Carbonium ions defined

Whereas the differentiation of trivalent carbenium and pentacoor-dinated carbonium ions serves a useful purpose in defining them as limiting cases, it should be clear that in carbocationic systems there always exist varying degrees of delocalization. This can involve participation by neighboring -donor atoms, 7r-donor gronps, or [Pg.148]

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Evidence that the actual methylation of the anion can be divided into SnI, Eq. (3), and Sx2 types, Eq, (4), is provided by a whole series of investigations. " The terms S l and 8 2 must be taken to mean reactions with, respectively less or greater nucleophilic participation of the anion in the transition state. The importance of oriented ion pairs" in the solvents of low polarity frequently used in reactions involving diazomethanc, e.g., the ions formed by a diazoalkane and benzoic acid in ether, should be emphasized. The expression oriented ion pair means that, because of insufficient solvation, the ions are not individually solvated but exist as ion pairs within a solvent cage. The orientation within the ion pair is defined electrostatically, and this orientation fixes the path for the productdetermining step. Several indications (cf, foo otes 22-24) in the literature indicate the occurrence of carbonium ions and oriented ion pairs in Broensted-type equilibria of the type of Eq. (2). 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

Carbonium ions are well-defined transition states in many reactions. Indeed, one of the earliest applications of MM method to the reactivity problem was concerned with carbonium ions. At present, only the Schleyer force field (26b) and its predecessor (253) are capable of handhng carbonium ions, although the parameterization principle used earlier can be readily improved upon to the present standards. Schleyer s measure of steric strain in carbonium ions, Ajtrain (the difference in steric energies of free carbonium ion and its... 

All the amendments to Bredt s rule that have been presented in the past decade have been more or less violated. One reason for these failures is that the past rules ignored the strain in parts of the molecule other than at the bridgehead double bond. Schleyer defines the strain at the bridgehead double bond, or olefin strain (OS), as the difference in strain between the olefin and the parent hydrocarbon, analogous to for carbonium ions (293). He... 

A carbonium ion is defined by this theory as a molecule containing a carbon atom which is deficient two electrons in its outer shell, and consequently carries a positive charge. The carbonium ion is highly reactive, and can undergo several transformations and reactions, among which are the following ... 

The authors presented no detailed discussion of the possible course of this reaction, beyond proposing a carbonium ion mechanism of the type shown in Eq. ( HW5) without precisely defining the role of the catalyst. 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required. ... 

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41... 

Winstein and coworkers14 have introduced a variable termed the driving force, L, attributable to participation of a neighboring group in the rate-determining step, which is a measure of the decrease in free energy attended by the conversion of an open carbonium ion (VIII) to the cyclic carbonium ion (VII). The driving force was defined as... 

For the most part we are concerned with acid catalysed reactions in which carbonium ions are the key intermediates. Carbonium ions are formed by the interaction of a feed molecule with an acid site on a catalyst surface. Carbonium ion chemistry is well defined and has several features relevant to the production of light olefins. These are illustrated in Figure 2.6. 

The immediate product from migration of the angular methyl group to Cp ) is a Cps)-carbonium ion (25), the fate of which varies according to the compound. Solvolysis of a secondary i7j -tosylate, or acid-catalysed dehydration of the i7j3 alcohol, leads to a mixture of olefins containing the isomer 26) as the major product, with minor amounts of j i3(i4) 27) and other olefins. An elaborate analysis by Johns [126] of the olefins obtained by the boric add-catalysed dehydration of oestradiol 3-methyl ether at high temperatures led to the characterisation of at least six isomeric olefins in the mixture. The precise mechanism of the action of boric acid has not been defined. It may involve thermal decomposition of a borate ester or merely the expulsion of the protonated... 

Many other substituent parameters have been developed to improve correlations for specific types of reactions. Brown and Okamoto (1958) developed substituent constants (electrophilic reactions based on hydrolysis rates of meta- and para-substituted 2-chloro-2-phenylpropanes (CPP), which react by electrophilic carbonium ion intermediates. Formation of these intermediates is facilitated by high electron density at the reactive carbon (i.e., by meta- or para-electron donors). The parameter meta substituents), k0 is the hydrolysis rate constant for unsubstituted CPP, and k is the rate constant for a substituted CPP. 

Again the conformational requirements are well defined for effective stabilization of the carbonium ion it is necessary for the empty p-orbital to be coplanar with the G-Si bond18. 

Since cation-radical formation in the chemisorption of hydrocarbons has not previously been considered in the catalytic literature, the nature, reactions, and mechanism for formation of such species should be of considerable importance to the elucidation of catalytic reaction mechanisms particularly in view of the fact that Webb (20) has found spectral evidence for the formation of species other than carbonium ions from butene-2 adsorbed on silica-alumina. It is not possible at the present time to define either the role of cation-radicals in acid catalysis or the chemical nature of the electrophilic surface sites involved in their formation. 

---