Carbenium ions isopropyl

The esters differ from each other in stability. To decompose the isopropyl ester, higher temperatures and higher acid strengths are needed than for decomposition of the s-butyl ester. It is claimed that the resulting carbenium ions are stabilized by solvation through the acid (25-27). Branched alkenes do not form esters. It is believed that they are easily protonated and polymerized (28). 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

Example The mass spectra of both acetone and butanone show typical acyiium ion peaks at m/z 43, whereas the signals in the spectra of isopropyl ethyl thioether (Fig. 6.9), of 1-bromo-octane, (Fig. 6.10), and of isomeric decanes (Fig. 6.18) may serve as examples for carbenium ion signals. The superimposition of both classes of ions causes signals representing an average pattern. The properties of larger carbenium ions are discussed in the section on alkanes (Chaps. 6.6.1 and 6.6.3). 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

In sill C MAS NMR spectroscopy has also been applied to characterize the scrambling in n-butene conversion on zeolite H-ferrierite (97), n-butane conversion on SZA (98), -butane isomerization on Cs2,5Ho.5PWi204o (99), n-pentane conversion on SZA (100), isopropylation of benzene by propene on HZSM-11 (101,102), and propane activation on HZSM-5 (103-105) and on Al2O3-promoted SZA (106,107). The existence of carbenium ions was proposed to rationalize the experimental scrambling results observed by in situ MAS NMR spectroscopy. 

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

The allyl cation has never been characterized as a persistent species in solution. If prepared, it would be the smallest carbenium ion universally accepted to have been formed in condensed media (this title has for many years been held by the isopropyl cation). No allyl cation derivatives, e.g., 10, were observed in a 1990 report of a CAVERN study of butadiene on HY and HZSM-5 (104). The same year, Hutchings reported a flow reactor study of allyl alcohol on HZSM-5 (105) and Gorte (106) reported a TPD study of allyl alcohol on the same zeolite. Hutchings and co-workers found that allyl alcohol had two reaction paths, one in which propanal was formed and another that formed hydrocarbons. The latter route was proposed to proceed through an allyl cation intermediate, but no claim for its persistence or spectroscopic observation was made or implied by Hutchings. Gorte... 

Attempts to cyclize the keto sulfide (282) with polyphosphoric acid at temperatures greater than 100 °C gave as the major product the 2-isopropyl-3-methylthiophene (284). When the cyclization was conducted at lower temperatures, the thiochromene (283) was the major product. Since 4-phenylthio-2-butanone cyclized to produce a stable thiochromene, the quaternary carbon in (283) which can form a stable carbenium ion (284a)... 

The observation of alkyl cations such as the ferf-butyl cation [trimethyl-carbenium ion, (CH3)3C+] 1 and the isopropyl cation [dimethylcarbenium ion, (CH3)2CH+] 2 was a long-standing challenge. The existence of alkyl cations in systems containing alkyl halides and Lewis acids has been inferred from a variety of observations, such as vapor pressure depressions of CH3C1 and C2H5CI in the... 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Theoretical and Experimental NMR techniques provide powerful tools for the investigation of heterogeneous catalysis. Recent advances in in situ NMR techniques are summarized, as are advances in theoretical methods. The utility of our combined theoretical/experimental approach is illustrated by studies of the pentamethylbenzenium cation and the 1,3-dimethylcyclopentenyl cation in zeolite HZSM-5, acetylene adsorption on MgO, and the isopropyl cation on frozen SbF5. We also discuss the role of the basicity of adsorbates in the formation of stable carbenium ions on zeolites. 

For our final example of theoretical NMR in catalysis, we again turn to carbenium ion chemistry. Here we study the formation of the isopropyl cation on frozen SbF5, a strong Lewis acid (27). In contrast to the studies presented earlier, with this system we were able to experimentally measure the chemical shift tensor. Because the full tensor is naturally obtained from NMR calculations, a comparison can readily be made. In addition, for the isopropyl cation we also studied the effect the medium (in this case, the charge balancing anion) had on the chemical shift tensor. 

