Magic Acid alkanes

Both methylcyclopentane and cyclohexane were found to give the methylcyclo-pentyl ion, which is stable at low temperature, in excess superacid.22 When alkanes with seven or more carbon atoms were used, cleavage was observed with formation of the stable ferf-butyl cation 4. Even paraffin wax (see Section 2.2.2.2 on Magic Acid) and polyethylene ultimately gave the ferf-butyl cation 4 after complex fragmentation and ionization processes. 

The reaction of branched-chain alkanes with hydrogen peroxide in Magic Acid-S02C1F solution has been carried out with various ratios of alkane and hydrogen peroxide and at different temperatures.602 Some of the results are summarized in Table 5.35. 

Because neither hydrogen peroxide nor Magic Acid-S02C1F alone led to any reaction under the conditions employed, the reaction must be considered to proceed via electrophilic hydroxylation. Protonated hydrogen peroxide inserts into the C H bond of the alkane. A typical reaction path is as depicted in Scheme 5.60 for isobutane. 

The mechanism was substantiated by independent treatment of alkane hydroperoxides with Magic Acid.603 Similarly, Baeyer-Villiger oxidation of several ketones in the presence of H2O2 and superacids gave similar product compositions. 

Table 5.37. Products of the Reaction of Branched Alkanes with Ozone in Magic Acid-S02C1F at 78 C599,642...

Although isobutane does not give any oxidation products in the absence of Magic Acid under the same low-temperature ozonization conditions, it was not possible for the authors to determine642 whether formation of intermediate oxidation products, such as alcohols, plays any role in the ozonization of alkanes in Magic Acid. There is no experimental evidence for reactions proceeding via the intermediacy of carbenium ions whether the initial oxidation step of alkanes to alcohols is important. This oxidation, indeed, was found to be extremely slow in the acidic media studied. 

Straight-chain alkanes also efficiently react with ozone in Magic Acid at —78°C in SO2CIF solution. Ethane gave protonated acetaldehyde as the major reaction product together with some acetylium ion (Scheme 5.62). Reaction of methane, however, is rather complex and involves oxidative oligocondensation to terf-butyl cation, which reacts with ozone to give methylated acetone (Scheme 5.63). 

A comparison of the reactivity of SbF5-treated metal oxides with that of HS03F-, SbCl5-, and HS03F-SbF5 (magic acid)-treated catalysts showed that the former was by far the best catalyst for reaction of alkanes (31, 32). Tracer studies of conversion of alkanes catalyzed by the superacids were performed it was suggested that the reactions proceeded by carbenium ion mechanisms in which the reactions were initiated by abstraction of H from the reactants (33). 

Methane reacts in superacid medium with the formation of its homologues (ethane, propane,. ..) and hydrogen [41 ]. Magic acids promote the oligomerization of alkanes (even methane) with the formation of oligomers ranging from 100 to 700. The ease of attack follows the trend [42j /iO-Cj > rr-Ca > Ci > C2 C t. 

Magic acid can abstract hydride from saturated alkanes, including straight-chain alkanes and branched and cyclic alkanes, at -125 to 25 °C to give alkyl cations [47,62], Its corrosive and toxic nature makes HF-SbFs a less frequently used acid system for the preparation of carbocations, compared to HSOsF-SbFs. It is, however, preferred in the generation of arenium ions, because high acidity is required for their formation (Eq. 25) [63],... 

Formed in superacid media species exhibiting electrophilic properties are able to attack alkanes primarily via electrophilic addition to C-H bond followed by other reactions. In particular, Olah et al. observed O atom insertion in hydrogen peroxide reaction with methane in Magic Acid above 0 °C to produce methanol with very high (>95%) selectivity [54a]. The particle (OH)"", which may be considered to be a protonated oxygen atom in the singlet state is apparently the active species in the reaction. Methyl alcohol formed is immediately protonated to methyloxonium ion, and this prevents further... 

Ozonization of alkanes in Magic Acid. Olah et a/. have studied the ozonization of alkanes in Magic Acid (5, 312) and SO2CIF and suggest that the reaction involves the insertion of OsH into a C H bond (equation T). 

G. A. Olah, R. H. Schlosberg, Chemistry in super acids. I. Hydrogen exchange and polycondensation of methane and alkanes in FSOjH-SbFs ( magic acid ) solution. Protonation of alkanes and the intermediacy of CHJ and related hydrocarbon ions. The high chemical reactivity of paraffins in ionic solution reactions, /. Am. Chem. Soc., 1968, 90, 2726-2727. 

Baeyer-Villiger oxidation of several ketones with hydrogen peroxide-Magic Acid systems . In the superacid-catalyzed hydroxylation of straight-chain alkanes such as ethane, propane, butane, etc. with hydrogen peroxide, related oxygenated products were identified . ... 

TABLE 6. Product of the reaction of branched alkanes with ozone in Magic Acid-S02C1F at — 78... 

Superacid-catalyzed reaction of ozone with straight-chain alkanes has also been investigated at low temperature. Magic Acid catalyzed ozonization of ethane is shown in equation 56. Reaction of methane under similar conditions was also investigated and discussed previously. Reactions of cycloalkanes have similarly been studied. ... 

Superacidic systems are able even to catalyze self-alkylative condensation of alkanes demonstrated here by the condensation of methane (63,78). The reaction requires the oxidative removal of hydrogen involving the pentacoordinate cation (17) (eq. 55). This can be attained by applying the oxidizing superacid Magic Acid (HSOsF-SbFs). The carbocationic pathway leading to C4-C6 alkanes also occurs through the involvement of pentacoordinate cations (18 eq. 56). 

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