Addition of a carbonium ion

In a recent study of this system Rozenberg (41) found that the rate of reaction of the dialkyl acyl oxonium ion with THF is less than the rate of propagation, probably as a result of conjugation of the oxonium ion with the carbonyl group. 

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Multiple-bond participation in solvolysis may be looked upon as a competition in a nucleophilic displacement between solvent and the tr electrons of the multiple bond or as an electrophilic addition of a carbonium ion, or carbonium ion like species, to the multiple bond. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

There are a few economical routes that can be employed for production of the largest-volume phosphines as specialty chemicals. The preparation of alkyl phosphines, where R > C2H, employs the addition of lower phosphines across an olefinic double bond. The reaction may be either acid-, base-, or radical-catalyzed. The acid-catalyzed addition probably proceeds through the generation of a carbonium ion intermediate which is attacked by the unshared... 

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.

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

The first step corresponds to a normal carbonyl addition, as is also observed with aliphatic aldehydes but here the equilibrium does not lie so far to the right. A strongly acidic medium is necessary for the next stage, the formation of a carbonium ion. For example, it is found that PH3 only reacts with acetone when the solution is more than 8-molar in hydrochloric acid. 

Many theories have been proposed but three have received considerable attention Whitmore s carbonium ion theory (26) postulates that a carbonium ion (positive hydrocarbon ion) adds to an olefin to form a higher molecular weight carbonium ion which then yields the olefin polymer by elimination of a proton (H+). With acid catalysts—for example, sulfuric acid—the initial carbonium ion is formed by addition of a hydrogen ion from the acid to the extra electron pair in the double bond of the olefin. A second pro-... 

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton to adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the rate-determining step. 

Keller and Heidelberger131 reported the kinetics of solvolysis of 1, 30, 38, and benz[a,/i]anthracene 5,6-oxide (228). These studies were carried out mostly in the pH < 7 region where nucleophilic addition ordinarily does not take place with either K-region or non-K-region epoxides. These authors found evidence for the formation of a carbonium ion and consequently were led to believe that the cell macromolecules react with arene oxides through a carbonium ion-trapping mechanism and not by a direct nucleophilic displacement on the oxides. 

The mechanism proposed in Reaction 3—i.e., the generation of a polymeric carbonium ion by the reaction of Et2AlCl with PVC and the addition of the carbonium ion to a double bond in cts-1,4-polybutadiene —would appear to be applicable to the polymer-polymer grafting reaction. The monomer-polymer grafting reaction may involve polymerization of butadiene on the polymeric carbonium ion site or the reaction between polybutadiene generated in situ and the polymeric carbonium ion. 

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. 

A reaction pathway involving heterolytic cleavage of the B—B bond might be expected to yield trans addition products if intermediates such as a coordinated boronium ion (VI) or a coordinatively stabilized carbonium ion (VII) were involved or nonspecific addition if a carbonium ion intermediate such as (VIII) were produced. The formally similar electrophilic addition of halogen to simple olefins is, of course, well known to lead to... 

In the case of AT-vinylcarbazole at 0 and —25°C, initiation has been shown to be the direct addition of the carbonium ion (C7H7) to monomer, a process which appears to take place virtually instantaneously. Under these circumstances the concentration of active centres is assumed equal to the initial salt concentration, [I]q. Significant termination appears to be absent during kinetic lifetimes, (up to 100% conversion) and polymer molecular weights are limited only by transfer reactions. The experimental rate law can be expressed as... 

Thus, the rate of addition of a hydrogen ion to a double bond depends upon the stability of the carbonium ion being formed. As we might expect, this factor determines not only the orientation of addition to a simple alkene, but also the relative reactivities of different alkenes. 

Since the reaction is catalyzed by acid, let us write as step (1) addition of a hydrogen ion to isobutylene to form the carbonium ion the tertiary cation would, of course, be the preferred ion. 

Thus far the reaction is like addition to alkenes an electrophilic particle, attracted by the rr electrons, attaches itself to the molecule to form a carbonium ion. But the fate of this carbonium ion is different from the fate of the ion formed from an alkene. Attachment of a basic group to the benzenonium ion to yield the addition product would destroy the aromatic character of the ring. Instead, the basic ion, HS04", abstracts a hydrogen ion (step 3) to yield the substitution product, which retains the resonance-stabilized ring. Loss of a hydrogen ion, as we have seen, is one of the reactions typical of a carbonium ion (Sec. 5.20) it is the preferred reaction in this case. 

In al this we have estimated the stability of a carbonium ion on the same basis the dispersal or concentration of the charge due to electron release or electron withdrawal by the substituent groups. As wc shall see, the approach that has worked so well for elimination, for addition, and for electrophilic aromatic substitution works for still another important class of organic reactions in which a positive charge develops nucleophilic aliphatic substitution by the S l mechanism (Sec. 14.14). It works equally well for nucleophilic aromatic substitution (Sec. 25.9), in which a negative charge develops. Finally, we shall find that this approach will help us to understand acidity or basicity of such compounds as carboxylic acids, sulfonic acids, amines, and phenols. 

The most generally satisfactory interpretation of this reaction 14 involves addition of the carbonium ion (V), formed by preliminary reaction of the olefin with sulfuric acid, to a molecule of unchanged olefin. 

Another type of catalysis is that on acid catalysts. The mechanism in this type of catalysis is based on the addition of a positively charged proton (H+) to an olefin compound. This results in the formation of a carbonium ion, a positively charged molecule that has only a very short life as an intermediate compound. It transfers the positive charge through the hydrocarbon. The reaction of the acid catalysts (HX) and olefin is depicted in the reaction equation (6.37). 

The rate determining step is the formation of a carbonium ion and as expected, substituents that are able to stabilize the carbonium ion will strongly accelerate the hydrolysis reaction. This effect is shown in Table 1. However, as also shown in Table 1, this is not true for ortho esters where substituent effects are much smaller than would be predicted by analogy with acetal or ketal hydrolysis, and in some cases are in the opposite direction. These data are consistent with the hypothesis that in ortho ester hydrolysis, there is very little carbonium ion character in the transition state and that in the hydrolysis of ortho esters, addition of a proton is concerted with the breaking of a C-O bond. 

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