Isobutene alkylation reactions

Butenes are used extensively in gasoline production to produce high-octane gasoline compounds. In alkylation reactions, butenes combine with isobutane to produce branched gasoline-range compounds (see Butane). Isooctane can be produced by dimerization of isobutene in the presence of sulfuric acid. Dimerization is the combination of a molecule with itself to produce a molecule called a dimer. The dimer has exactly twice the number of atoms in the original molecule. Therefore the dimerization of isobutene produces two dimers with the formula C H,... 

Good hydrocarbon dispersion in hydrofluoric acid is an impxjrtant factor In producing alkylate rich in trimethylpentanes and thus favors olefin isomerization (to isobutene), isobutene dimerization, and maximizes hydrogen transfer and primary alkylation reactions. Excess olefin polymerization to form residue is suppressed by good dispersion. 

Olefin Isomerization. Olefin isomerization plays an important role in butene-isobutane alkylation reaction mechanisms. Normal butenes are largely isomerized to isobutene before alkylation. This is believed to take place in ionic form, i.e., immediately following olefin protonation, since a number of olefins have been found to odd HF across their double bonds quite readily at room temperature (Grosse and Linn, 1938). Thus, the likelihood of olefin molecules being present for very long under alkylation conditions is not great. 

As shown in Table I, the equilibrium compKisition for dimethylhexanes is nearly eight times os great os actually found in isobutene alkylate whereas, the trimethylpentane concentration of isobutene alkylate exceeds the equilibrium concentration by about eight times. When considering only the trimethylpentanes, the 2,2,4 content of isobutene alkylate is very near that for equilibrium (70.37 percent vs 69.77 percent). Agreement for the other three trimethylpentanes on this basis is pxxsr. The conclusion is that the alkylation reactions are quite specific, and that isomerization of alkylation products is minor. 

Ionisation is followed by different types of interactions, depending upon the presence in the system of a monomer (e.g., isobutene), of a non polymerisable olefin (e.g., 2,4,4-trimethylpentene-2), or of no further reagent. In the latter instance the ion pair produced in reaction (iii) collapses by an alkylation reaction involving the migration of an alkyl group onto the cation, leaving a more halogenated aluminium alkyl, e.g. ... 

The alkylation reaction of isobutane with a mixture of C4 linear olefins was carried out in liquid phase at temperatures between 25 and 80°C, and at 30 Kg/cm, in a fixed-bed reactor. The space velocity was WHSV = 1 ft referred to the olefins. The isobutane is premixed with the olefins. The molar ratio used in this study was 15. The C4 olefins fraction contains 38% 1-butene, 22% trans-butene, 14% cis-2-butene and 26 % isobutene. In order to analyze the products coming out of the reactor, a ten-loop valve was used to collect the sample to be analyzed after the mn. Products are analyzed by GC, using a 100 m squalane column. Prior to the reaction, catalysts were pretreated in-situ, heating up to 250 C in an air stream. 

The following is an example of an alkylation reaction that is important in the production of isooctane (2,2,4-trimethylpentane) from two components of crude oil isobutane and isobutene. Isooctane is an antiknock additive for gasoline. 

It is interesting to note, that in the alkylation of phenol with isobutene, a change in temperature seems also to change the course of the alkylation reaction with the polysiloxane based catalysts. The principal reaction pathways to the main substitution products (according to the theory for electrophilic substitution, only very small amounts of meta-alkylation products are observed) are given in fig. 5. 

Figure 4.8-10 Phase effect on the alkylation reaction. Conditions isobutene/isobutane = 1/50 ...

Figure 4.8-11 Alkylation reaction in different supercritical solvents. Conditions isobutene/ isobutane = 1/50 W/F = 40 g h/mol 450°C-calcined H-USY (1 atm = 1.013 bar).

