Isoalkane formation

Carbenium ions have been shown to isomerize readily (Fig. 4.71). As long as no primary carbenium ions are involved, isomerization of carbenium ions occurs with low activation energy and at low temperatures. Isoalkane formation occurs by reaction with another alkane and transfer of a hydride ion (Fig. 4.72). 

Protonation of formic acid similarly leads, after the formation at low temperature of the parent carboxonium ion, to the formyl cation. The persistent formyl cation was observed by high-pressure NMR only recently (Horvath and Gladysz). An equilibrium with diprotonated carbon monoxide causing rapid exchange can be involved, which also explains the observed high reactivity of carbon monoxide in supera-cidic media. Not only aromatic but also saturated hydrocarbons (such as isoalkanes and adamantanes) can be readily formylated. 

Thermal, Thermooxidative, and Photooxidative Degradation. LLDPE is relatively stable to heat. Thermal degradation starts at temperatures above 250°C and results in a gradual decrease of molecular weight and the formation of double bonds in polymer chains. At temperatures above 450°C, LLDPE is pyrolyzed with the formation of isoalkanes and olefins. 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

In a series of investigations of the cracking of alkanes and alkenes on Y zeolites (74,75), the effect of coke formation on the conversion was examined. The coke that formed was found to exhibit considerable hydride transfer activity. For some time, this activity can compensate for the deactivating effect of the coke. On the basis of dimerization and cracking experiments with labeled 1-butene on zeolite Y (76), it is known that substantial amounts of alkanes are formed, which are saturated by hydride transfer from surface polymers. In both liquid and solid acid catalysts, hydride transfer from isoalkanes larger than... 

Isoalkanes can also be synthesized by using two-component catalyst systems composed of a Fischer-Tropsch catalyst and an acidic catalyst. Ruthenium-exchanged alkali zeolites288 289 and a hybrid catalyst290 (a mixture of RuNaY zeolite and sulfated zirconia) allow enhanced isoalkane production. On the latter catalyst 91% isobutane in the C4 fraction and 83% isopentane in the C5 fraction were produced. The shift of selectivity toward the formation of isoalkanes is attributed to the secondary, acid-catalyzed transformations on the acidic catalyst component of primary olefinic (Fischer-Tropsch) products. 

Besides the rearrangement of carbocations resulting in the formation of isomeric alkylated products, alkylation is accompanied by numerous other side reactions. Often the alkene itself undergoes isomerization prior to participating in alkylation and hence, yields unexpected isomeric alkanes. The similarity of product distributions in the alkylation of isobutane with n-butenes in the presence of either sulfuric acid or hydrogen fluoride is explained by a fast preequilibration of n-butenes. Alkyl esters (or fluorides) may be formed under conditions unfavorable for the hydride transfer between the protonated alkene and the isoalkane. 

Formylation of isobutane with carbon monoxide in the presence of an excess of A1C13 was first reported by Nenitzescu to yield, among others, methyl isopropyl ketone (31%).168 A new highly efficient superelectrophilic formylation-rearrange-ment of isoalkanes by Olah and coworkers has been described.282 Selective formation of branched ketone in high yield with no detectable branched acids, that is, the Koch products, was achieved. A particularly suitable acid is HF—BF3, which transforms, for example, isobutane to methyl isopropyl ketone in 91% yield. The... 

Because the reaction is catalytic in ferf-butyl cation and the deprotonation/ reprotonation steps are very fast, extensive regioselective deuteriation of the isoalkane is observed at room temperature as shown by GC-MS analysis. The absence of mass 68 (d10-isobutane) and the presence of mass 64 due to S02 formation in the oxidative process are typical features in accord with the oxidative activation of the alkane and the Markovnikov-type addition of deuterons on the intermediate isobutylene (14). However, the exchange process does not take place in the presence of carbon monoxide, which traps the ferf-butyl cation and prevents deprotonation (Scheme 5.7). 

The products obtained from isobutane and isoalkanes (Table 5.37) are in accord with the above-discussed mechanism. However, the relative rate of formation of the dimethylmethylcarboxonium ion from isobutane is considerably faster than that of the ferf-butyl cation from isobutane in the absence of ozone under the same conditions.642 Indeed, a solution of isobutane in excess Magic Acid-SCECIF solution showed only trace amounts of the ferf-butyl cation after standing for 5 h at —78°C. Passage of a stream of oxygen gas through the solution for 10 times longer a period than in the ozonization experiment showed no effect. It was only when ozone was introduced into the system, that rapid reaction took place. 

A problem that is characteristic of sulfuric acid-catalyzed alkylation is its capabihty to oxidize hydrocarbons. H2SO4 decomposes in the presence of isoalkanes to form water, SO2, and alkenes. This is a slow process, and so it occurs predominantly when the acid is in contact with hydrocarbons for a longer period. Higher temperatures favor the formation of SO2 (10). Some irreversible reactions between acid and hydrocarbons also take place during alkylation. Sulfone, sulfonic acid, and hydroxy groups have been detected in conjunct polymers produced with H2SO4 as the catalyst (8,96). Kramer (97) reported that... 

By-product formation is due chiefly to two side reactions, namely destructive alltylation and a hydrogen-transfer reaction, the net result of which is the hydrogenation of alkene and the self-condensation of the isoalkane. 

Formation of an intermediate alkylcarbenium ion which is the key step in superacid-catalyzed reaction of ozone with alkanes is considered to proceed by two mechanistic pathways as illustrated in Scheme 12. The carbenium ions subsequently undergo nucleophilic reaction with ozone as discussed previously. Reactions of ozone with alkanes giving ketones and alcohols as involved in mechanism b have been reported in several instances " . The products obtained from isobutane and isoalkanes (Table 6) are in accordance with the mechanism discussed above. 

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