Formation of Carbonium Ions by Addition Reactions

Formation of carbonium ions from olefins alkenes). Many industrial reactions of olefins involve protonation to give a carbonium ion, which is subject to nucleophilic attack, followed by proton transfer from the product to olefin. The ease of protonation follows the stability of the carbonium ion formed in the sequence tertiary > secondary > primary. Additional proton exchanges can occur at any stage in the overall process, leading to doublebond shifts in the olefinic feedstock and mixed products in some cases. (At high temperatures, products with terminal substituents may also be detectable). 

At the end of the polymerisation, when species IA has disappeared and ions are present, the addition of styrene makes the ions vanish instantaneously and they remain absent whilst polymerisation proceeds. Moreover, this polymerisation has the same rate constant as the first. This means that it cannot have been initiated only by the acid that was free at that time and that the acid bound as ions must also have become available. These facts are represented by the reaction paths leading to esters IB and IC, which complete the cycle whereby eventually ions are formed again, and can be destroyed again by addition of more monomer. Of course, reaction of the freshly added monomer with the then free acid leads to formation of ester IA. The maximum concentration of carbonium ions increases after each addition because of the increasing double bond concentration, as the polymer concentration increases. Thus the final value of the equivalent conductance and... 

The charged end of a polymer and its counter-ion may recombine and form a stable covalent bond thus terminating the propagation of polymerization. Such a termination is frequently observed in carbonium ion polymerizations. For example, polymerization of a vinyl monomer if initiated by hydrochloric acid produces a carbonium ion and a chlorine-counter-ion. These two ions recombine readily forming a stable covalent C-Cl bond which does not propagate further the polymerization and forms, therefore, the dead end of a polymeric molecule. Actually, the recombination of carbonium ion with Cl- ion is such a rapid reaction that usually it follows immediately the formation of the relevant carbonium ion. This prevents the formation of a polymeric molecule and gives instead an addition product of HC1 to the reactive C=C double bond. A polymeric product can be obtained if the ions recombination is slowed down by sufficiently powerful solvation. For example, a solution of styrene in nitromethane, but not in a hydrocarbon, can be polymerized by HC1 (2), since the recombination of the solvated ions is sufficiently slow to permit the formation of a polymer. 

Formation of vinylic carbonium ions by various routes has been suggested in recent years by several workers. Addition of electrophiles, mostly protons, to various acetylenes is the most investigated pathway (Whitlock and Sandvick, 1966 Richey and Buckley, 1964 Noyce et ah, 1965 Letsinger et ah, 1965 Bott et ah, 1964, 1965 Peterson and Duddey, 1966 Peterson and Kamat, 1966 Fahey and Lee, 1966), but their formation was also suggested in the reaction of vinyltriazenes in acidic solution (Jones and Miller, 1966) or in the deamination of vinyl-amines (Curtin et al., 1965). However, solvolytic formation of vinyl cations has been investigated in very few cases. 

The failure of difluoramine to appear among the final products is not particularly surprising. In the presence of nitric acid and/or nitrogen oxides, it might easily be oxidized and may well constitute the source of the silicon tetrafluoride. The formation of a carbonium ion from trityl-difluoramine would be favored by resonance stabilization. In the tert-butyl case, on the other hand, this driving force is not present and formation of the ion would be expected to occur less readily. In addition, both the tert-butyl carbonium ion and the difluorammonium ion from which it is derived would be more subject to a variety of side reactions than the corresponding trityl species. 

Evidence for the presence of organic cations was provided by bright red or purple colors observed immediately upon addition of the carbonyl compounds to the catalyst-aromatic mixtures, and by isolation of side products derived from hydride shifts to intermediate carbonium ions. Mechanistically, these reactions are visualized as proceeding by initial Bideal-like attack of aromatic on the adsorbed conjugate acid derived from the carbonyl compound, with the formation of an intermediate tert-benzylic carbinol ... 

The mechanisms of alkylation reactions appear to be very complex. Analyses of typical alkylation products show that on basis of known reactions, secondary reactions of isomerization, cracking, and disproportionation, hydrogen transfer and polymerization must occur in the reaction. All these reactions are almost certain to occur by means of carbonium ion complexes including formation, addition, rearrangement, and proton and hydride ion transfer. The following reactions are at present beheved to occur as the main reactions in the alkylation of butene-1 with isobutane ... 

The question of carbonium ion formation from saturated hydrocarbons was considered in (,1) by the writer when the possibility of participation by olefins from thermal cracking was mentioned. However, it was only somewhat later that this suggestion was seriously adopted ( ). Then it was postulated that even traces of unsaturated hydrocarbons can activate saturated hydrocarbons by first forming a carbonium ion by proton addition. This ion can then extract a hydride ion by hydrogen transfer from the paraffin or cycloparaffin. This initiates a sort of chain reaction in which new carbonium ions are formed by hydrogen transfer with a steady-state population of ions on the catalyst surface. 

Rapid collapse of an ion pair of carbonium ion (28) and fluoride should also result in cu-addition of HF across the allene 1,2-double bond such addition could give rise either to c(r-2/f-nonafluoropent-2-ene (25) or to its trawr-isomer (26), depending on the direction of approach of the reagent to the reaction site. Preferential formation of the c/s-product is ascribed to partial shielding of one side of the 1,2-bond by the pentafluoroethyl group (a Courtauld molecular model reveals the distinct possibility of such hindrance to attack). 

Pepper and Reilly s views on the mechanism of this polymerisation implied that with the concentrations of perchloric acid used by them, it should be possible to estimate the concentration of polystyryl ions spectrophotometrically and so to test whether initiation did indeed give carbonium ions in concentration equal to that of the acid. When Gandini and Plesch [5, 29-31] carried out the appropriate measurements, they found from spectroscopic, conductimetric and kinetic studies that no ions were present during the polymerisations, but that they were formed once the styrene concentration had been reduced by polymerisation to less than four times the concentration of acid. Addition of more styrene instantly removed the ions, which reappeared once again when polymerisation had reduced the styrene concentration sufficiently. This formation of ions after polymerisation had misled some workers into concluding that they were also present during that reaction. 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

The polymerization of olefins in the presence of halides such as aluminum chloride and boron fluoride but in the absence of hydrogen halide promoter may also be described in terms of the complex carbonium ion formed by addition of the metal halide (without hydrogen chloride or hydrogen fluoride) to the olefin (cf. p. 28). These carbonium ions are apparently more stable than those of the purely hydrocarbon type the reaction resulting in their formation is less readily reversed than is that of the addition of a proton to an olefin (Whitmore, 18). Polymerization in the presence of such a complex catalyst, may be indicated as follows (cf. Hunter and Yohe, 17) ... 

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