Cations, t-butyl

The reaction of t-butyl cation with carbon monoxide was foimd to be very much faster than that with hydrogen. Both this carbonylation and... 

There is direct evidence, from IR and NMR spectra, that the re/T-butyl cation is quantitatively formed when tert-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 984) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenyhnethyl chloride and 1-chloroadamantane alkylate activated... 

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. 

In 1962 Olah repotted the NMR spectrum of the t-butyl cation in superacid solution, [1] and NMR was thenceforth the ex rimental method of choice for studies of intermediates in solution acid chemistry. The inhomogeneous nature and diversity of solid acid systems will ensure that no one experimental technique will so completely dominate as NMR has in solution studies, but the contributions and potential of NMR to solid acid studies are clearly such as to put it on an equal footing with reaction studies, infrared, TPD, diffraction methods and calorimetry. 

Benzyl, methyl, and t-butyl esters are rapidly cleaved, but secondary esters react more slowly. In the case of t-butyl esters, the initial silylation is followed by a rapid ionization to the t-butyl cation. 

This value is shielded compared to the experimental and calculated shift of the tert-butyl cation 7. This was taken as evidence that the dication 8 even in strong superacids is only in a very limited equilibrium with the t-butyl cation. 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

Fig. 4. Potential energy profiles for the isobutane/t-butyl cation hydride transfer reaction in various media (25,64).

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) ... 

The near-uniqueness of isobutene arises from the fact that in the substituted t-butyl cation II the charge is much more concentrated than in any of the other ions shown here, and yet it is stable because it cannot undergo any energetically favourable isomerisations, such as that of the ion VI derived from 3-methylpent-l-ene [6] ... 

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. 

Table 7 contains shift information for a number of other compounds that have been used to analyse the norbomyl spectra. Included among these are data for the isopropyl ion which has been used as the principal model for the secondary ion, and for the 1,2-dimethylnorbomyl cation which has been reported to be an example of an equilibrating classical cation, the Wagner-Meerwein shift being too fast to be stopped at —140°. In the table it may also be noticed that the t-butyl cationic centre experiences a shift of about 11 p.p.m. on being transferred from neat SbFs to SbFs—SO2. 

Inspection of the shift in Tables 6 and 7, however, raises immediate doubt of the quantitative relation between charge density and chemical shift. Thus, whereas an uncharged carbon appears near +165 p.p.m. vs. CS2) and the tertiary carbon of the t-butyl cation at —135-4 p.p.m. the cationic centre of the isopropyl ion is found at... 

Schleyer (1972) found that their calculations also support the notion that the tertiary carbon in the t-butyl cation is more positively charged than the secondary carbon in the isopropyl iort but this conclusion and the enormous discrepancy with Pfeiffer and Jewett are not readily understood. 

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. 

High activity associated with x = 0.5 composition demonstrates an optimum concentration of acid-base sites is needed for phenol adsorption and subsequent polarization of both phenol and isobutene as in the ease of other alkylations. It was proposed that in the phenol t-butylation, t-butyl carbocation ean attaek phenol from the adsorbed as well as from the gaseous state resulted in the formation of para t-butylated products such as 4-tBP and 2,4-tBP. The steric hindrance of t-butyl group prevents the sequential attack of t-butyl cation at ortho position for dialkylation and that demonstrated the negligible formation of 2,6-di-t-butyl phenol. 

The comparison with the dihydrogen binding energy of the methonium ion is obvious the more stable the cation, the smaller the binding energy to a hydrogen molecule. This applies to the itmer and outer complexes. Thus, one can extrapolate that even more stable cations, like the isopropyl and t-butyl cations, do not benefit from an association with a neutral a-electron donor. 

An important antioxidant for many products is butylated hydroxytoluene (BHT), more properly named 4-methyl-2,6-di-t-butylphenol. Acid-catalyzed electrophilic aromatic substitution of a t-butyl cation at the activated positions ortho to the hydroxy group of /)-cresol yields this product, p-Cresol is obtained from coal tar or petroleum. 

