Isobutene alkylation

When alkylating isobutane, chain tennination forms primarily, but not entirely, 2,2,4-trimethylpentane the alkylate from chain termination very closely resembles isobutene alkylate. The similarity of alkylate compositions, particularly their C0 fractions, originating from various olefins and the distance from thermodynamic equilibrium composition indicates that alkylate molecules, once formed, are relatively stable under alkylation conditions and undergo little isomerization. Undesirable side products, e.g., dimethylhexanes and residue, are probably formed by buter e isomerization and polymerization (rather than by isomerization of alkylate or by isomerization of the C3 carbonium Ion which subsequently becomes alkylate). 

TABLE I. OCTANES FRACTION FROM ISOBUTENE ALKYLATE... 

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. 

Examination of 3 fractions from propylene and Isobutene alkylates (see Table I) shows a great degree of simi larlty. Data are summarized In Table II. 

Olefin + olefin = polymer Olefin + isobutene = alkylate... 

Scheme IILll. The classical mechanism of isobutene alkylation with butene.

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. 

The use of supercritical conditions is most perspective in the alkylation process where the lifetime of the catalyst is a burden. The comparison of the catalytic performances and stabilities of H-Beta and H-USY zeolites for isobutene alkylation with butenes in the supercritical isobutane phase [190] clearly shows that the zeolite structure strongly influences the performance and deactivation behavior of the zeohte in the alkylation process in supercritical (SC) conditions. H-Beta outperformed H-USY zeohte because of its lower deactivation and the stable quality of the alkylate, composed mainly of isoparaffins, whereas olefin di-/oligomerization was responsible for the H-USY deactivation. The stable activity of H-Beta was attributed to the effective extractive effect of the supercritical iC media to clean the acid sites located on the external surface of the small crystallites of H-Beta or located at the pore mouth. On the contrary, the deactivation observed for H-USY was caused by olefin oligomerization inside the zeolite supercages. The supercritical isobutane media cannot prevent the oligomers formation inside the zeolite supercages and extract these oligomers. 

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). 

A somewhat similar method to that mentioned above involves alkylation of 4,4-dimethyl-1,3-oxathiolane 5,5-dioxide 32 at the 2 position followed by FVP at 400°C, which results in fragmentation with loss of SO2 and isobutene to give the aldehydes 33. Other electrophiles that may be used include aldehydes, ketones, and McsSiCl, making this a convenient formyl anion equivalent (79TL3375). 

Other olefins that are commercially alkylated are isobutene and 1- and 2-butenes. Alkylation of isobutene produces mainly 2,2,4-trimethylpen-tane (isooctane). 

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...

Highly alkylated l-chloro-2-(trimethylsilyl)cyclopentenes 44, which are of interest as possible cyclopentyne precursors, were prepared by reacting 3-chloro-3-methyl-l-(trimethylsilyl)but-l-yne (45) with 1,1-dialkylated or 1,1,2-trialkylated ethylenes in the presence of titanium tetrachloride35. Because of the low S/v 1 reactivity of 45, the yields of the products were moderate. The stepwise [3 + 2]-cycloaddition mechanism discussed above was proven by the isolation of the intermediate acyclic adduct (in 74% yield) when 45 and isobutene were reacted in the presence of BCI3. Under these conditions, the intermediate 46 could be trapped by Cl since BCI4 is more nucleophilic than TiC.15 (equation 16). 

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. 

Notes The uncertainty in the proton affinities of the alkylenes is probably 3 kcal/mole, in those of butadiene and styrene rather greater The proton affinities of multiply alkyl substituted ethylenes probably all lie within 3 kcal/mole of that of isobutene [4c] ... 

When isobutene is polymerised in an inert solvent such as w-hexane by a metal halide with water as co-catalyst, the same end-groups are formed [9, 10] However, with other solvents, especially alkyl halides, transfer reactions may also introduce end-groups derived from the solvent [11, 12], for example ... 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

Polymerisations in alkyl chlorides. In Figure 3 of Reference 43 it was shown that the DP of the polymers at first increased with monomer concentration, and then fell off steeply to a quite low value characteristic of the polymerisation of undiluted monomer. The exact nature of the diluent ( alkyl halide ) and catalyst were not disclosed, but it is now known that the diluent was methyl chloride and the catalyst aluminium chloride. Kennedy and Thomas have investigated in some detail this interesting phenomenon [56] Experiments were carried out at -78° in a dry-box To 7.1 g of isobutene and the appropriate quantity... 

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