General Concepts of Carbocations

It is well known that trivalent carbenium ions play an important role in electrophilic reactions of 7t- and -donors systems. Similarly, pentacoordinate carbonium ions are the key to electrophilic reactions of o-donor systems (single bonds). The ability of single bonds to act as o-donors lies in their ability to form carbonium ions via delocalized two-electron, three-center (2e-3c) bond formation. Consequently, there seems to be in principle no difference between the electrophilic reactions of n- and O-bonds except that the former react more easily even with weak electrophiles, whereas the latter necessitate more severe conditions. 

Trivalent ( classical ) carbenium ions contain an, vp2-hybridized electron-deficient carbon center that tends to be planar in the absence of constraining skeletal rigidity or steric interference. (It should be noted that ip-hybridized, linear acyl cations and vinyl cations also show substantial electron deficiency on carbon.) The carbenium carbon contains six valence electrons, and thus it is highly electron-deficient. The stmcture of trivalent carbocations can always be adequately described by using two-electron, two-center bonds (Lewis valence bond structures). 

Lewis s concept that a chemical bond consists of a pair of electrons shared between two atoms became the foundation of structural chemistry, and chemists still tend to name compounds as anomalous when their structures cannot be depicted in terms of such bonds alone. Carbocations with too few electrons to allow a pair for each bond came to be referred to as nonclassical, a label still used even though it is now recognized that, like many other substances, they adopt the delocalized structures appropriate for the number of electrons they contain. 

The direct observation of stable penta(or higher)-coordinate species with eight electrons around the carbon center in solutions was not reported until recent studies of long-lived nonclassical ions in superacid solvent systems. 

Neighboring group interactions with the vacant p orbital of the carbenium ion center can contribute to ion stabilization via charge delocalization. Such phenomena can involve atoms with unshared electron pairs (w-donors), C—H and C—C hyperconjugation, bent o-bonds (as in cyclopropylcarbenium ions), and 7t-electron systems (direct conjugative or allylic stabilization). Thus, trivalent carbenium ions can show 

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On the basis of my extensive study of stable, persistent carbocations, reported in more than 300 publications, I was able to develop the general concept of carbocations referred to in Chapter 9. Accordingly, in higher-coordinate (hypercoordinate) carbonium ions, of which pro-... 

The generalized concept of carbocations and electrophilic reactions indicates that the initial interactions of electrophiles with 7r-donor systems (olefins, acetylenes, aromatics) involves three-centered bond carbonium ion formation. The 7r-bond provides the bonding electron pair which interacts with the empty orbital of the electrophile. Thus in principle there is no difference between the electrophilic reactivity of... 

It was Meerwein and van Emster who, in 1922, while studying the kinetics of the rearrangement of camphene hydrochloride (1) to isobomyl chloride (2) [Eq. (5.1)], noticed that the reaction rate increased in a general way with the dielectric constant of the solvent. Further, they found that metalhc chlorides such as SbCh, SnCb, FeCb, AlCb, and SbCf (but not BCh or SiCh), as well as dry HCl (all of which promote ionization of triphenylmethyl chloride by the formation of ionized complexes), also considerably accelerate the rearrangement of camphene hydrochloride. Meerwein concluded that the conversion of camphene hydrochloride to isobornyl chloride actually does not proceed by way of migration of the chlorine atom, but by a rearrangement of a cationic intermediate. Thus, the modern concept of carbocation intermediates was born. 

As we have seen so far, there exist different methodologies to characterize hydridicities of transition metal hydrides, namely the electronic, kinetic, and thermodynamic approaches. However, up to now there is no unified view, which could combine all aspects of these different methods. It would be valuable to have in hand a general concept of hydridicity in the same way as it was established for electrophylicity in carbocation chemistry by Mayr and coworkers [19]. 

Gas-phase intracomplex substitution in (R)-(- -)-l-arylethanol/CHs OH2 adducts. It is well established that bimolecular Sn2 reactions generally involve predominant inversion of configuration of the reaction center. Unimolecular SnI displacements instead proceed through the intermediacy of free carbocations and, therefore, usually lead to racemates. However, many alleged SnI solvolyses do not give fully racemized products. The enantiomer in excess often, but not always, corresponds to inversion. Furthermore, the stereochemical distribution of products may be highly sensitive to the solvolytic conditions.These observations have led to the concept of competing ° or mixed SNl-SN2 mechanisms. More recently, the existence itself of SnI reactions has been put into question. ... 

On the basis of the study of carbocations by direct observation of long-lived species, it became increasingly apparent that the carbocation concept is much wider than previously realized and necessitated a general definition.21 Therefore, such a definition was offered based on the realization that two distinct, limiting classes of carbocations exist (Figure 3.1). 

There was no perception of reactive intermediates in the mind of most German chemists. The only thing that mattered in a reaction was the starting material and the product. In line with this, the carbocation concept of Meerwein had not yet found general acceptance. To postulate such a novel reactive intermediate back in those days could easily have put an academic career at risk. 

The energy required to proceed from reactants to products is AG, the free energy of activation, which is the energy at the transition state relative to the reactants. We develop the theoretical foundation for these ideas about reaction rates in Section 3.2. We first focus attention on the methods for evaluating the inherent thermodynamic stability of representative molecules. In Section 3.3, we consider general concepts that interrelate the thermodynamic and kinetic aspects of reactivity. In Section 3.4, we consider how substituents affect the stability of important intermediates, such as carbocations, carbanions, radicals, and carbonyl addition (tetrahedral) intermediates. In Section 3.5, we examine quantitative treatments of substituent effects. In the final sections of the chapter we consider catalysis and the effect of the solvent medium on reaction rates and mechanisms. 

Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions. Olah, G.A. 

Frank Whitmore in the United States in the 1930s in a series of papers, generalized these concepts to include many other organic reactions. Carbocations, however, were generally considered to be unstable... 

Mechanism. The mechanism of alkylation and of other related Friedel-Crafts reactions is best explained by the carbocation concept. The alkylation of benzene with isopropyl chloride may be used as a general example ... 

Mechanism. The proven acidity of the effective alkane isomerization catalysts suggests that carbocations are involved in acid-catalyzed alkane isomerization. Such a mechanism was first proposed by Schmerling and coworkers54 on the basis of the pioneering ideas of Whitmore55 for the skeletal isomerization of alkanes and cycloalkanes in the presence of aluminum chloride and a trace of olefin or other promoter. Subsequently these concepts were used to explain the mechanism of the acid-catalyzed isomerizations in general. 

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