2-Butene, isomers

Taking into account the double bond, an olefin situation is encountered that is much more complex than that of the preceding families. For exampie, the C4Hg butene isomers have many arrangements ( B), ... 

FIGURE 6 2 Heats of hydro genation of butene isomers All energies are in kilojoules per mole... 

The effect of butene isomer distribution on alkylate composition produced with HF catalyst (21) is shown in Table 1. The alkylate product octane is highest for 2-butene feedstock and lowest for 1-butene isobutylene is intermediate. The fact that the major product from 1-butene is trimethylpentane and not the expected primary product dimethylhexane indicates that significant isomerization of 1-butene has occurred before alkylation. 

Table 1. HF Alkylation Products from Pure Butene Isomers ...

Closely related catalytic systems have also been used for the selective dimerization of ethene to butenes [99]. Dupont et al. dissolved [Ni(MeCN)<3][BF4]2 in the slightly acidic [BMIM]Cl/AlCl3/AlEtCl2 chloroaluminate system (ratio = 1 1.2 0.25) and obtained 100 % butenes at -10 °C and 18 bar ethylene pressure (TOF = 1731 h Y Unfortunately, the more valuable 1-butene was not produced selectively, with a mixture of all linear butene isomers (i.e., 1-butene, cis-2-butene, trans-2-butene) being obtained. 

Dehydrogenation of butanes is a second source of butenes. However, this source is becoming more important because isobutylene (a butene isomer) is currently highly demanded for the production of oxygenates as gasoline additives. 

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. 

Figure 30.4 Electrocyclic interconversions of 2,4-hexadiene isomers and 3,4-dimethylcyclo-butene isomers.

Variation of the CN/Co ratio brings about a striking change in the ratio of butene isomers produced 107, 170, 168). Compare, for example, the following percentages (107) ... 

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

In an effort to further support the proposed mechanisms for the Y+propene reaction, we have examined the reactions of Y with four isomeric butenes, which are essentially propene molecules with one additional methyl group (Fig. 31). Based on estimated potential energy barrier heights22 and thermodynamics (Fig. 32))22-31-34,i56,i57 q js eXpected that analogous product channels to those observed for propene should be seen for the butenes. Therefore, a comparison of reactions of butene isomers to reactions with propene should allow us to further test the validity of the proposed mechanisms. Here we briefly summarize our most notable conclusions from this work. 

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 ... 

Data were also recorded for the reaction of Y with all four butene isomers at a lower collision energy of 11.0 kcal/mol. Time-of-flight spectra were taken at the CM angle for each isomer. As shown in Fig. 39 for Y + cis-2-butene, only YC4H6 products were observed. This collision energy corresponded to the thermodynamic threshold for YCH2 formation, and was only slightly above threshold for the YH2 channel (Fig. 32). 

Upon examining the data for the reactions of all four butene isomers (Fig. 37), the most striking observation is that the data for all four isomers are quite similar, except that there is no YH2 formed from isobutene. In addition, the branching ratios for each isomer are similar, except that 4>ych2 OyCiHe, is approximately a factor of two greater for isobutene than for the other isomers, and for propene, YCH2 is a much more important channel than is YH2 (Fig. 40), a situation that is exactly the opposite to that for the butene reactions (Fig. 37). 

Fig. 37. Lab angular distributions for all reactive products from reactions of Y with four butene isomers at con = 26.6 kcal/mol. Products are YC4H6 (open circles), YH2 (open triangles), and YCH2 (open squares). Solid-line fits generated using CM distributions shown in Fig. 38. Corresponding product yields given in the upper right corner of each graph. Each distribution is scaled to the same number of scans (2).

Owing to the similarity in structures (Fig. 31) and the magnitude of reactive / non-reactive for propene and the four butene isomers, the... 

Ziman et al. [Kinetika i Kataliz, 9 (117), 1968] have studied the kinetics of the catalytic oxidative dehydrogenation of various butene isomers to form 1,3-butadiene. Over a Bi-Mo catalyst the following reactions are important. 

Figure 7.1 An energy diagram for the three butene isomers. The order of stability is trans-2-butene > cw-2-butene > 1-butene.

Methylpropene cannot be compared directly with other butene isomers. 

Krupp-Koppers (2) A process for separating butane and butene isomers from their mixtures by extractive distillation. The added solvent (Butenex) is a morpholine derivative, possibly N-formyl morpholine. 

In the case of the butene isomers, the addition will lead to different isooctyl cations, depending on the isomer and the type of carbenium ion. The reactions involving s-butyl ions are likely to be negligible for liquid acid catalysts and of minor importance for zeolites. 

Recent studies of the kinetics and mechanism of n-butene isomerization over lanthanum oxide by Rosynek et al. (28) indicate that for this catalyst interconversion of the two 2-butene isomers (s4 in Example 8) is very slow and in that case the system could be described by mechanism m3. Studies by Goldwasser and Hall (29) indicate that as temperature is increased, there is appreciable direct conversion via s4 so that one or both of the other two direct mechanisms may be involved. These authors suggest that further studies with all three isomers, at several temperatures and with tracers, would be desirable. 

Table II. Yields of Butene Isomers from the Radiolysis of Poly (1-Butene Sulfone) at 30 °C...

Figure 5. Mechanism proposed for formation of butene isomers by rearrangement of intermediate carbonium ion.

Bell et al. (81) presented forced diffusion calculations of butene isomers in the zeolite DAF-1. DAF-1 (82) is a MeALPO comprising two different channel systems, both bounded by 12-rings. The first of these is unidimensional with periodic supercages, while the other is three-dimensional and linked by double 10-rings. The two channel systems are linked together by small 8-ring pores. It is a particularly useful catalyst for the isomerization of but-l-ene to isobutylene (S3) its activity and selectivity are greater than those of ferrierite, theta-1, or ZSM-5. 

Favorable sorption sites for the butene isomers were found to be the double 10-rings of the three-dimensional channel system. Thus, diffusion was investigated between adjacent 10-rings. Results showed that the diffusion was an activated process the lowest barrier was 17.5 kJ/mol (but-l-ene) and the largest 22.5 kJ/mol (m-but-2-enc). The authors concluded that all of the three-dimensional channel system is accessible by the four butene isomers, since the diffusion barriers are small enough to be overcome at ambient temperatures. 

A further development of this successful technology was achieved to take advantage of the available feedstock base of butene isomers (raffinate II) for the preparation of n-C5 products (n-valeraldehyde, n-isoamyl alcohol, and n-valeric acid). In December 1995 production of n-valeraldehyde was started up in a new plant at Hoechst/Ruhrchemie (138). Generally, there are strong restrictions in the application of the two-phase catalytic processes to higher alkenes (Section IV.B.l), but the adaptation to butenes was possible with little modification of the process developed for propene. 

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