Characteristic examples of industrial fast chemical reactions are the electrophilic polymerisation of isobutylene [7], its copolymerisation with isoprene [10], chlorination of olefins [17] and butyl rubber [18], ethylene hydrochlorination [17], sulfation of olefins [19], neutralisation of acidic and basic media [20], isobutene alkylation (production of benzines) [21-23], and so on. These examples of fast liquid-phase reactions and a variety of such processes assume a formal approach for their calculation and modelling, based on material and heat balance in the industrial implementation of respective products. It is a priori acknowledged that is not difficult to achieve an isothermic mode for fast chemical exothermic processes if you are aware of the process behaviour and can control it. 

Fan et al. (312) report the alkylation on Y-type zeolite catalysts of two olefin/paraffin systems (a) isobutene with isopentane and (b) isobutene with isobutane. The isopentane and isobutane, respectively, were also used as the solvents in this work, and results were compared for gas-, liquid-, and SCF-phase conditions. Alkylations conducted in the SCF phase were reported to exhibit higher activity and longer catalyst lifetimes in comparison with reactions conducted in the gas and liquid phases. Catalyst deactivation observed in the gas-and liquid-phase results was attributed to deposition of high-molecular-weight olefinic oligomers on the Lewis acidic sites that were determined to be active for the alkylation reactions. Operation under SCF-phase conditions resulted in successful extraction of these oligomers in situ and extended the catalyst life. Additional aspects of this report are discussed in a subsequent commentary (313) and rebuttal (314). 

Friedel-Crafts and Other Alkylation Reactions. A variety of r-butyl-substituted aromatics have been prepared with isobutene in the presence of acids (eqs 19-21). Due to steric factors, the regiochemistry is enhanced. Isobutene can be alkylated by benzylic and allylic halides in the presence of Zinc Chloride (eqs 22-24). ... 

The HF is a good and cheap catalyst for isobutene alkylation, however its use caused significant concern because its high vapors pressure and tendency to form aerosol. The fluoride anion is highly toxic and ecological problems are generated when fluoride is not completely removed after the alkylation reaction (Weitkamp Traa, 1999). 

The preparation of mono- and di-tm-butylcyclopentadienes 1 and 2 starting from monomeric cyclopentadiene was reported first in 1963 [23]. It was noted that the nucleophilic attack of the cyclopentadienide anion on ferf-alkyl halide has to compete with elimination reaction giving isobutene. The yield of the di- and tri-fer/-butylcyclopentadienes 2 and 3 was therefore reported to be modest to low [23, 24], Recently an elegant improvement for this synthesis using phase transfer catalysis was presented (Eq. 1), but the availability of the tri-substituted derivative... 

Polarization also occurs in coupling and disproportionation reactions of Grignard reagents with alkyl halides. The vinyl protons of isobutene produced in the reaction of t-butylmagnesium chloride with t-butyl bromide show A/E polarization as do the methyl protons of isobutane (Ward et al., 1970). Similar results arise in the reaction of diethyl-magnesium with organic halides (Kasukhin et al., 1972). 

Figure 9.14. Alkylation of isobutane and propene is a chain reaction with the isobutene carbenium ion as the chain carrier (indicated...

Hydride transfer from alkenes was also proposed to occur during sulfuric acid-catalyzed alkylation modified with anthracene (77). Then the butene loses a hydride and forms a cyclic carbocation intermediate, yielding—on reaction with isobutene—trimethylpentyl cations. This conclusion was drawn from the observation of a sharp decrease in 2,2,3-TMP selectivity upon addition of anthracene to the acid. 

While the formation of multiadducts in the above reactions clearly demonstrates the difficulties confronted in terms of controlling the reaction, the issue of whether C6o and C70 undergo addition by carbon electrophiles is of great interest, because such a reaction would provide a useful method for carbon-carbon bond formation for the derivatization of fiillerenes. Initial attempts to test the possibility of electrophilic alkylation of C6o with terf-butyl chloride and AICI3 gave only polymeric products, probably formed via isobutene, indicating the insufficient reactivity of C60 towards terf-butyl cation. 

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