Anisochrony due to axial chirality of the diastereotopic methylene protons in H2C = C=C(Me)COCHBrR (cf. Fig. 24) has been observed the chemical shift differences may be as high as 0.13 ppm.34) Also related to axial chirality are several cases of anisochronous methyl groups in isopropyl moieties which are diastereotopic through being part of a chiral allene of the type Me2CHCR=C=CR R" 35,49). These cases resemble that shown in Fig. 27 where a prochiral center (Me2CH—C. ..) is attached to a chiral ferrocenyl moiety it should be noted that the ferrocenylmethyl-carbenium ion fragment is chiral only if rotation about the Cp—C+ bond (marked a in Fig. 27) is slow on the NMR time scale 42 . 

This cation can be drawn either as an oxonium ion or as a primary carbenium ion. The oxonium ion structure is the more realistic. Primary carbenium ions are not known in solution, let alone as isolable intermediates, and the proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 p.p.m. instead of at the 13.0 p.p.m. of the true carbenium ion. 

The rate constants of propagation in bulk polymerizations of several alkenes initiated by y-rays are presented in Table 15. The rate constant of propagation of isobutene is estimated to be 1000 times lower in chlorinated solvents than in bulk [134]. The rate constant of vinyl ether propagation decreases a few times by adding only 1 mol% of methylene chloride [238]. This may be due to either an error in the estimate of Gh or to specific interactions between growing carbenium ions and solvent molecules both explanations assume that much less reactive, but still conducting carbenium ions are formed. Nevertheless, recently determined rate constants of propagation of isopropyl and isobutyl vinyl ethers initiated with trityl salts [217] are within a factor of 2 of those calculated from y-irradiated systems. 

The energies of activation of vinyl ether polymerizations are much larger isobutyl and isopropyl vinyl ether E = 21 kJmol-1 ethyl vinyl ether Ea 54 kJ mol This indicates that carbenium ions of vinyl ethers are less reactive, probably due to an equilibrium with dormant oxonium ions formed by an intramolecular cyclization [Eq. (67)]. The overall activation energies should also increase to more positive values if formation of the active carbenium ions is endothermic. 

The H NMR spectrum of 155 in superacid had the right number of peaks to fit the symmetry of a static 2-adamantyl cation but the chemical shift value of the CH proton at the C(2) presumed carbocation center was only 5 H 5.1. This is 8 ppm to higher field than expected for typical static secondary carbenium ions such as the isopropyl cation."... 

Finally, strong solvation of the carbenium ion intermediate also causes a marked reduction in secondary (5-deuterium KIE.84 For example, the (5-deuterium KIE in the solvolysis of isopropyl tosylate is reduced from 1.13/(5-D in trifluoroethanol, which does not solvate the developing carbenium ion strongly, to 1.08/ p-D in water where the carbenium ion is strongly solvated. Again, hyperconjugation is reduced because the solvent stabilizes the developing carbenium ion in the transition state. 

Furthermore, if it is true that the 1,2 shift leaves a planar carbenium ion IX, a process that erases the configurational identity of the carbon atom bearing the isopropyl group (and therefore apparently any further use of the advantages first introduced by the stereoselective step VII - VIII), it is also true that the ensuing 1,2-methyl migration (IX -> II) must proceed in a suprafacial fashion... 

Such an interaction is probable. However, according to the available data (see Sect. IV.2.Q, a change in the acid medium does not usually cause great changes in the rate constants of carbenium ion rearrangements (cf., however, the rearrangement of hydroxytenzenium ions in aqueous acids. Sect. IV.2.C). One can, therefore, assume the equilibrium between the isomeric ions not to be very sensitive to the nature of acid medium either. The equilibrium constants (K) for isomeric isopropyl-trimethylcyclopentenyl cations at 25 "C in different acids i - i > are as follows ... 

Superacids such as Magic Acid and fluoroantimonic acid have made it possible to prepare stable, long-lived carbocations, which are too reactive to exist as stable species in more basic solvents. Stable superacidic solutions of a large variety of carbocations, including trivalent cations (also called carbenium ions) such as t-butyl cation 1 (trimethyl-carbenium ion) and isopropyl cation 2 (dimethylcarbe-nium ion), have been obtained. Some of the carbocations, as well as related acyl cations and acidic carboxonium ions and other heteroatom stabilized carbocations, that have been prepared in superacidic solutions or even isolated from them as stable salts are shown in Fig. 1. 

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