Interpretation carbenium ions formation

We wish to emphasise that the formation of esters (E) from alkenes (M) and acids (HA), the catalysis of the reactions of E by HA or MtXn, and the activation of E, such as organic chlorides, by the co-ordination of a Lewis acid, such as A1C13, are all very familiar chapters in conventional organic chemistry. It follows that the pseudo-cationic theory is nothing more than a generalisation of conventional organic-chemical ideas and a revival of some pre-Whitmore interpretations which had become occulted by the usefulness and novelty of the carbenium ion concept. 

The formation of both isomeric chlorides 147 and 148 and the corresponding methoxy adducts 149 and 150 in methanol is at variance with the behavior observed in AcOH/ LiC104, where only the acetoxy species 140 is formed. This has been interpreted by taking into account the possible role of a specifically solvated carbenium ion pair, such as 145, prior to the formation of a free carbenium ion of type 144. 

In early 1993, Haw and co-workers (107) reported in situ studies of allyl alcohol-/-13C on HZSM-5 and CsHX. No persistent carbenium ions were observed, but 1,3 label exchange was observed for the alcohol on the weakly acidic zeolite. We interpreted this as support for a transient allyl cation. The low stability of this cation was invoked to explain the failure to observe this species as a persistent species. Downfield signals observed in that study were attributed to the formation of propanal. Later in 1993, Biaglow, Gorte, and White (BGW) (108) reported similar studies conducted at different loadings and assigned a downfield resonance (variously reported at 216 and 218 ppm by BGW) to the allyl cation in HZSM-5. 

When R—X is enantiomerically enriched exo-2-norbornyl 4-bromobenzenesulfonate (brosylate), 1 in Fig. 4.5, and SOH is aqueous ethanol, for example, solvolysis is accompanied by racemisation. However, the rate of racemisation is faster than the rate of product formation, and starting material isolated before completion of the solvolysis is partially racemised [14]. The most economical interpretation of these results (and much other evidence, see Chapter 7) is that R+ X- includes the achiral nonclassical carbenium ion (3 in Fig. 4.5), and the ion pair undergoes internal return faster than nucleophilic capture. In other words, k- > k2 in Scheme 4.4, the precise value of the ratio k- lk2 depending upon the particular solvent, and the greater this ratio, the closer the initial reversible process approaches a pre-equilibrium. 

LeBorgne and Souverain have in fact found that the interaction of these two reactants in approximately equal concentrations in CH2CI2 leads to the formation of an amount of 1,1-diphenylethylium ions corresponding to only one third to one fourth of the amount of acid used. The interpretation of these results involves the necessity of free acid to provide adequate solvation for the carbenium ion and to account for the homoconjugation of the anion formed in the reaction. 

A few years later, Eckard et al. studied its polymerisation by tropylium hexachlor-oantimonate and showed that only with very high salt concentrations was it possible to obtain complete conversion. Chain termination was found to be an important reaction probably because of the formation of stable, non-propagating carbenium ions as end groups. No attempt was made to characterise an addition product between monomer and initiator. The development of a red colour during the polymerisation was also reported and attributed to a stable tropylium-indene carbenium ion. This interpretation did not take into account a simpler alternative explanation which Prosser and Young put forward a year later as a feature common to all cationic polymerisations involving indene. It consists in the formation of an allylic-type carbenium ion formed in a termination reaction between an active species and an unsaturated polymer molecule (hydride-ion transfer). This type of termination is now well known in the cationic polymerisation of several monomers ... 

These authors interpreted their results in terms of a cationic mechanism involving the following reactions 1. The carbenium ion abstracts a /J-hydrogen from an ethyl group linked to aluminum with the simultaneous formation of ethylene. 2. The carbenium ion adds to ethylene. 3. The carbenium ion abstracts an ethyl group or a chloride ion from the counter anioa... 

Oligomerization of olefins and their homologation by methanol (or methanol -derived species) appear to be essential features of methanol conversion over ZSM-5 zeolite. A self-consistent-interpretation of the entire process is possible in terms of Rr nsted-acid ZSM-5 zeolite catalysing proton transfer and methylation reactions, and the formation of carbenium ions, and their various oligomerization, cracking, rearrangement and hydride-transfer reactions. 

This interpretation is further supported by the decrease in benzene to C7/C8-aromatics with time on stream. The formation of benzene from a Ce molecule on an acid site would probably involve a primary carbenium ion intermediate, which explains the low propensity for benzene formation over H/ZSM-5. The ability of metal sites to aromatise Ce molecules from the olefin pool led to an increase in benzene selectivity over Zn/ZSM-5 when compared to H/ZSM-5.The observed decrease in benzene formation relative to C7/C8-aromatics therefore points toward deactivation of the metal sites. 

These results were interpreted as implying that the reaction of the exo-substrate occurred solely via a nonclassical carbocation, while the endo-substrate reacted by initial formation of a classical carbenium ion, which then rearranged to the nonclassical carbocation, but not before a small amount had reacted with solvent (attack being sterically directed to the exo-face). 

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