Frans-2-Butene

C16-0046. Redraw the graph for the reaction in Problem, showing what happens if the system initially contains pure frans-butene at a pressure of 1.0 bar. 

Cfs-butene should lead initially to the anti form trrms-butene should lead initially to the syn form and 1-butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs greatly from the equilibrium distribution of cis- and frans-butene for cobalt complexes 59, 60) the syn form, precursor of irans-butene, is by far the most stable. By way of contrast for the corresponding carbanion, the cis anion seems by far the more stable. This preference for the cis carbanion is presumed to be the source of the high initial cis-to-trans ratio in the initial products of base catalyzed isomerization. In the base catalyzed isomerization of more complex cf-s-olefins (cfs-S-methyl-stilbene), the ions corresponding to syn and anti are not interconvertible and cis-trans isomeriza-... 

Figure 23a shows the C—H region of the spectrum about 10 min (solid line) and 60 min (dotted line) after admission of 8 mm of butene-1 to a sample of zinc oxide. These spectra, which are primarily due to physically adsorbed and gas-phase butenes, show sizeable changes as a function of time. The region above 3000 cm-1 is particularly clear cut. Initially, a band is observed only at 3082 cm-1 this corresponds closely to the 3086 cm-1 band for gaseous butene-1 (64). After 1 hr the band at 3082 cm-1 is gone and two new bands (above 3000 cm-1) have appeared at 3035 and 3018 cm-1 which correspond within the experimental uncertainty to the expected bonds for cfs-butene (64) (3030 cm-1) and frans-butene (64) (3021 cm-1). Other bands are consistent with these changes. Thus, it is evident that double-bond isomerization has occurred. 

Extrapolation of the rate data in Fig. 24 to zero conversion shows that the initial ratio of butene-1 to frans-butene formation is about unity. Thus, butene-1 is not an intermediate in the cis-trans isomerization and direct cis-trans isomerization occurs. Similar results are found for the heterogeneous base catalyzed isomerization over sodium on alumina (17). 

When nonstereospecific addition of dimesityl- and bis(2,4,6-triisopropylphenyl)silylene to trans-2-butene, and to a lesser extent to d.v-2-butene, was reported, a stepwise addition mechanism was suspected131. However, the apparent nonstereospecificity was caused by a d.s-butene impurity in the frans-butene and by photoisomerization of the cis- and trans-siliranes132. 

The rate of H addition to 1-pentene is roughly equal to the addition to 1-butene, of the H addition to cis and trans isomers of 2-pentene as to cis-and frans-butenes. Cycloalkenes add a H atom in a similar way to simple alkenes of comparable structure. H attacks either the terminal or the internal C atom of 1,3-butadiene the first way predominates, probably due to allylic or hyperconjugative stabilization of the generated radical. 

The reaction appears to be stereoselective because the reaction of methyimagnesium bromide with cis and /ran. -1-bromopropene affords only ds- and fran. -butene-2, respectively. 

S) Cis- and frans-butene-2 are readily obtainable in high states of purity. 

Diphenylcyclobutanones. Contrary to earlier reports, diphenylketene undergoes cycloaddition with simple alkenes to give 2,2-diphenylcyclobutanones.1 However, a long reaction time is needed for satisfactory yields. cw-Olefins react much faster than trans-olefins. Thus the reaction with cis-butene-2 requires 3 days, but the reaction with frans-butene-2 requires 3 months. With low-boiling alkenes, the reaction is carried out in a sealed tube or in an autoclave. 

We can measure the energies of the two molecules by measuring the heat of hydrogenation of each isomer to give butane—the same product from both- The difference between the two heats of hydrogenation will be the difference in energy of cis- and frans-butene. 

Fig. 6. Chromatogram of separation of unsaturated C -C compounds. Column, 2 m X 1.75 mm I.D. stationary phase, 15% 0.01 Af dicarbonylrhodium 3-(trifluoroacetyl) camphorate on Chromosorb P. I emperature, 30°C. Peaks 1 = acetylene 2 = isobutene 3 = frans-butene-2 4 = cw-butene-2 5 = butene-1. From ref. 26.

Fig. 8.6. Chromatogiam of mixed butenes (A) without concentration and (B) with concentration. Peaks 1 = butene-1 2 = cw-butene-2 3 = frans-butene-2. From ref. 26.

Show as many separation schemes as you can for the problem of Fig. 1.16, assuming that, in the presence of furfural as an MSA, n-butane is more volatile than 1-butene and that ordinary distillation cannot be used to separate n-butane from frans-butene-2. Furfural is less volatile than -pentane. One such scheme is shown in Fig. 1.20. 

Early IR and UV-VIS spectroscopic studies on the formation of carbonium ions from triphenyl methyl compounds on zeolites, titania and alumina were carried out by Karge [111]. In 1979, upon interaction of olefins Hke ethene and propene with zeoHtes CoNaY, NiCaNaY, PdNaY and HY, the appearance of electronic bands between 230 and 700 nm was observed by Garbowski and PraHaud and attributed to an allylic carbenium ion which upon thermal treatment transforms into polyenyl carbenium ions and/or aromatic compounds [112]. These findings were corroborated and extended by studies of the interaction of propene, cyclopropane and frans-butene on zeoHtes NaCoY and HM [30]. In spite of the obscuration of the spectrum in the range between 450 and 700 nm by the threefold spHt d-d band of tetrahedraUy coordinated Co(II) ions in the case of zeoHte NaCoY,the development of bands near 330,385 and 415 nm was assigned to unsaturated carbocations. 

An example of such a monomolecular reaction is the acid-catalyzed isomerization of cw-butene to fran -butene in Figure 2.7. The acid site is a proton as present on the surface of a solid acid (see sections 3.3 and 6.4.2.1). The adsorption complex /ads formed upon protonation is a carbenium ion. 

Estimate the equilibiium composition at 1 bar of a gas mixture containing the following isomers 1-butene (1), cis-butene (2), and frans-butene (3). 

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