2- Butyl cation branching

As for the dehydrogenation reaction, IRC calculations [67] indicate that the protolytic cracking of linear and branched alkanes follows different mechanisms. For ethane and propane the products of the reaction are methane and the proper alkoxide. For isobutane, as one follows the reaction path towards the products, the t-butyl cation decomposes into propene and a proton which restores the acid site of the zeolite. 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

Secondary or tertiary carbenium ions are formed, depending on the branching of the reactant olefin. In the case of 1-butene and 2-butene, the overall reaction scheme for the skeletal isomerization is identical. Protonation of 1-butene or of 2-butene generates x-butyl cations. 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

The stability of carbocations increases for alkyl cations with the number of alkyl groups that surround the positive charge and thereby stabilize it by their inductive effects. Thus, a methyl carbocation CH3 is the most unstable and reactive one while the tert-butyl cation [(CH3)3C]+ is the most stable and least reactive. This stability order is also the reason why carbocations frequently undergo isomerization and rearrangement reactions after formation, a reactivity that is very important for all isomerization reactions in refineries (here branched hydrocarbons are highly desired due to their higher octane number - see Chapters 6.9 and 6.10). 

Star-branched butyl rubber, 4 437-438 copolymers, 4 445-446 Starch(es), 4 703-704, 20 452-453 as blood substitute, 4 111-112 cationic, 18 114-115 in cereal grains, 26 271-274 in cocoa shell from roasted beans, 6 357t compression effects in centrifuges, 5 513 depolymerization, 4 712 in ethanol fermentation, 10 534—535 etherified, 20 563 as a flocculant, 11 627 high-amylose, 26 288 Mark-Houwink parameters for, 20 558t modified and unmodified, 12 52-53 in paper manufacture, 18 122-123 performance criteria in cosmetic use, 7 860t... 

The poly(vinylpyridine) and poly(tert-butyl methacrylate) copolymers can easily be converted to either cationic or anionic polyelectrolytes by protonation of the pyridine rings or by base hydrolysis of the tert-butyl ester units, respectively. The highly branched structure of the molecules, in combination with the polyelectrolyte effect, should confer useful properties to these materials in solution for applications such as pH-sensitive reversible gels. 

Unwanted branching of many polymers probably occurs through such isomerizations. PP, formed using cationic polymerization, has methyl, ethyl, w-propyl, w-butyl, isopropyl, gem-dimethyl, isobutyl, and t-butyl groups connected to the main chain. 

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

Protonated pyrroles are currently believed to be kinetic intermediates, precursors of a-protonated species. However, it has been found that some A -vinylpyrroles in superacids afford exclusively 3-protonated species (Scheme 65) <1998MI30>. For example, 2-/-butyl-l-vinylpyrrole 288 in the superacid at —70°C gives a stable cation 289 having the proton attached to the position 3 and the double bond intact. It requires several hours at 0 °C to rearrange this ion, via a [l,2]-hydride shift, to the a-protonated isomer 290 with an unexpected structure (with a proton at the position where the highly branched substituent is attached). 

Similarly, 2-methylpropene (isobutene) is an important monomer. It only polymerizes by a cationic mechanism, and its copolymers with dienes are known as butyl rubber. Higher 1-alkenes (1-butene, 1-hexene, 1-octene) are important copolymerization components [4, 5] they produce tailored branching of some polyethylene types prepared by a coordination mechanism. Longer-chain alkenes (Cjq, C,2, Cj ) are also sometimes used as comonomers... 

Thus, i-butyl carbonium ion is more stable by 21 kcal. than isobutyl carbonium ion. For this reason, there is a tendency for all cationic hydrocarbon reactions to produce branched-chain structures, if possible. 

Polymers are produced by one of two processes, addition or condensation polymerization. Addition polymerization occurs by one of three mechanisms, radical (e.g., low density branched polyethylene), cationic (e.g., butyl rubbers), or anionic (e.g., polystyrene). Condensation polymerization is used to produce Nylon 6,6 from adipic acid and hexamethylenediamine with the elimination of water. Industrially,... 

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