Carbonium ions general reactions

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

Since the metal sulfate catalyst has both Bronsted and Lewis acid sites, it is expected that many n bases with nonbonding electrons such as —O— and —Cl and tt bases hke olefin will undergo acid-base equilibria, thus initiating carbonium ion or carbonium ion-like reactions. Table II summarizes the acid- catalyzed reactions on metal sulfate catalysts and shows the versatility of these systems. Although this table includes some examples of industrial work, our results and others clearly show the general trend in the strength of acid sites required for each specific reaction. But detailed discussion of correlation between the catalytic activity and the acidic property is reserved for the next section. 

Alkylation is the paramount electrophilic substitution reaction in industrial aromatic chemistry, for example, in the production of ethylbenzene, cumene, diisopropylbenzenes and diisopropylnaphthalenes. A carbonium ion generally acts as the electrophilic agent and is produced by reaction of a Lewis add with an olefin. The most stable of the possible carbonium ions normally predominates in the reaction nevertheless, attention must also be paid to the formation of isomers. 

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

Although in general azoles do not undergo Friedel-Crafts type alkylation or acylation, several isolated reactions of this general type are known. 3-Phenylsydnone (120) undergoes Friedel-Crafts acetylation and Vilsmeier formylation at the 4-position, and the 5-alkylation of thiazoles by carbonium ions is known. 

The reaction is generally thought to involve carbonium-ion intermediates but several puzzling features remain. Secondary aliphatic amines give nitrosamines without evolution of N2 ... 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

The corresponding reaction of CDI (A) with tertiary alcohols, the carbonium ions of which are relatively stable like, for example, triarylmethanols, are carried out at much lower temperature (often room temperature, CH3CN, CH2C12, or CHC13, several hours) and are therefore of general preparative value. 

The experimentally observed pseudo-first order rate constant k is increased in the presence of DNA (18,19). This enhanced reactivity is a result of the formation of physical BaPDE-DNA complexes the dependence of k on DNA concentration coincides with the binding isotherm for the formation of site I physical intercalative complexes (20). Typically, over 90% of the BaPDE molecules are converted to tetraols, while only a minor fraction bind covalently to the DNA bases (18,21-23). The dependence of k on temperature (21,24), pH (21,23-25), salt concentration (16,20,21,25), and concentration of different buffers (23) has been investigated. In 5 mM sodium cacodylate buffer solutions the formation of tetraols and covalent adducts appear to be parallel pseudo-first order reactions characterized by the same rate constant k, but different ratios of products (21,24). Similar results are obtained with other buffers (23). The formation of carbonium ions by specific and general acid catalysis has been assumed to be the rate-determining step for both tetraol and covalent adduct formation (21,24). 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

One of the most interesting parts of this publication is a report by G. Salomon of Delft on a Symposium on Carbonium Ion Reactions held at Leiden in March 1952, just a few days before our symposium. It is clear that this marks, as nearly as can be, the general... 

Since the condition for formation of high polymers is that the propagation reaction must be faster than all other reactions of the growing species, and since carbonium ions are highly reactive, it is evident that very special conditions are required for the formation of high polymers by cationic polymerization. The general conditions which must be satisfied are ... 

Concerning the (generally complicated) polymerisation of higher alkenes, it is shown that the transfer of CHf, analogous to H transfer, may play a significant part, except for isobutene. The energetic reasons for the distinctive polymerisation behaviour of isobutene are analysed, with special reference to the energetics of the transfer of protons or carbonium ions to monomer. The hypothetical termination reaction for the isobutene-BF3 polymerisation. 

Gaseous carbonium ions from the decay of tritiated molecules, 8, 79 General base and nucleophilic catalysis of ester hydrolysis and related reactions, 5, The Gomberg Century Free Radicals 1900-2000, 36, 1 Gomberg and the Nobel Prize, 36, 59... 

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