside the carbon chain of polyfluoroolefin, the double bond always migrates to these groups, following the rule of a minimum number of vinylic fluorine substituents [7] ... 

The nickel-catalyzed hydrocyanation of butadiene is a two-step process (Figure 3.32). In the first step, HCN is added to butadiene in the presence of a nickel-tetrakis(phosphite) complex. This gives the desired linear product, 3-pente-nenitrile (3PN), and an unwanted branched by-product, 2-methyl-3-butenenitrile (2M3BN). The products are separated by distillation, and the 2M3BN is then isomerized to 3PN. In the second step, 3PN is isomerized to 4PN (using the same nickel catalyst), followed by anti-Markovnikov HCN addition to the terminal double bond. The second step is further complicated by the fact that there is another isomerization product, CH3CH2CH=CHCN or 2PN, which is thermodynamically more stable than 4PN. In fact, the equilibrium ratio of 3PN/2PN/4PN is only 20 78 1.6. Fortunately, the reaction kinetics favor the formation of 4PN [95],... 

The thermodynamics and kinetics of the thermal equilibrium between previtamin D3 and vitamin D3 have been studied (34,35). The isomerization of previtamin D3 to vitamin 63 is an exothermic first order reaction. The vitamin D3/previtamin D3 equilibrium ratio depends on the temperature and can be calculated from the appropriate equilibrium and kinetic constants reported by Hanewald et al. (36). The rate constants for the equilibrium have been shown to be independent of the nature of the solvent, of acidic or basic catalysis and of factors known to affect free radical process (37,38). The percentages of vitamin D3 in equilibrium with previtamin D3 ranges from 98% at -20° to 78% at 80°. Thus, when vitamin D3 is stored in the cold, the equilibrium constant hinders the conversion to previtamin D3. 

In Section 5.2.5, we discussed the Friedel-Crafts alkylation of benzene with 2-chloropentane. This reaction includes a Wagner-Meerwein reaction in conjunction with other elementary reactions. The Lewis acid catalyst A1C13 first converts the chloride into the 2-pentyl cation A (Figure 11.3). Cation A then rearranges into the isomeric 3-pentyl cation B, in part or perhaps to the extent that the equilibrium ratio is reached. The new carbenium ion B is not significantly more stable than the original one (A),... 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

Free radicals, R, are formed in these reactions only if R could be trapped before any isomerization occurs would the stereochemistry of the organic halide be retained. The equilibration of radicals proceeds at a faster rate than reaction of R with [ArH]"Li to give RLi. The ratios of organo-Li products reflect the equilibrium ratios of the intermediate free radicals. Table 1 provides further examples. 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

Under basic conditions, the hydrolysis of imidate salt UZ, at 0°C gave a mixture (2 B) of the amidoalcohol rotamers 115A and USB as the kinetic products of the reaction. Isomerization followed to yield the equilibrium ratio (4 6) of USA and USB. Imidate first anti imidate salt which does not give the anticipated product, i.e. the aminoester U4. However, being a formate, thus a reactive ester, it is possible that under the reaction conditions. U recyclizes rapidly to give new tetrahedral intermediates which then yield a 2 B mixture of amide rotamers USA and USB. This was proven by showing that the treatment of ester ammonium salt U1 under the same basic conditions at 0°C led directly to a 2 8 mixture of USA and USB. 

Figure 5. Isomerization of the products from FDP aldolase reactions to aldolases catalyzed by glucose isomerase. The numbers in parenthesis indicate the equilibrium ratio.

Reactions of internal olefins can even generate terminal alkylnitriles by a pathway that involves isomerization of intermediate cyanometal-alkyl complexes. Tlus isomerization is similar to the isomerization that occurs during the hydroformylation of internal olefins discussed in Chapter 17. In fact, the nickel catalyst rapidly isomerizes hexene to the equilibrium ratio of olefins faster than it adds HCN to the C=C bond. Thus, internal hexenes generate the terminal alkane nitrile. 

According to early results of Calderon and others the cis/trans ratio of the reaction products approaches the thermodynamically controlled equilibrium value. This, however, is a consequence of isomerization reactions, since the equilibrium values derivate from those found upon extrapolation to O a conversion. 

The acid strength of aluminum phosphorous oxide is enhanced by the addition of SO4 up to 3 wt% Enhancement of acid strength is suggested by high catalytic activities for 1-butanol dehydration and cyclohexene skeletal isomerization. However, addition of excess S04 ions reduces the activity for the reactions. The catalytic activity of the oxide prepared from aluminum sulfate is different from those prepared from chloride and nitrate for 1-butanol dehydration equilibrium mixture of butene isomers is produced over the oxide from sulfate whereas the ratios of l-butene/2-butenes and cis/trans art larger than the equilibrium ratios over the oxides from chloride and nitrate. The catalytic behavior of the oxide from sulfate different from the other oxides is caused by the presence of a small amount of SO4 ions generating strong acid sites. 

The reaction data are summarized in Table 4.4. With unmodified H — ZSM-5 zeolite catalyst, near equilibrium ratio of the meta and para isomers was observed. However, lower than equUibrium amounts of o-ethyltoluene were produced. A dramatic increase in selectivity compared with unmodified catalysts was obtained for the para isomer (up to 98%). A corresponding decrease in meta isomer and virtual elimination of 0-ethyltoluene was also observed. Thus, a completely different isomeric mixture ofethyltoluenes was obtained with modified ZSM-5 zeolites compared with HCl/AlCb catalyst presently used for commercial vinyltoluene production. 

The question of the existence of an equilibrium in enamines derived from 3-aIkyl and 3-aryl cyclohexanones has been investigated. The equilibrium ratio (282) (283) is 45 55, whereas (284) dominated the corresponding isomer ratio to the virtual exclusion of (285). However, reaction with 3-nitrostyrene (which reacts quantitatively with some trisubstituted enamines) under conditions of kinetic control gives adducts formed in the first isomeric pair, to the extent of 80% from (283) in the second pair adducts are also formed to the extent of 80 % from (285). This result is rationalized in terms of a rapid equilibrium between the enamine pairs and the high selectivity shown in adduct formation is accounted for in terms of steric and stereoelectronic effects. 

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