Butene forms

Small olefins, notably ethylene (ethene), propene, and butene, form the building blocks of the petrochemical industry. These molecules originate among others from the FCC process, but they are also manufactured by the steam cracking of naphtha. A wealth of reactions is based on olefins. As examples, we discuss here the epoxida-tion of ethylene and the partial oxidation of propylene, as well as the polymerization of ethylene and propylene. 

Figure 7 The effect of diethyl ether on the specific conductivity of solutions of AlCl3 in C2H5Cl (2.5 x 10"2 mole/l) at -78.5°, and on the DP of poly iso butenes formed in...

The effect of crown ethers on the geometrical orientation in base-promoted E2 eliminations has been studied by several groups. Bartsch et al. (1973) have investigated the effect of several parameters on the potassium alkoxide-promoted eliminations of HBr from 2-bromobutane (Table 45). In the absence of crown ethers the relative amount of 1-butene formed increases, while the trans/cis ratio of the 2-butenes decreases, with decreasing solvent polarity. Furthermore, the proportion of 1-butene increases and the trans/cis ratio decreases on increasing the base concentration. These effects were explained in terms of steric interactions between the base and a- and /7-alkyl groups in the... 

All obtainable three-dimensional data sets were used to test this methodology. They were the following Aluminium Iron alloy [20], Brucite (Mg(OH)2), CNBA, DMABC, DMACB, Copper Perchlorophthalocyanine, Poly(l-butene) form III, Polyethylene, Silicon surface, Poly (1,4- trans-cyclohexanediyl dimethylene succinate) (T-cds)... 

Problem 7.54 How is conformational analysis used to explain the 6 1 ratio of trans- to ci5-2-butene formed on dehydrochlorination of 2-chlorobutane M... 

TABLE 1. Copolymers of Trans-l,4-bromo-2-butene Formed by Condensing with Selected nucleophiles in AyV-dimethyl Acrylamide Using a 1 1 Reagent Charge"... 

At low conversions (10-25%) high yields can be achieved (85-95%). 1,4-Dichloro-2-butenes formed may be used in the manufacture of other products or isomerized to give the desired 3,4-dichloro-l-butene. This is done in the presence of a catalytic amount of CuCl2 with the equilibrium shifted to the formation of 3,4-dichloro-1-butene by distilling it off the reaction mixture. Finally, 3,4-dichloro-l-butene undergoes alkaline dehydrochlorination to produce chloroprene (5-15% NaOH, 80-110°C). 

In developing the mechanism, it was assumed that the adsorbed state of buta-1 2-diene active in hydrogenation was the di-7r-adsorbed species C in Fig. 32. The close similarity in the ZV-profiles for each butene formed in a given reaction suggests that each was formed as a primary product and the general mechanism shown in Fig. 33 was proposed. The difference in behaviour of the type A and B catalysts was explained by proposing that, at the type A surface, a-bonded and 7r-olefinic species are of importance as... 

In the rhodium- and platinum-catalysed reactions [167], it is of significant interest that the 7V-profiles calculated from the distributions of deuterium in the n-butane (Table 30) appear to bear no clear relationship to the 7V-profiles of the n-butenes formed simultaneously. This observation has been interpreted as indicating that either the butene which undergoes further hydrogenation never desorbs as butene, or that the sites responsi-... 

Further information about the direction of dehydrohalogenation was obtained with threo- and eryt/iro-2-deutero-3-bromobutanes on Si02 and Si02—KOH catalysts [193,195]. The determination of deuterium content in the butenes formed allowed the estimation of the extent of the syn-and anti-eliminations. The values of the deuterium kinetic isotope effect showed that the C —H (or C —D) bond is split in the rate-determining step. Over KOH—Si02, the anti-elimination was preferred, but at 300°C, syn-elimination was the peferential reaction mode. With Si02, syn-elimination was favoured under all conditions. 

For specific cases such as olefin oxidation over Bi-Mo oxide combinations some information concerning the oxidation mechanism is available. The work of Adams and Jennings (2), of Sachtler (16), and of Adams (1) has led to the general acceptance of an allylic intermediate. The discoverers of the Bi-Mo catalyst system (21) showed that propene is converted to acrolein, while Hearne and Furman (9) proved that butene forms butadiene. The allylic intermediate therefore can in principle react in two different ways (1) formation of a conjugated diene... 

Scheme 9. Deuterium distribution in n-butenes formed by dissociative and associative mechanisms.

The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

The molecular mechanism of the selective oxidation pathway is believed to be the one shown in Scheme 2 (Section I). Adsorbed butene forms adsorbed 7r-allyl by H abstraction in much the same way as xc-allyl is formed from propene in propene oxidation (28-31). A second H abstraction results in adsorbed butadiene. Indeed, IR spectroscopy has identified adsorbed 71-complexes of butene and 7t-allyl on MgFe204 (32,33). On heating, the 7r-complex band at 1505 cm 1 disappears between 100-200°C, and the 7t-allyl band at 1480 cm-1 disappears between 200-300°C. The formation of butadiene shows a deuterium isotope effect. The ratio of the rate constants for normal and deuterated butenes, kH/kD, is 3.9 at 300°C and 2.6 at 400°C for MgFe204 (75), 2.4 at 435°C for CoFe204, and 1.8 at 435°C for CuFe204 (25). The large isotope effects indicate that the breaking of C—H (C—D) bonds is involved in the slow reaction step. 

