Reactions of Carbonium Ions

Triisopropylcarbinyl chloride, for example, is hydrolyzed in acetone-water solution seven times as fast as J-butyl chloride. (See p. 88.) 

After a carbonium ion has been formed, its reactions depend largely upon the structure of the ion, the nature of the groups attached to it, and the medium in which the reaction is being carried out. Four courses of reaction are open.2 

This mode of reaction is also characteristic of solvolytic and first-order nucleophilic substitution reactions ( 1). They will be discussed in Chapter 4. 

This type of reaction, called an elimination reaction, will be discussed in Chapter 5. 

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Perhaps the most important single function of the solution environment is to control the mode of decomposition of reaction intermediates and hence the final products. This is particiflarly true in the case of electrode reactions producing carbonium ion intermediates since the major products normally arise from their reaction with the solvent. It is, however, possible to modify the product by carrying out the electrolysis in the presence of a species which is a stronger nucleophile than the solvent and, in certain non-nucleophilic solvents, products may be formed by loss of a proton or attack by the intermediate on further starting material if it is unsaturated. The major reactions of carbonium ions are summarized in Fig. 6. 

The reactions of carbonium ions occur via transition states having precise stereochemistry in which the electron pair of the attacking nucleophile must be col inear with the empty p-orbital of the electron-poor carbon atom. Thus, powerful stereoelectronic effects control these reactions. 

As indicated in Table 5, the [1,2] reaction of carbonium ions is allowed and is energetically favored (Table 9). The migration of alkyl groups to... 

Condensation reactions of carbonium ions compete with sulfonation and their frequency is increased with increasing acidity. Carbon-carbon bonds are formed most commonly when the benzylium ions react with the weakly nucleophilic 1- and 6-(or 5-)positions of other phenylpropane units (Fig. 

The catalyst diagnostician can justifiably inquire if the carbonium ion participation and migration by some push-pull mechanism is the only mechanism on which catalyst construction can be based, or do we have some alternatives. Up to the present no alternatives have been published, however, the relatively simple mechanism described above, can be modified in that the initial reaction of carbonium ion formation can be also looked upon as a two step process, whereby a paraffin is first dehydrogenated and the resulting olefin promptly forms a carbonium ion. The overall result is the same. 

C. Reactions of Carbonium Ions in the Gas Phase at Normal Pressure. ... 

The next Section describes the results obtained from the study of the reactions of carbonium ions directly formed from the decay of tritiated molecules in organic systems at normal pressure, while Section V deals... 

C. Reactions of Carbonium Ions in the Oas Phase at Normal Pressure 1. Reactions of CT ions with alkanes The reactions of tritiated methyl ions from the decay of methane-<4 were investigated in pure CH4, and in mixtures of CH4 and CaHg by Cacace et al. (1966), whose results are illustrated in Table 12. It was... 

In contrast with the mechanisms proposed for the formation of neutral species from the gas phase, or liquid phase reactions of carbonium ions, the formation of the neutral radical (4) was attributed to the intervention of a direct neutralization process. Such discrepancy can be easily explained by taking into account the different rate of the neutralization process. This can be expected to be extremely rapid in the semiconductor lattice of the solid aromatic hydrocarbon, whilst it is known to be relatively insignificant in the gas phase, owing to the competition of exceedingly fast ion-molecule reactions. It appears that the technique introduced by Lloyd et al. affords a unique tool for producing free... 

The Schmidt reactions discussed above involve the reaction of a carbonium ion with an azide. There has been a report of the reaction of carbonium ions, generated either from triethyloxonium tetra-... 

The commonly accepted mechanism of this alkylation is based on the study of many related reactions and involves in step (3) a reaction of carbonium ions that we have not previously encountered. 

But this picture of the reaction is not satisfactory, and for two reasons. First, to account for the complete stereospecificity of addition, we must assume that attack at the bottom face of the cation is not just preferred, but is the only line of attack conceivable, but—especially in view of other reactions of carbonium ions (Sec. 14.13)—not likely. Then, even if we accept this exclusively bottom-side attack, we are faced with a second problem. Rotation about the carbon-carbon bond would convert cation I into cation II bottom-side attack on cation II would... 

One or two examples of the use of these concepts will illustrate the ideas and help to formulate appropriate rate equations. The acidic catalysts, such as silica-alumina, can apparently act as Lewis (electron acceptor) or Br0nsted (proton donor) acids, and thus form some sort of carbonium ion from hydrocarbons, for example. Note the analogy between this hydrogen deficient entity and a free radical. However, the somewhat different rules for the reactions of carbonium ions apply from organic chemistry and permit miquantitative predictions of the products expected see Table 2.1-2 from Oblad, et al. [11 ]. 

The reactions to be described in this chapter have in common the formal involvement of even-electron intermediates having unfilled orbitals of low energy. The most familiar of these intermediates are carbonium ions. Sections 8.4 and 8.5 contain examples of fragmentation and rearrangement reactions of carbonium ions that are of synthetic value the discussion of these reactions supplements the mechanistically oriented discussion presented in Part A, Chapter 5. Also important are the neutral divalent carbon and monovalent nitrogen species, carbenes and nitrenes, respectively. 

This mechanism explains the observed formation of the more highly substituted alcohol (Markownikoft s rule). A number of other points must be considered in order to provide a more complete picture of this mechanism. Is the protonation step reversible Is there a discrete carbonium ion, or does the nucleophile become involved before carbonium ion formation is complete Can other reactions of carbonium ions, such as rearrangement, compete with capture by water ... 

Brouwer, D. M., and H. Hogeveen The Importance of Orbital Orientation as a Rate-Controlling Factor in Intramolecular Reactions of Carbonium Ions. Rec. trav. chim. Pays-Bas 89, 211 (1970). 

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