Carbocations controversy

The "classical" - "non-classical" carbocation controversy concerned the Wagner-Meerwein rearrangement of norbomyl systems ... 

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

Clearly there was no lack of devoted adversaries (perhaps a more proper term than enemies) on both sides of the norbornyl ion controversy. It is to their credit that we today probably know more about the structure of carbocations, such as the norbornyl cation, than about most other chemical species. Their efforts also resulted not only in rigorous studies but also in the development or improvement of many techniques. Although many believe that too much effort was expended... 

The most studied hypercoordinate carbocation is the 2-norbornyl cation, around which the nonclassical ion controversy centered (Chapter 9). 

The existence of trivalent silicenium cations as reactive species in solution is more controversial. Many early attempts to demonstrate the solution-phase existence of stable silicenium ions by using techniques analogous to those successfully applied to carbocation formation failed.34-36 Other reports of attempts... 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

The essential features of the mechanism for aliphatic nucleophilic substitution at tertiary carbon were established in studies by Hughes and Ingold." ° However, as chemists probed more deeply, the problems associated with the characterization of borderline reaction mechanisms were encountered, and controversy remains to this day about whether these problems have been entirely solved." What is generally accepted is that ferf-butyl derivatives undergo borderline solvolysis reactions through a ferf-butyl carbocation intermediate that is too unstable to diffuse freely through nucleophilic solvents such as methanol and water. The borderline nature of substitution reactions at tertiary carbon is exemplihed by the following observations. 

In summary, controversy concerning the mechanism for solvolysis at tertiary carbon is semantic and can be avoided by making a clear distinction between (a) nucleophilic solvation, which is stabilization of the transition state for stepwise solvolysis through carbocation or ion pair intermediates by charge-dipole interactions with nucleophilic solvents (Scheme 2.8A) and, (b) nucleophilic solvent participation, which is stabihzation of the transition state for concerted solvolysis by formation of a partial covalent bond to the solvent nucleophile (Scheme 2.8B). 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

As indicated in note 5, there is still controversy over the existence of primary carbocations in solution. For cases when some participation by neighboring hydride or alkyl in the formation of secondary or tertiary carbocations has been suggested see, for example, (a) V. J. Shiner, Jr., and J. 

The concept of an intermediate phenonium ion was, at first, controversial, and its chief detractor was H. C. Brown.25 Although 3-phenyl-2-butyl tosylate showed the stereochemical behavior expected if an intermediate phenonium ion were formed, it did not, in his opinion, show the rate acceleration that should attend anchimeric assistance to ionization of the tosylate.26 Brown said that the stereochemical results could be accounted for by invoking rapidly equilibrating open carbocations (15). According to his explanation, ionization of the tosylate... 

More recently, however, it has been suggested that carbocation intermediates might result from the addition of an amminium ion (through the aromatic ring) to a jr-bond114. Recently, this controversy seems to have been definitively settled Most hole-catalyzed Diels-Alder reactions actually do involve cation radicals115, except for the addition of tetracyanoethylene to electron-rich alkenes116. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

The fundamental step in acid-catalyzed hydrocarbon conversion processes is the formation of the intermediate carbocations. Whereas all studies involving isomerization, cracking, and alkylation reactions under acidic conditions (Scheme 5.1) agree that a trivalent carbocation (carbenium ion) is the key intermediate, the mode of their formation of this reactive species from the neutral hydrocarbon remained controversial for many years. 

Saunders and coworkers have developed a powerful tool which makes it possible to discriminate between nonclassical ions and rapidly equilibrating ions. It seems to have the capacity to resolve much of the controversy about the structures of nonclassical carbocations. The method is based upon a combination of equilibrium isotope effects and nmr spectroscopy and is also used for accurate determination of different types of equilibrium isotope effects. Below the essentials and applications of this new tool will be reviewed. 

Avoid any controversy by calling all cations where the charge is on a carbon atom carbocations. 

This is the synthesis of a somewhat controversial food preservative called BHT. It is an antioxidant that has been put in packaging to keep foods from becoming rancid from air oxidation. A balanced reaction would require two molecules of the alkene for each of the starting aromatic. The aromatic ring is relatively electron rich with two donors attached to it. With sulfuric acid present, the medium is definitely acidic. The first step is to generate the excellent electrophile needed for electrophilic aromatic substitution. For simplicity, let s symbolize sulfuric acid as H-A. The Markovnikov addition of a proton to isobutylene gives the tert-butyl carbocation, an excellent electrophile. 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

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