Alkanes protolysis

Further evidence for the pentacoordinate carbonium ion mechanism of alkane protolysis was obtained in the H-D exchange reaction observed with isobutane. When isobutane is treated with deuterated superacids (DS03F SbF5 or DFiSbFs) at low temperature (—78°C) and atmospheric pressure, the initial hydrogen-deuterium exchange is observed only at the tertiary carbon. Ionization yields only deuterium-free t-butyl cation and Recovered isobutane from the reaction mixture shows at low... 

Protoiiation (and protolysis) of alkanes is readily achieved with superacids. The protonation of methane itself to CH5, as discussed earlier, takes place readily. 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

Protolysis can involve not only C—H but also C—C bonds [Eq. (1.23)] this explains why alkanes can be directly cleaved protolytically by superacids, which is of significance, for example, in hydrocleaving heavy oils, shale oil, tar-sand bitumens, and even coals ... 

Treating alkanes with superacids such as FS03H-SbF5, Olah and coworkers observed both C—H and C—C bond protolysis 303... 

In 1946 Bloch, Pines, and Schmerling17 observed that w-butane (1) isomerizes to isobutane (2) under the influence of pure aluminum chloride only in the presence of HC1. They proposed that the ionization step takes place through initial protolysis of the alkane as evidenced by formation of minor amounts of hydrogen in the initial stage of the reaction [Eq. (5.1)]. 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

This reaction is much faster than the carbon-carbon cleavage in neopentane, despite the initial formation of secondary carbenium ions. Norbomane is also cleaved in a fast reaction, yielding substituted cyclopentyl ions. Thus, protonation of alkanes induces cleavage of the molecule by two competitive ways (i) protolysis of a C—H bond followed by /3-scission of the carbenium ions and (ii) direct protolysis of a C—C bond yielding a lower-molecular-weight alkane and a lower-molecular-weight carbenium ion. 

The formation of C6 and C7 acids along with some ketones was reported in the reaction of isopentane, along with methylcyclopentane and cyclohexane with CO in HF-SbF5 at ambient temperatures and atmospheric pressure.406 Yoneda et al.407 have also found that other alkanes can be carboxylated as well with CO in HF-SbF5. Tertiary carbenium ions, which are produced by protolysis of C—H bonds of branched alkanes in HF-SbF5, undergo skeletal isomerization and disproportionation before reacting with CO. Formation of the tertiary carboxylic acids in the... 

The hydrogen peroxonium ion may be considered as an incipient OH+ ion capable of electrophilic hydroxylation of single (effect reactions similar to such previously described electrophilic reactions as protolysis, alkylation, chlorination (chlorolysis), and nitration (nitrolysis). 

Since the early 1960s, superacids have been known to react with saturated hydrocarbons to yield carbocations, even at low temperature [41]. This discovery initiated extensive studies devoted to electrophilic reactions and conversions of saturated hydrocarbons. Thus, the use of superacidic activation of alkanes to their related carbocations allowed the preparation of alkanecarboxylic acids from alkanes themselves with CO. In this respect, Yoneda et al. have found that alkanes can be directly carboxylated with CO in an HF-SbFs superacid system [42]. Tertiary carbenium ions formed by protolysis of C-H bonds of branched alkanes in HF-SbFs undergo skeletal isomerization and disproportionation prior to reacting with CO in the same acid system to form carboxylic acids after hydrolysis (eq. (9)). 

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

For the reaction of ozone with alkanes under superacidic conditions, two mechanistic pathways may be considered. The first is the formation of an alkylcarbenium ion via protolysis of the alkane prior to quenching of the ion by ozone (Scheme 6.26, route a). Alkylcarbenium ions may also be generated via initial transformation of the alkane to an alcohol followed by protonation and ionization (Scheme 6.26, route b). There have already been a number of reports of ozone reacting with alkanes to give alcohols and ketones. °" In both cases, intermediary alkylcarbenium ions would then undergo nucleophilic attack by ozone as mentioned earlier. 

The ease of protolysis of tertiary (and to a lesser degree secondary) aliphatic nitro compounds explains in part the complex reaction mixtures obtained in mixed acid nitrations of alkanes. 

The rapid protonation-deprotonation process can be monitored by using either a deuterated alkane or a deuterated superacid. When the reaction is carried out in the presence of carbon monoxide, it is also possible to compare the isotope exchange process with the protolysis reaction which produces a smaller alkane and a smaller carbenium ion which is trapped by CO, forming stable oxocarbenium ions that are easily converted into esters for analysis (e.g.. Scheme 2). 

G. A. Olah, Y. Halpem, J. Shen, Y. K. Mo, Electrophilic reactions at single bonds. XII. Hydrogen-deuterium exchange, protolysis (deuterolysis), and oligocondensation of alkanes with superacids, J. Am. Chem. Soc., 1973, 95, 4960-4969. 

V. ACID-CATALYZED CLEAVAGE (CRACKING) REACTIONS OF ALKANES (p-CLEAVAGE vs C—C BOND PROTOLYSIS)... 

When hydrogen peroxide is protonated in superacid media, hydrogen peroxonium ion (HaOa ) is formed. Certain peroxonium salts have been well characterized and even isolated as stable salts ". Hydrogen peroxonium ion is considered as the incipient OH ion, a strong electrophile for electrophilic hydroxylation at single (cr) bonds of alkanes. The reactions are thus similar to those described previously for electrophilic protonation (protolysis), alkylation and formylation. 

The polarity of metal-carbon and metal-hydrogen bonds allows protolysis to occur with alcohols under moderate conditions. Consequently carbanionic metal alkyls react with alcohols to give alkoxides, in excellent yields. The reaction assumes special importance as it forms the basis for the calorimetric measurements of a large number of metal-alkyl bond dissociation energies. This preparative route tends to be more convenient owing to the volatility of the hberated alkane side-products. Organometal... 

---