Cis and trans-2-Butene

The goal of this project is to determine the enthalpies of formation of cis- and trans-2-butene and to calculate the enthalpy of isomerization between them. 

Hydnde shifts often occur during the dehydration of primary alcohols Thus although 1 butene would be expected to be the only alkene formed on dehydration of 1 butanol It IS m fact only a minor product The major product is a mixture of cis and trans 2 butene... 

FIGURE 5.5 Ball-and-spoke and space-filling models of cis- and trans-2-butene. The space-filling model shows the serious van der Waals strain between two of the hydrogens in c/s-2-butene. The molecule adjusts by expanding those bond angles that increase the separation between the crowded atoms. The combination of angle strain and van der Waals strain makes c/s-2-butene less stable than trans-2-butene. 

There are four butene isomers three unbranched, normal butenes (n-butenes) and a branched isobutene (2-methylpropene). The three n-butenes are 1-butene and cis- and trans- 2-butene. The following shows the four butylene isomers ... 

The industrial reactions involving cis- and trans-2-butene are the same and produce the same products. There are also addition reactions where both 1-butene and 2-butene give the same product. For this reason, it is economically feasible to isomerize 1-butene to 2-butene (cis and trans) and then separate the mixture. The isomerization reaction yields two streams, one of 2-butene and the other of isobutene, which are separated by fractional distillation, each with a purity of 80-90%. Table 2-3 shows the boiling points of the different butene isomers. 

The three isomers constituting n-hutenes are 1-hutene, cis-2-hutene, and trans-2-hutene. This gas mixture is usually obtained from the olefinic C4 fraction of catalytic cracking and steam cracking processes after separation of isobutene (Chapter 2). The mixture of isomers may be used directly for reactions that are common for the three isomers and produce the same intermediates and hence the same products. Alternatively, the mixture may be separated into two streams, one constituted of 1-butene and the other of cis-and trans-2-butene mixture. Each stream produces specific chemicals. Approximately 70% of 1-butene is used as a comonomer with ethylene to produce linear low-density polyethylene (LLDPE). Another use of 1-butene is for the synthesis of butylene oxide. The rest is used with the 2-butenes to produce other chemicals. n-Butene could also be isomerized to isobutene. ... 

Figure 6.5 Energy diagrams for hydrogenation of cis- and trans-2-butene. The cis isomer is higher in energy than the trans isomer by about 2.8 kJ/mol and therefore releases more energy in the reaction.

The isomerization of 1-butene to cis- and trans- 2-butene onPd/C/Nafion and Pd-Ru/Nafion electrodes is one of the most remarkable and astonishing electrochemical promotion studies which has appeared in the literature.39,40 Smotkin and coworkers39,40 were investigating the electrocatalytic reduction of 1-butene to butane on high surface area Pd/C and Pd-Ru cathodes deposited on Nafion 117 when, to their great surprise, they observed at slightly negative overpotentials (Fig. 9.31) the massive production of 1-butene isomerization, rather than reduction, products, i.e. cis- and trans-2-butenes. This is extremely important as it shows that electrochemical promotion can be used also to enhance nonredox catalytic reactions such as isomerization processes. 

Figure 9.31. Effect of cell potential on the rates of cis- and trans-2-butene and butane formation upon electrochemical reduction of 1-butene on Pd/C/Nafion electrodes at room temperature.35 Reprinted with permission from the American Chemical Society.

E. Smotkin The group of Professor Smotkin at Illinois (IIT) was first to demonstrate NEMCA for an isomerization reaction (1-butene to cis-and trans-2-butene) over a Pd/Nafion catalyst at room temperature. This important and spectacular discovery underlines the great potential ofNafion for inducing NEMCA at low temperatures for numerous important organic synthesis reactions. 

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

The reactions of Y with four butene isomers, namely 1-butene, cis-2-butene, trans-2-butene, and isobutene, were studied at a collision energy (/ ycoii) of 26.6kcal/mol (see Table 2). In reactions with 1-butene and cis- and trans-2-butene, four processes were observed ... 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

Considerations of mixed mechanisms and mixed reactive states might apply to these cases also, but there is no direct evidence that cyclo-hexenone itself reacts from other than the lowest zn-n state. Both cis and trans products are produced, except possibly in the acrylonitrile reaction 97>, and cis and trans-2-butene give the same mixture of products. A predominant biradical mechanism would seem the logical choice. 

Recent Raman studies of cis and trans 2-butenes show that the torsional frequency of the cis isomer is greater than that of the tram (cis = 394 cm-1, tram = 294 cm-1) which is in agreement with our expectations253,254. Furthermore, IR and Raman studies of the cis and tram isomers of 1,2-difluoroethylene... 

The final stereochemistry of a metathesis reaction is controlled by the thermodynamics, as the reaction will continue as long as the catalyst is active and eventually equilibrium will be reached. For 1,2-substituted alkenes this means that there is a preference for the trans isomer the thermodynamic equilibrium at room temperature for cis and trans 2-butene leads to a ratio 1 3. For an RCM reaction in which small rings are made, clearly the result will be a cis product, but for cross metathesis, RCM for large rings, ROMP and ADMET both cis and trans double bonds can be made. The stereochemistry of the initially formed product is determined by the permanent ligands on the metal catalyst and the interactions between the substituents at the three carbon atoms in the metallacyclic intermediate. Cis reactants tend to produce more cis products and trans reactants tend to give relatively more trans products this is especially pronounced when one bulky substituent is present as in cis and trans 4-methyl-2-pentene [35], Since the transition states will resemble the metallacyclobutane intermediates we can use the interactions in the latter to explain these results. 

