Carbonium ions Subject

In the presence of strong acid, formic acid decomposes to water and carbon monoxide. In the process, reactive intermediates form which are capable of direct carboxylation of carbonium ions. Since many carbonium ions are readily generated by the reaction of alcohols with strong acid, the process of elimination and carboxylation can be conveniently carried out in a single flask. The carbonium ions generated are subject to the... 

In order to overcome these difficulties a number of other kinds of supporting evidence for carbonium ion intermediates have been used. Even so, the status of many nucleophilic displacement reactions remains in doubt, and this type of reaction has probably been the subject of more mechanistic man hours of work than any other.211-212... 

Reaction with solvent - The solvent influences the course of cationic reactions not only through its dielectric constant, but also because many substances used as solvents are far from inert in these reactions [22, 23]. Although much more experimental material is required before a full treatment of the subject becomes possible, at least one example, the cationic polymerisation of styrene in toluene, is amenable to quantitative discussion. Experiment shows that polymerisation is rapid and complete, the molecular weight is low and the polymer contains para-substituted rings which are almost certainly tolyl endgroups [22]. Theoretically, a polystyryl carbonium ion can react with toluene in six different ways, only two of which (a.l and b. 1 below) can lead to tolyl endgroups in the first case the tolyl group is at the end of the terminated chain, in the second it is the start of a new chain. The alternative reactions can be represented as follows... 

Low basicity, even in combination with a reasonably stable carbonium ion intermediate, will not permit general acid catalysis if the leaving group is not sufficiently good. Thus, hydrolysis of benzaldehyde methyl S-phenyl thioacetals [67] is not subject to general acid catalysis (Fife and Anderson, 1970) even though... 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

Lewis acids readily isomerize both 1,3-dioxolanes and 1,3-oxathiolanes in ether solution. The reaction proceeds by coordination with the oxygen atom in the latter case since 1,3-dithiolanes do not isomerize under the same conditions. With trityl carbonium ion, an oxidative cleavage reaction takes place as shown in Scheme 6. Hydride extraction from the 4-position of 2,2-disubstituted 1,3-dioxolanes leads to an a-ketol in a preparatively useful reaction. 1,3-Oxathiolanes are reported to undergo similar cleavage but no mention of products other than regeneration of the ketone has been made (71CC861). Cationic polymerization of 1,3-dioxolane has been initiated by a wide variety of proton acids, Lewis acids and complex catalytic systems. The exact mechanism of the polymerization is still the subject of controversy, as is the structure of the polymer itself. It is unclear if polymerization... 

Assuming a single-ion (carbonium ion) propagation step and based on scavenger studies with ammonia and amines and on electrical conductance measurements, rate constants for the propagation reaction (ion-molecule reaction) can be estimated. These estimates are free of the correction that one has to apply (or ignore) for chemically generated ion pair ionic polymerization, but are subject to other limitations imposed by different assumptions in the treatment of the data. 

The scope of this review is a detailed survey of reactions proceeding through vinyl cations and an attempt of a systematic definition of the properties of these intermediates with reference to those of saturated carbonium ions. Although attention will be particularly devoted to linear cations, bridged unsaturated species will be considered as alternative structures of vinyl cations rather than as a distinct type of reactive intermediates. The 77--complex terminology (Dewar, 1949) widely abused in the past decades to indicate especially cyclic cations and recently reassessed by Banthorpe (1970) will be generally avoided. The most recent Btudies not covered by published reviews on the subject (Rappoport, 1969 Richey and Richey, 1970 Richey, 1970 Hanack, 1970) are discussed in greater detail than others and data are collected in pertinent Tables. 

Aryl cations are related to vinyl cations insofar as they are disubsti-tuted carbonium ions, although their chemistry is more related to the aromatic properties of the nucleus. This subject has recently been reviewed (Richey and Richey, 1970). 

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. 

The nature and role of carbonium ions as intermediates in molecular rearrangements has been the subject of a vast literature including many reviews [i]. Reduced to its simplest terms, the overall reaction may be represented as ... 

This subject has already arisen in connection with the stabilities of olefinic bonds at various positions in the steroid nucleus (p. 14) and as a side-reaction in certain hydrogenation processes. Rearrangements of allylic systems will be covered in Chapter 9, and photolytic reactions in Chapter ii. We are concerned here with the double-bond migrations which involve carbonium ions generated by protonation of simple olehns. 

Others contend that the stability can be more conventionally related to a resonance effect via the cyclopentadienyl rings and the metal (XXI) 103, 240). In either case, the positive charge which would develop on the exocyclic carbon atom would be delocalized considerably, which would aid the carbonium ion to form more easily. The differences between these interpretations are subtle but distinct, and the topic will no doubt be the subject of considerable future research. 

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