Hydrocarbons carbocations

Figure 2. The most stable all-hydrocarbon carbocations with their pKR+ values...

The pKR+ value for the parent tri(l-azulenyl)methyl cation (2a+) is 11.3. Hydrocarbon-based carbocations, which comprise only of carbon and hydrogen, are generally very reactive species. Some extremely stable hydrocarbon carbocations, which exist even under basic conditions, were reported in the literature (5). However, most of these examples are cyclic cations, such as cyclopropenylium or tropylium ions (Figure 8). The tropylium ion 8+ annelated to three bicyclo[2.2.2]octane units is one of the most stable hydrocarbon-based carbocation ever reported (9). 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

It was the study of hypercarbon-containing nonclassical carboca-tions that allowed us to firmly establish carbon s ability in a hydrocarbon system to bind simultneously with five (or six or even seven) atoms or groups. It should be emphasized that carbocations represent... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

Chemists make compounds and strive to understand their reactions. My own interest lies in the chemistry of the compounds of the elements carbon and hydrogen, called hydrocarbons. These make up petroleum oil and natural gas and thus are in many ways essential for everyday life. They generate energy and heat our houses, fuel our cars and airplanes and are raw materials for most manmade materials ranging from plastics to pharmaceuticals. Many of the chemical reactions essential to hydrocarbons are catalyzed by acids and proceed through positive ion intermediates, called carbocations. 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

More often than what has been mentioned above regarding the cyclization of paraffins over the platinum catalyst, the formed olefin species reacts with the acid catalyst forming a carbocation. Carbocation formation may occur by abstraction of a hydride ion from any position along the hydrocarbon chain. However, if the carbocation intermediate has the right configuration, cyclization occurs. For example, cyclization of 1-heptene over the alumina catalyst can occur by the following successive steps ... 

The new carbocation may experience another beta scission, rearrange to a more stable carbonium ion, or react with a hydrocarbon molecule in the mixture and produce a paraffin. 

An isopropyl carbocation cannot experience a beta fission (no C-C bond beta to the carbon with the positive charge).It may either abstract a hydride ion from another hydrocarbon, yielding propane, or revert back to propene by eliminating a proton. This could explain the relatively higher yield of propene from catalytic cracking units than from thermal cracking units. 

The synthetic procedures for isolation of the salt appear to be rather simple. First, one prepares a solution in which the carbocation and carbanion coexist free from any combination reactions. Then, the hydrocarbon cation-anion salt is isolated after separation of the concomitant inorganic salt and evaporation of the solvent. For the purification of the crude salt recrystallization or reprecipitation with proper solvents is used. 

Sufficient stability of the hydrocarbon ions, as the salt or in the solution, is an obvious prerequisite for these procedures, and, in practice, selecting or designing the stable ions and choosing a proper solvent are tasks of primary importance. As an ordinary stability index for the ions, thermodynamic scales referred to the water molecule, i.e. p CR+ and pKa values, are chosen for the carbocation and carbanion, respectively. 

In the course of the salt synthesis, it was found that a hydrocarbon [3-2], which was formed by an unfavourable cation-anion combination reaction, dissociates into the original carbocation and carbanion in a polar aprotic solvent (Okamoto et ai, 1985) (1). This was the first example of ionic dissociation of the carbon-carbon a bond in genuine hydrocarbons, although a few cases of heterolytic dissociation of carbon-carbon tr bonds had been reported by Arnett (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985) for compounds bearing cyano and nitro groups, e.g. [4-6] and [5-6] as in (2). 

Structural factors necessary for carbocation and carbanion stability are discussed briefly. The mechanistic problems of dissociation of the carbon-carbon a bond are then discussed, and finally the physicochemical nature of hydrocarbon salts and the related novel hydrocarbons. 

The Stability of hydrocarbon ions is discovered intuitively by observing whether the hydrocarbon ion can be isolated as a salt, for example, a sodium salt of the carbanion or a tetrafluoroborate salt of the carbocation. Conversely, a single hydrocarbon ion produced in the gas phase is obviously an unstable and short-lived species. Thus, many of the aliphatic carbocations in the gas phase are merely observable species but are not usable for synthesis. 

Three significant hydrocarbon ions, [S" ], [9 ] and [10 ], should be mentioned as additional examples of the formation of stable salts of the hydrocarbon ions. The triphenylmethylium ion [8" ], as a prototypical aromatic carbocation, reacts with the chloride ion to give the chloride, a... 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

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