Active carbenium

The initiator stability must therefore be balanced with its required reactivity. For example, stable carbenium ions such as tropylium or trityl react very slowly with alkenes and are therefore not optimal initiators. Protonic acids may be sufficiently reactive but often have anions that are basic and cause transfer by j3-proton abstraction from the growing carbenium ion anions resulting from Lewis acid initiators are less basic. Many of the new efficient initiators are prepared by mixing two stable components, such as an alkyl halide and a Lewis acid, which provide reversibly active carbenium ions. 

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. 

Carbenium ions react with neutral nucleophiles to produce onium ions. A favorable equilibrium between active carbenium ions and temporarily inactive onium ions can be used to produce well-defined polymers (Chapter 4). However, rather than reacting directly with carbenium ions, nucleophiles may also react with Lewis acids to form strong complexes, thereby reducing their activity and ability to ionize covalent compounds. A third reaction that basic nucleophiles may be involved in is /3-proton elimination this transfer reaction may subsequently result in termination if it involves a strong base [Eq. (131)]. 

Thus, the extent of termination depends on the equilibrium positions inEq. (131) and on subsequent reactions. For example, termination occurs when very stable onium ions are generated which do not isomerize back to active carbenium ions during the time of the polymerization or when... 

Penczek discussed at Kyoto the interesting problem of carbenium ion — onium ion equilibria and summariKd data and concepts elaborated recently in his laboratory. The aspect most relevant here has to do with the cationic polymerisation of rr — and n-donor monomers in which the active carbenium ions can equilibrate with the corre onding onium ions both intramolecularly (isomerisation of the active species) and intermolrani-larly (reaction with the n-donor atom of a polymer chain). Owing to the marked differences in stability and reactivity of the two types of cations, these interactions would of course bring about important kinetic complications in sterns involvii such monomers as vinyl ethers. 

Examples of the different types of living cationic polymerization systems are listed in Table 3. All involve relatively fast initiation and optimal equilibria between a low concentration of active carbenium ions and a high concentration of dormant species (Scheme 9). Only hydroiodic acid initiated polymerizations of A-vinyl carbazol are controlled in the absence of a Lewis acid activator or a nulceophilic deactivator [115]. 

Broensted sites, the product distributions can usually be explained on the basis of cafbeniiun ion chemist involving isomersation, scission, bimolecular hydrogen transfer oligomerisation etc., suggesting that the olefin eolite complexes are precursors for active carbenium ions. 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Protonic acids are less suitable because the conjugate base is too active a nucleophile. HC1, for example, will not initiate polymerization because chloride ion adds immediately to the carbenium ion before the latter can propagate. Other initiators which have been studied include electroinitiation, photoinitiation and ionizing radiation. 

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. 

The hydrocarbon catalytic cracking is also a chain reaction. It involves adsorbed carbonium and carbenium ions as active intermediates. Three elementary steps can describe the mechanism initiation, propagation and termination [6]. The catalytic cracking under supercritical conditions is relatively unknown. Nevertheless, Dardas et al. [7] studied the n-heptane cracking with a commercial acid catalyst. They observed a diminution of the catalyst deactivation (by coking) compared to the one obtained under sub-critical conditions. This result is explained by the extraction of the coke precursors by the supercritical hydrocarbon. 

The alkylation reaction is initiated by the activation of the alkene. With liquid acids, the alkene forms the corresponding ester. This reaction follows Markovnikov s rule, so that the acid is added to the most highly substituted carbon atom. With H2S04, mono- and di-alkyl sulfates are produced, and with HF alkyl fluorides are produced. Triflic acid (CF3S020H) behaves in the same way and forms alkyl triflates (24). These esters are stable at low temperatures and low acid/hydrocarbon ratios. With a large excess of acid, the esters may also be stabilized in the form of free carbenium ions and anions (Reaction (1)). 

There are substantial differences between gas-phase and liquid-phase hydride transfer reactions. In the latter, the hydride transfer occurs with a low activation energy of 13-17 kJ/mol, and no carbonium ions have been detected as intermediates when secondary or tertiary carbenium ions were present (25). 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

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