To calculate L values using Table I we need forward reaction rates, the number of molecules converted per unit area per second. These reaction rates are most easily obtained at low conversions. But frequently—often for practical purposes—high conversions are reported. We have developed a method for the determination of site densities in a certain class of systems even though the conversion and back reaction are both large. Also, the method can be used under certain conditions even if product isomerization is involved. We shall describe the theory here and in Section III,B apply it to the isomerization of 1-butene to cis- and trans-2-butene over silica-alumina. Although in this article we do not use this method to analyze any other systems, we present it in some detail because it may have potential for further use. 

Butene from 1-butene, isobutene, cis- and trans-2-butene KX 1-Hexene/ cyclohexane [147, 148]... 

SCHEME 3. Transition states for the reaction of cis- and trans-2-butenes with O2... 

The preparation of acyclic allylic hydroperoxides has been described before (3, 7, 9), but it is not clear how the reactivities differ from the better known saturated hydroperoxides and cyclic allylic hydroperoxides. Dykstra and Mosher prepared allyl hydroperoxide by the reaction of allyl methanesulfonate with hydrogen peroxide and alcpholic potassium hydroxide and purified the hydroperoxide by gas chromatography. It detonated on heating and decomposed on exposure to light but was relatively stable in the cold and dark. The isomeric allylic hydroperoxides formed from the autoxidation of the branched olefin, 4-methyl-2-pentene, have also been isolated and were not abnormally reactive (3). In the present study, cis- and trans-2-butene were photooxidized in the presence of methylene blue as a sensitizer (14), and the product, l-butene-3-hydro-peroxide, was isolated by preparative chromatography. 1-Butene proved unreactive and 2-butene-l-hydroperoxide could be formed only by isomerization of the secondary hydroperoxide. 

The lifetime of the excited N03-alkene adducts is sufficiently long that rotation about the C-C bond leads to the same yields of trans- and cw-epoxides regardless of the configuration of the reactant alkene for example, the reactions of both cis- and trans-2-butene give about 80% of the trans form of the product epoxide and 20% of the cis form (Benter et al., 1994). 

Skov, H Th. Benter, R. N. Schindler, J. Hjorth, and G. Restelli, Epoxide Formation in the Reactions of the Nitrate Radical with 2,3-Dimethyl-2-butene, cis- and trans-2-Butene, and Isoprene, Atmos. Environ., 28, 1583-1592 (1994). 

Photolysis of azide 15e in cyclohexene, or in cis- and trans-2-butene, produced the corresponding aziridines in 39, 18 and 18% yields respectively. ... 

A unique mechanism was suggested to interpret the difference observed in the isomerization and hydrogenation of 1-butene and ds-2-butene over a stepped Pt(775) surface.360 It was observed that the hydrogenation rates were insensitive to surface structure for both 1-butene and ds-2-butene. The isomerization rates of cis-2-butene to give only trans-2-butene on the stepped Pt(775) surface, however, was double that of 1-butene to yield both cis- and trans-2-butenes. The Horiuti-Polanyi associative mechanism, that is, the involvement of the 2-butyl intermediate (see Section 4.3.2), cannot explain this difference. However, a facile dehydrogenation of ds-2-butene to 2-butyne followed by a rehydrogenation is consistent with the experimental observations ... 

The addition of B2C14 to multiple bonds was shown to be stereoselective. The initial diborane products isolated after the addition of B2C14 to cis- and trans-2-butene were oxidized to yield meso- and racemic-2,3-butanediols, respectively.478 480 A similar treatment of the addition product of cyclohexene resulted in cis-1,2-cyclohexanediol 480 These observations are consistent with syn addition and the involvement of four-center transition state 42. Similar studies with addition... 

Stereospecific Epoxidation of 2-Butene. The hydroperoxide epoxidation reaction is stereospecific. Pure cis- and trans-2-butene were epoxi-dized separately by cumene hydroperoxide. The cis olefin gave exclusively cis epoxide, and the trans olefin gave exclusively trans epoxide. In both cases, the epoxide was the sole product formed from the olefin. They can be distinguished easily by their different retention times on a gas chromatography column of 20% diisodecyl phthalate on Chromosorb W(60-80 mesh). They were also identified by comparing their infrared spectra with authentic samples. 

Reaction (21) occurs with cyclohexene and both cis- and trans-2-butene, without isomerization of the alkenes,139 while the reduction of Ni(acac)2 with Al(alkyl)3 in the presence of PCy3 and 2-butene (cis and trans) affords the complex bis(tricyclohexylphosphine)(l-butene)nickel(0)>... 

FIGURE 5.2 Interconversion of cis- and trans-2-butene proceeds by cleavage of the -it component of the double bond. The yellow balls represent methyl groups. 

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