Carbenium ions transition states

Isobutyl species formed on the catalyst in the initiation steps do not react in unimolecular processes since such reactions (e.g., the formation of n-butyl species) involve primary carbenium ion transition states. Therefore, propagation steps in the conversion of isobutane include oligomerization reactions... 

In general, we have outlined how the conversion of isobutane on sohd acid catalysts takes place according to well-established carbenium ion transition state chemistry. The difficulty with using isobutane conversion as a probe of catalyst performance is that many combinations of oligomcrization// -scission processes with isomerization steps are possible, resulting in a wide variety of adsorbed species and observable reaction products. For example, the following products are observed from isobutane conversion in the presence of ultrastable Y zeolite at temperatures near 520 K (where the reaction is initiated by the addition of isobutylene to the feed) ... 

An example is the disproportionation of /w-xylene to toluene and trimethyl-benzenes in the wide-pored zeolite Y (Fig. 7-5 c). In the large zeolite cavity, bulky diphenylmethane carbenium ion transition states can be formed as precursors for methyl group rearrangement, whereby the less bulky carbenium ion B is favored. Thus the reaction product consists mainly of the imsymmetrical 1,2,4-trimethylben-zene rather than mesitylene (case A). In contrast, in ZSM-5, with its medium sized pores, monomolecular xylene isomerization dominates, and the above-mentioned disproportionation is not observed as a side reaction. 

It is worth mentioning that under the conditions applied the silanediols 15d,g could not be obtained the final products of the acid hydrolysis of 9d,g were the 1-hydroxyalkylsilanols 14d,g. Similarly, 9d and 9g were reluctant to take part in a Cl/OSiMes exchange and the formation of lld,g and their resultant products. We suppose that the tert-butyl group in 9d prevents the molecule from adopting a suitable conformation for the replacement of the respective substituents, and the isomerisation of 9g fails as the result of the electronic effect of the dichlorophenyl substituent, destabilizing a necessary carbenium ion transition state. Interestingly, also in case of the related equally substituted 1-hydroxyalkyl tris(trimethylsilyl)silanes 1 (R = H, = tert-butyl R = H, R = 2,6-dichlorophenyl, respectively) no acid-induced isomerization could be performed. 

In zeolite catalysis, carbenium- or carbonium-ion intermediates are energetically located at the top of the reaction energy barriers. In contrast, in superacid solutions, these protonated intermediates are ground-state reactants. The zeolite carbonium- and carbenium-ion transition state concepts are illustrated for C-C activation and olefin isomerization reactions below. ... 

These differences were explained by solvation effects in the liquid phase. The carbenium ions are more efficiently stabilized by solvation than carbonium ions, because the former have unsaturated trivalent carbon atoms. In this way, the energy barrier between the (solvated) carbenium ion and the carbonium ion transition state increases. 

More recently, it has been proposed (78, 79) that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state. This can then eliminate H2 or a short alkane molecule leaving an adsorbed carbenium ion on the zeolite (protolytic cracking), as shown below ... 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

In line with Higashimura s view34 that carbenium ions are not strictly sp2 hybridized and that they retain some sp3 character, a more nucleophilic G would be expected to induce more sp3 character to the growing cation. Pronounced sp3 character of the carbenium ion would prevent orbital overlap, i.e., the formation of transition state leading to transfer, and thus to increase in molecular weight. 

Operando DRIFTS examination of the working zeolite catalysts shows adsorbed hexane but do not support the presence of bound alkoxide/olefin/carbenium ion species. Data substantiate that alkanes may be activated without full transfer of zeolite proton to the alkane, i.e., without generation of any kind of real carbocation as transition state or surface intermediate. 

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

To explain the very varied behaviour patterns shown by the various monomers in various solvents, use has been made of a further, hitherto unrealized, implication of the model, namely that the rate of the isomerization-propagation must depend upon the electrochemical environment of the complex. This vague idea has been given precision by concentrating attention on the species which occupies the site at the back-side of the near-planar carbenium ion, the front-side of which is 7t-bonded to the double bond of the monomer. The idea is that the stronger the dipole at the back, the weaker is the Jt-bond, and the lower is the energy of the transition state, and therefore the greater is the rate. 

It is appropriate to point out here just why it is not valid to assume (as is commonly done) that throughout the propagation step a paired cation will remain paired and that the resulting newly formed carbenium ion will therefore start its life paired (see, e.g., Mayr et al. [13]). On the contrary, if we follow the assumption made by the founders of Transition State Theory that the transition state can be treated as a thermodynamically stable species, it follows that because in the transition state the positive charge is less concentrated than in the ground state and because therefore the Coulombic force holding... 

From the fundamental reaction-mechanistic point of view, the essential difference between the cationic and the pseudo-cationic mechanisms is this the attack of an ion on the double bond of an alkene to form a carbenium ion generally involves a 3-centred transition state in... 

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