The most stable of all alkyl cations is the t-butyl cation. Even the relatively stable f-pentyl and f-hexyl cations fragment at higher temperatures to produce the t-butyl cation, as do all other alkyl cations with four or more carbons so far studied.21 Methane,22 ethane, and propane, treated with super acid, also yield r-butyl cations as the main product (see 2-18). Even paraffin wax and polyethylene give f-butyl cation. Solid salts of t-butyl and f-pentyl cations, e.g., Me3C SbF4, have been prepared from super-acid solutions and are stable below -20°CP... 

Carbocations such as t-butyl cation which are sufficiently electrophilic to react efficiently with styrene and linear dienes are themselves highly unstable and cannot be isolated as stable salts. They tend to oligomerise or react readily even with the most stable anions. Of course they can be generated in situ in a polymerisation reaction (1,95),... 

Figure 6.15 Carbon lr electron spectrum (ESCA) of the t-butyl cation. From G. A. Olah, Angew. Chem. Int. Ed., 12, 173 (1973). Reproduced by permission of Verlag Chemie, GMBH.

Apeloig and coworkers studied solvolysis rates of Me2CX and (Me3Si)Me2CX in 97% trifluoroethanol and derived a value of 4.8 kcalmol-1 for the stability of the t-butyl cation over the 2-trimethylsilylpropyl cation323. 

Solvent relaxation, hydration equilibria, and the t-butyl cation... 

For such unstable carbocations, an alternative approach to pAR can be developed, by recognizing the relationship that exists between pATR and pAa implied in Equation (15) (p. 30). For carbocations with [3-hydrogen atoms, loss of a proton normally yields an alkene. Then, as discussed by Richard, pATR and pAa form two arms of a thermodynamic cycle, of which the third is the equilibrium constant for hydration of the alkene, pAH2o, as already indicated in Scheme 1. The relationship between these equilibrium constants is shown for the t-butyl cation in Scheme 4. In the scheme the equilibria are... 

The equilibrium constants Ka, K, and h2o are conveniently summarized in Scheme 10 in the form of a cycle similar to that shown above for the a-phenethyl and t-butyl cations (Schemes 1 and 4). It is worth noting that P h2o measures the stability of the double bond relative to the alcohol (hydrate). If p fR was converted to HIA, p h2o in the cycle would be replaced by the energy of hydrogenation. The latter provides the conventional measure of double bond stability, save that here free energy in aqueous solution is measured rather than the more usual heat of hydrogenation in the gas phase. 

The more negative pAR is preferred for two reasons. Firstly, as will be clear below, it is correlated better with the gas-phase stability of the isopropyl cation. Secondly, derivation of pAR 16.5 for the cyclohexyl cation from the rate constant for protonation of cyclohexene98 gives a value which, for a secondary alkyl cation, seems too close to that of the t-butyl cation (pAR = -16.4), even though the difference is increased by 2.1 log units if allowance is made for the more favorable geminal interactions of an OH... 

Structural modifications of the reactive intermediates also alter selectivity. The alkylating agent in the isopropylation of toluene, approximating the i-propyl cation, yields 28.5% ro-i-propyltoluene (Condon, 1948, 1949). The reaction of toluene with t-butyl halides under Friedel-Crafts conditions results in the formation of only 7% ro-t-butyltoluene (Schlatter and Clark, 1953). More precisely, the parajmeta ratio is greater for the more selective tertiary ion than for the more reactive secondary species. These results are in agreement with the expectation of a depressed reactivity for the t-butyl cation as compared to the less stable i-propyl cation. 

Bromination in the absence of free radical catalysts, for example, gives high yields of 1-bromoadamantane (Eq. (53)) 18s The Koch 189> and Ritter 188> 19°) reactions, which involve the initial generation of the 1-ada.-mantyl cation either by means of hydride transfer to the t-butyl cation or... 

This reaction gives two products 21 and 22 but neither contains the /-butyl group. Both contain instead the z-butyl group. The intermediate complex rearranges by hydride shift 19 into the t-butyl cation 20 as the primary cation 17 is too unstable. 

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