Hydrocarbons protolytic

As expected, superacids were found to be extremely effective in bringing about protolytic transformations of hydrocarbons. 

This realization led me to study related possible intermolecular electrophilic reactions of saturated hydrocarbons, Not only protolytic reactions but also a broad scope of reactions with varied electrophiles (alkylation, formylation, nitration, halogenation, oxygenation, etc.) were found to be feasible when using snperacidic, low-nucleophilicity reaction conditions. 

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. 

In another experiment tritiated adamantane diazirine fixed to the hydrocarbon core of a membrane gave rise to carbene insertion into the catalytic subunit of ATP-ase. After protolytic degradation adjacent areas of the original structure became evident (80JBC(255)860). 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the <7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion ... 

Consequently, the reverse reaction of protolytic ionization of hydrocarbons to carbenium ions—that is, the reduction of carbenium ion by molecular hydrogen — can be considered as alkylation of H2 by the electrophilic carbenium ion through a pentacoordinate carbonium ion. Indeed, Hogeveen and Bickel have experimentally proved this point by reacting stable alkyl cations in superacids with molecular hydrogen [Eq. (5.7)]. 

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

Among the hydrocarbons the weakest protolytes are the paraffins. The branching of the carbon chain reduces their strength as acids and enhances their strength as bases, since the stability of potential carbanions and carbonium ions changes in opposite directions. 

The interaction between a and -it electrons of single and multiple bonds (or the -tt electrons in the aromatic ring) is usually interpreted (Baker, 1952) as a, -conjugation (hyperconjugation). In this process the hydrocarbons become stronger protolytes. For example, the acidic and basic behaviour of propene and toluene are more pronounced than the corresponding properties of ethylene and benzene. 

According to Bronsted (1928), hydrocarbons cannot participate in protolytic reactions. The incorrectness of this conclusion (which we have already indicated above) and the many departures from the quantitative theory of Bronsted (Izmailov, 1959) show that the theory is inadequate for the description of protolytic reactions. 

Protio and dipolar aprotic solvents, rates of bimolecular substitution reactions in, 5, 173 Protolytic processes in H2O-D2O mixtures, 7, 259 Pyrolysis, gas-phase, of small-ring hydrocarbons, 4, 147... 

Abstract The ab initio pseudopotential plane wave DPT simulation of the structure and properties of zeolite active sites and elementary catalytic reactions are discussed through the example of the protonation of water and the first step in the protolytic cracking mechanism of saturated hydrocarbons. 

Cracking of small saturated hydrocarbons, catalyzed by zeolites, can proceed via two mechanisms, both involving carbocations the bimolecular chain reaction, which involves carbenium ions that are further transformed by / -scission, and the unimolecular protolytic mechanism, involving alkanium ions that are formed by the direct protonation of the alkane by the Br0nsted acid OH groups of the catalyst. This latter mechanism, originally proposed by Haag and Dassau, is the predominant one at about 800 K in medium-pore zeolites, like HZSM-5, which favor monomolecular reactions. While rela-... 

For hydrocarbon transformation on H-ZSM-5, it has been generaUy accepted that the carbenium ions are the common intermediates, but the initiation of the reaction might vary. Possible initiation routes for the reaction include a direct protonation of the n-heptane molecule on Bronsted acid sites to produce a carbonium ion (protolytic route) [9], according to ... 

The first evidence for the protonation of alkanes under highly acidic (superacid) conditions was independently reported by Olah and Lukas, as well as by Hogeveen and coworkers. " Protolytic reactions of hydrocarbons in superacid media were interpreted by Olah as an indication of the general electrophilic reactivity of covalent C-H and C-C single bonds of alkanes and cycloalkanes. The reactivity is due to the donor ability of the a-bond electron pairs via a three-center, two-electron (3c-2e) bond formation and follows the trend tertiary C-H > C-C > secondary C-H primary C-H. The transition state for protolytic ionization of hydrocarbons was presumed to be linear. It was later suggested that such 3c-2e interactions in carbocations generally tend to be nonlinear (even in stericaUy crowded cases) in nature (see also Chapter 1) [Eqs. (6.3) and (6.4)]. That is, such interactions are similar to transition states proposed for frontside Se2 reactions. 

The direct reduction of SbFs in the absence of hydrocarbon by molecular hydrogen necessitates, however, more forcing conditions (50 atm, high temperature), which suggests that the protolytic ionization of alkanes proceeds probably via solvation of the pentacoordinate carbocation by SbFs and concurrent ionization-reduction [Eq. (6.13)]. 

El Tanany et al. (ref. 22) have investigated the n-heptane hydroconversion at 428 K and 1013 mbar on H-erionite. According to Haag et al. (ref. 26) and their results the formation of C3/C4 species should take place via a pentacoordinated carbonium ion by protolytic cracking. In a following chain process higher hydrocarbons may result. 

Novel oxidations of hydrocarbons in superacids with ozone or hydrogen peroxide have been investigated. Proto-nated ozone (O H) or hydrogen peroxide (H3OJ) attacks the single a-bond, resulting in oxygen insertion. These can be followed by protolytic transformation, such as the conversion of isobutane into acetone and methyl alcohol. 

The crucial step in acid-catalyzed conversions of hydrocarbons is the formation of the intermediate trivalent or classical sp hybridized carbocation (car-benium ion). In the case of saturated hydrocarbons, this is interpreted by the interaction of the proton of the superacid and the bonding electron pair of the C—H a bond (a similar interaction between the proton and the C—C a bond would result in the cleavage of the carbon-carbon bond). This is based on the concept of a-basicity developed by Olah (65), which describes the ability of a bonds to share their bonded electrons with electrophiles. A hypervalent, pentacoordinate non-classical carbocation (carbonium ion) is formed, which possesses a three-center, two-electron (3c-2e) bond. This is transformed to the classical trivalent carbocation by the loss of hydrogen, that is, protolysis (protolytic cleavage) of the carbon-hydrogen bond occurs. The process is illustrated by the conversion of hexane to yield the 3-hexyl cation through the pentacoordinate carbonium ion (1) (eq. 45). 

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