The other secondary reaction that we observed is isomerization of butene formed in reaction +V (and, perhaps, in the supposed reaction IV). Table II shows the isomeric distribution of butenes that are formed from w-butyl alcohol upon dehydration at 423 K in the HZSM-5 zeolites with different crystallite sizes. It is seen that, for Sample 1 (with the smallest crystallite size), the isomeric distribution is very far from equilibrium (Table II). However, with the increase of the crystallite size or by inserting rather bulky Na+ ions into the... 

Isomeric Distribution of Butenes Formed upon Dehydration of n-Butyl Alcohol at 423 K... 

Epitaxial Crystallization of Isotactic Poly(l-Butene), Form I. 26... 

Epitaxial crystallization of helical polymers may involve three different features of the polymer chain or lattice. These are (a) the interchain distance (as for stretched out polymers), (b) the chain axis repeat distance, and (c) the interstrand distance - the distance between the exterior paths of two successive turns of the helix. The two former periodicities are normal and parallel to the chain axis direction, and are therefore not usually sensitive to the chirality of the helix (unless the substrate topography is asymmetric and favors a given helical hand). However, the interstrand distance is oblique to the helix axis (it is normal to the orientation of the outer chain path) and therefore has different, symmetric orientations relative to the helix axis for left-handed and right-handed helices (Fig. 2). In other words, epitaxies that involve the interstrand distances are discriminative with respect to helix chirality. This discrimination becomes visible if the crystal structure is based on whole layers of isochiral helices. Such a situation does indeed exist for isotactic poly(l-butene), Form I, that will be considered soon. 

Figure IS. Percent of 1-butene formed in reactions of 0 1 M 2-bromobutane with 0 5 M potassium t-butoxide in t-butyl alcohol in the presence of added dimethyl sulfoxide at 50 0° (Bartsch etal., 1973a).

The results in Table II show the effect of adding hexene to thiophene in singleshot experiments. Cyclohexene had a similar effect cyclohexane, n-hexane, and benzene had no effect, though all were shown to adsorb on the catalyst. Thus the effect of adding 3 moles of hexene per mole of thiophene was to lower thiophene conversion by 21% 1 mole per mole of thiophene cut conversion by only 4.5%, so that assuming that butene and hexene behave similarly, the 1 mole of butene formed from each mole of thiophene probably would not retard the reaction appreciably. 

Later studies by Wells and co-workers, however, showed that the translcis ratios of the 2-butene formed from hydrogenation of 1,3-butadiene over nickel and cobalt catalysts depended on the reduction temperature employed for catalyst activation. High translcis ratios of 3.5-8 were obtained over the catalyst reduced at 400°C, while the ratios decreased to 2 with the catalysts activated below 350°C.119,120 The characteristic properties of the nickel and cobalt catalysts activated at 400°C were attributed to a modification of the catalysts caused by the sulfur compounds contained in the support that occurred at such a high reduction temperature as 400°C.121... 

Allylic halides can undergo slow dissociation to form stabilized carbocations (SnI reaction). Both 3-bromo-l-butene and l-bromo-2-butene form the same allylic carbocation, pictured above, on dissociation. Addition of bromide ion to the allylic carbocation then occurs to form a mixture of bromobutenes. Since the reaction is run under equilibrium conditions, the thermodynamically more stable l-bromo-2-butene predominates. 

The latest industrial application of metathesis was developed by Phillips who started up a plant in late 1985 at Cbannelview, Texas, on the L ondell Petrochemical Complex with a production capacity of 135,000 t/year of propylene from ethylene. This facility carries out the disproportionation of ethylene and 2-butenes, in the vapor phase, around 300 to 350°C, at about 0.5.10 Pa absolute, with a VHSV of 50 to 200 and a once-througb conversion of about 15 per cent 2-butenes are themselves obtained by the dimerization of ethylene in a homogeneous phase, which may be followed by a hydroisomerization step to convert the 1-butene formed (see Sections 13.3.2. A and B). IFP is also developing a liquid phase process in this area. 

Later, Brown and co-workers developed the method described above for the preparation of enantiomerically pure Ipc2BH (>99% ee) and applied the reagent in the asymmetric hydroboration of prochiral alkenes. Oxidation of the trialkylboranes provided optically active alcohols. In the case of cis-alkenes, secondary alcohols were obtained in excellent enantiomeric purity (Figure 1). The reaction is general for most types of cw-alkene, e.g. C(S-2-butene forms (R)-2-butanol in 98.4% ee, and c(s-3-hexene is converted to (R)-3-hexanol in 93% ee. However, the reagent is somewhat limited in reactions with unsymmetrical alkenes e.g. c/s-4-methyl-2-pentene yields 4-methyl-2-pentanol with 96% regioselectivity but only 76% ee (Figure 1). ... 

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