Butyl cation observation

An obvious candidate for a stable noncyclic carbenium ion is the tert-butyl cation observed in superacidic media. Even if the proton affinity of isobutene (Table 22.1) does not make it very likely that tert-butyl cations will exist in zeolites, several quantum chemical studies have localized stationary points for tert-butyl cations in zeolite and found that they are less stable than the adsorption complex, but are similar in stability to surface butoxides. Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the... 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

Does the alkyl group effect on proton affinity depend on the position of substitution Is the proton affinity oftrans-2-butene (leading to 2-butyl cation) larger, smaller or about the same as that of its isomer, 2-methylpropene Rationalize what you observe. 

From Fig. 4 it is seen that the free-enthalpy of activation for the rearrangement of tertiary butyl to secondary butyl cation is 30-4 — 3.9 = 26-5 kcal mole . As the reverse rearrangement has been found by direct observation to have JG cl7-18 kcal rnole" (Saunders et al., 1968), it follows that the difference in stabilization between tertiary and secondary butyl cations is indeed 9 + 1 kcal mole . This value is in excellent agreement with a previous experimental value of 10 + 1 kcal mole (Brouwer and Hogeveen, 1972). 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

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. 

The isopentene produced will either be protonated or be added to another carbenium ion. With a butyl cation, this would lead to a nonyl cation. The resultant carbenium ion fragment can accept a hydride and form a product heptane, or it can possibly add a butene to form a Cn cation. With hydride transfer, another alkane with an odd number of carbon atoms is produced. Just this example is sufficient to show the huge variety of possible reactions. By means of gas chromatographic analysis, Albright and Wood (82) found about 100-200 peaks in the C9-C16 region, regardless of the alkene and acid employed. A similar number of products can be observed for solid acid-catalyzed alkylation. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

An ab initio study of the l-azabicyclo[1.1.0]butyl cation (97) and its isomers shows that (98) and (99) are much less stable than (97), and that the transition states between (97), (98), and (99) are too high in energy to allow (99) to form. The 3-halobicyclo[l.l.l]pent-l-yl cation (101) has been shown to be an intermediate in the addition of halogens to (100). The only product observed was (102) no rearranged products were detected. The Diels-Alder-type reaction of (103) to give (104) is said to involve several carbenium ion intermediates. 

The behaviour of the butyl system provides important information on the nature of the intermediate formed during the rearrangement of the isobutyl to the 2-butyl cation. Thus, from the observation that isobutyl chloride yields n-butane which has exchanged one proton with the acid, while the solvolysis of 2-butyl chloride in the same acid (2% HjO), yields unexchanged n-butane one might deduce that an intermediate was formed during the former s solvolysis which exchanged one proton with the acid before it converted to a secondary butyl ion. A reasonable mechanism is shown in Scheme 1. 

Proton and C-nmr, ESCA, and Raman studies provide a wealth of information which unfortunately is not subject to a unique interpretation. The main conclusion to be drawn therefore is that the structure of the solvent stabilized cation is still unproven. Gas phase estimates of the heat of formation of the norbomyl cation imply a rather marked stability of the stmcture relative to other secondary ions (Kaplan et al., 1970). When combined with other estimates of the heat of formation of the t-butyl cation, however, these data suggest that hydride transfer from isobutane to the norbomyl ion will be endothermic by 6 to 15 kcal mole . This is contrary to experience in the liquid phase behaviour of the ion, and the author s conclusion that their observation of enhanced stability is evidence of stabilization by bridging deserves further scmtiny. 

Maciel (117) described the formation of the trityl cation 16 on silica-alumina in a 1984 symposium in retrospect it is surprising that no one followed up on this work until much later. 16 easily forms from triphenylcar-binol and other precursors in solutions of modest acid strength. The early observation of such an easy cation had the unintended effect of suggesting that real carbenium ions would not so easily be detected in NMR studies of solid acids. Over a decade elapsed before we characterized the rert-butyl cation on A1C13 powder (43). 

Figure 24 reports 13C MAS spectra of the ferf-butyl cation (43) and the methylcyclopentyl cation 17 (45) on the solid metal halides A1C13 and AlBr3 the asymmetry parameters, CSAs, and isotropic shifts (Table III) are unambiguous for the species indicated. Repeated attempts in various laboratories to observe the ferf-butyl cation as a persistent species in a zeolite have thus far been unsuccessful. Detailed theoretical work will be required to determine whether or not the ferf-butyl cations are local minima (i.e., true intermediates) on typical reaction pathways in zeolites. The ease with which these cations form in true superacids (liquid or solid) should be contrasted with the history of negative observations in zeolites. 

The reactivity of aromatic side chains to undergo dealkylation is in line with the stability of the corresponding carbocations. This indicates the possible involvement of carbocations in dealkylation, which was proved to be the case. The intermediacy of the rm-butyl cation in superacid solution was shown by direct spectroscopic observation.228,229 Additional proof was provided by trapping the ferf-butyl cation with carbon monoxide during dealkylation 230... 

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... 

The 13C NMR shift in the ferf-pentyl cation [C2H5(CH3)2C+] 3 is at 813C 335.4, which is similar to the that of the ferf-butyl cation. The shift difference is much smaller than the 17 ppm found in the case of the related alkanes, although the shift observed is in the same direction. The 13C NMR chemical shifts and coupling constants 7C-h of C3 to Cg alkyl cations 1-13 are shown in Tables 3.2 and 3.3.95... 

Both methylcyclopentane and cyclohexane were found to give the methylcyclo-pentyl ion, which is stable at low temperature, in excess superacid.22 When alkanes with seven or more carbon atoms were used, cleavage was observed with formation of the stable ferf-butyl cation 4. Even paraffin wax (see Section 2.2.2.2 on Magic Acid) and polyethylene ultimately gave the ferf-butyl cation 4 after complex fragmentation and ionization processes. 

Because the reaction is catalytic in ferf-butyl cation and the deprotonation/ reprotonation steps are very fast, extensive regioselective deuteriation of the isoalkane is observed at room temperature as shown by GC-MS analysis. The absence of mass 68 (d10-isobutane) and the presence of mass 64 due to S02 formation in the oxidative process are typical features in accord with the oxidative activation of the alkane and the Markovnikov-type addition of deuterons on the intermediate isobutylene (14). However, the exchange process does not take place in the presence of carbon monoxide, which traps the ferf-butyl cation and prevents deprotonation (Scheme 5.7). 

As is apparent in the last step, isobutane is not alkylated but transfers a hydride to the Cg+ carbocation before being used up in the middle step as the electrophilic reagent (tert-butyl cation 4). The direct alkylation of isobutane by an incipient tert-butyl cation would yield 2,2,3,3-tetramethylbutane,142 which indeed was observed in small amounts in the reaction of ferf-butyl cation with isobutane under stable ion conditions at low temperatures (vide infra). 

The alkylating ability of methyl and ethyl fluoride-antimony pentafluoride complexes has been investigated by Olah et al.,143,144 who showed the extraordinary reactivity of these systems. Self-condensation was observed as well as alkane alkylation. When CH3F-SbF5 was reacted with excess of CH3F at 0°C, at first only an exchanging complex was observed in the H NMR spectrum. After 0.5 h, the starting material was converted into the ferf-butyl cation 4 (Scheme 5.19). 

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