Isobutylene protonation

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

Otvos suggested that under the reaction conditions a small amount of t-butyl cation is formed in an oxidative step, which then deprotonates to isobutylene. The reversible protonaton (deuteration) of isobutylene was responsible for the H-D exchange on the methyl hydrogens, whereas tertiary hydrogen is involved in intermolecular hydride transfer from unlabeled isobutane (at the CH position). Under superacidic conditions, where no olefin formation occurs, the reversible isobutylene protonation cannot be involved in the exchange reaction. On the other hand, a kinetic study of hydrogen... 

In this connection, the aim of the present work is quantum-chemical study of the mechanism of isobutylene protonation by classical semi-em-pirical method MNDO. 

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

The strong catalytic activity of anhydrous hydrogen fluoride results from the abiUty to donate a proton, as in the dimerization of isobutylene (see Butylenes) ... 

Addition to cis- and /n t-2-butene theiefoie yields different optical isomers (10,11). The failure of chlorine to attack isobutylene is attributed to the high degree of steric hindrance to approach by the anion. The reaction intermediate stabilizes itself by the loss of a proton, resulting in a very rapid reaction even at ambient temperature (12). 

Alkylation of isobutylene and isobutane in the presence of an acidic catalyst yields isooctane. This reaction proceeds through the same mechanism as dimerization except that during the last step, a proton is transferred from a surrounding alkane instead of one being abstracted by a base. The cation thus formed bonds with the base. Alkylation of aromatics with butylenes is another addition reaction and follows the same general rules with regard to relative rates and product stmcture. Thus 1- and 2-butenes yield j -butyl derivatives and isobutylene yields tert-huty derivatives. 

The desired extraction process is the exothermic proton-catalyzed hydrolysis of isobutylene to tert-huty alcohol. This alcohol is further dehydrated to yield pure isobutylene. At low concentrations the hydrolysis reaction is favored ... 

Model experiments with 2,4,4-trimethyl-l-pentene (C8H16, TMP) and H20 / AlBr3/MeBr at —80 °C, ia, with a conventional Lewis acid system which would give AEjjv = —6.6 kcal/mole in isobutylene polymerization, gave exclusively a dimer (C16H32) by proton elimination, ia, by a mechanism which mimics transfer in polymerization ... 

CH3)2 C=CH2] can be polymerised by BF3 to yield polymers of molar mass well in excess of one million in just a few seconds. The first step in the reaction is between BF3 and the co-catalyst, usually water. The resulting complex readily protonates an isobutylene molecule to yield the carbenium ion C(CH3)3, which is the species that starts the polymerisation proper. For energetic reasons, the process can only yield head-to-tail polymer. 

NMR Spectroscopy. All proton-decoupled carbon-13 spectra were obtained on a General Electric GN-500 spectrometer. The vinylldene chloride isobutylene sample was run at 24 degrees centigrade. A 45 degree (3.4us) pulse was used with a Inter-pulse delay of 1.5s (prepulse delay + acquisition time). Over 2400 scans were acquired with 16k complex data points and a sweep width of +/- 5000Hz. Measured spin-lattice relaxation times (Tl) were approximately 4s for the non-protonated carbons, 3s for the methyl groups, and 0.3s for the methylene carbons. 

Example 2. Vinviidene Chloride Isobutylene Copolymer. The next example is for the carbon-13 spectrum of copolymer vinylidene chloride isobutylene. Figure 5 shows the full spectrum and the peak assignment listing for the non-protonated vinylidene chloride carbon in the 84-92 ppm range. Triad assignments were made (Crowther, M. W., 1987, Syracuse University, unpublished data) using the two-dimensional COLOC (20) experiment. There are ten v-centered pentads representing different environments for the vinylidene chloride carbon. The i represents the non-protonated carbon in the isobutylene polymer unit. 

It is much more likely that initiation involves transfer of a proton, or possibly some other cation, to the monomer. Thus, the mechanism proposed by Evans and Polanyi and others to account for the polymerization of isobutylene in the presence of boron trifluoride monohydrate is represented as follows ... 

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261... 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

The most unexpected features of our results are the slow increase of conductivity after the polymerisations of isobutylene to a definite, stable maximum and the finding of approximately one tritium atom, i.e., one C-Al bond, per polymer molecule. These observations are puzzling because at first thought it appears that only those polymer molecules which had been started by initiation (and not those started by proton transfer)... 

Ionic polymerization may also occur with cationic initiations such as protonic acids like HF and H2SO4 or Lewis acids like BF3, AICI3, and SnC. The polymerization of isobutylene is a common example, shown in Fig. 14.5. Note that the two inductively donating methyl groups stabilize the carbocation intermediate. Chain termination, if it does occur, usually proceeds by loss of a proton to form a terminal double bond. This regenerates the catalyst. 

Calculation of the proton affinities of the carbon atoms of the doubly-bonded pairs in propylene and isobutylene has shown that the proton affinity of the end carbon atom is greater in each case (Evans and Pol-anyi, 14). This means that the proton will add to the double bond at the CH2 more readily than at the CHCH3 or C(CH3)2. Hence, if the addition of HX to a double bond proceeds by way of initial addition of a proton, the hydrogen atom will become attached to the carbon atom holding the greater number of hydrogen atoms. Markownikoff s rule has thus been interpreted in terms of proton affinities which in turn are calculated from bond strengths and ionization potentials. 

Formation of XXVII by loss of a proton from the tertiary carbon atom in the neopentyl group of XXVI rather than from the tertiary carbon atom in the secondary isopentyl group is, however, hardly to be expected. Furthermore, the addition of a tertiary olefin to a secondary carbonium ion is also unexpected (compare the results on the copolymerization of n-butylene with isobutylene, page 46). A somewhat more likely combination consists of the addition of the tertiary carbonium ion. XXVIII to fert-butylethylene (XXIX) followed by a 1,3-shift of a... 

The complex, XLYI, may add to another molecule of isobutylene to yield a higher polymer complex or eliminate aluminum chloride to yield the dimer in the latter case intramolecular migration (a 1,5-shift 1) of hydrogen must be postulated in order to form an olefin. On the other hand, cyclization may readily occur (particularly after a 1,2-shift of a proton from a methyl group) with the resultant formation of a naphthene. 

The experimental observations were explained by means of a mechanism which postulated that the catalyst acts as a proton transfer agent thereby converting an isobutylene molecule to the tert-butyl carbonium ion which is the chain initiator (Evans and Polanyi, 84) ... 

The chain continues to grow by addition of isobutylene molecules until chain termination by proton transfer occurs (cf. discussion of titanium tetrachloride mechanism, p. 74). At higher temperatures, the loss or transfer of a proton occurs faster than does the addition reaction and lower molecular weight products are obtained. 

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