Branched alkanes alkylation

In summary, the alkylation of alkenes (jr-donor nucleophiles) in the presence of conventional acid catalysts occurs through a trivalent carbocation, follows Markovnikov addition and gives the corresponding branched alkanes. Alkylation of alkanes (a-donors), in turn, proceeds through a five-coordinate carbocation without involvement of trivalent carbenium ions and thus branched alkanes are not formed. 

Model 2 Naming Branched Alkanes (Alkyl Groups)... 

By combining the basic principles of lUPAC notation with the names of the various alkyl groups we can develop systematic names for highly branched alkanes We 11 start with the following alkane name it then increase its complexity by successively adding methyl groups at various positions... 

Normal alkanes (n-alkanes, n-paraffms) are straight-chain hydrocarbons having no branches. Branched alkanes are saturated hydrocarbons with an alkyl substituent or a side branch from the main chain. A branched... 

Alkylation of isobutane with C3-C5 alkenes in the presence of strong acids leads to the formation of complex mixtures of branched alkanes, called alkylate, which are excellent blending components for gasoline. Alkylate has a high octane... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Tab. 10.7 summarizes the results of the application of rhodium-catalyzed allylic etherification to a series of ortho-substituted phenols. The etherification tolerates alkyls, including branched alkanes (entries 1 and 2), aryl substituents (entry 3), heteroatoms (entries 4 and 5), and halogens (entry 6). These results prompted the examination of ortho-disubstituted phenols, which were expected to be more challenging substrates for this type of reaction. Remarkably, the ortho-disubstituted phenols furnished the secondary aryl allyl ethers with similar selectivity (entries 7-12). The ability to employ halogen-bearing ortho-disubstituted phenols should facilitate substitutions that would have proven extremely challenging with conventional cross-coupling protocols. 

GC of fraction 7 (Figure 5" I) has alkanes as small as The ratio of peak heights of pristane to C. increases in the uC of this fraction compared to previous fractions as expected from its shorter linear molecular size. The smaller peaks between n-alkane peaks are alkylated phenols and branched alkanes. 

The radical-forming reactions are suggested to take place mostly after an Si T type ISC the reactions have nonactivated character. The homolytic split to H atom and alkyl radical has a considerable yield in the photolysis of n-alkanes and cycloalkanes, while the scission to two radicals is characteristic of the decay of excited branched alkane molecules. 

Isomerization of straight-chain to branched alkanes also increases the octane number, as do alkylates produced by alkene-isoalkane alkylation (such as that of isobutane and propylene, isobutylene, etc.). These large-scale processes are by now an integral part of the petroleum industry. Refining and processing of transportation fuels became probably the largest-scale industrial operation. 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

Ipatieff and coworkers observed first that A1C13 catalyzes the destructive alkylation of aromatics with branched alkanes.179 For example, rm-butylbenzene (35%), p-di-rm-butylbenzene (25%), and considerable isobutane are the main products when benzene is reacted at 20-50°C with 2,2,4-trimethylpentane. Toluene and biphenyl are alkylated at 100°C in a similar way.180 Straight-chain alkanes required more severe reaction conditions. n-Pentane reacted at 175°C to yield 8% propylbenzene, 25% ethylbenzene, and 20% toluene.181 Phosphoric acid afforded similar products at higher temperature (450°C).182 Pentasil zeolites and dealumi-nated pentasils have been found to promote alkylation of benzene with C2—C4 alkanes to form toluene and xylenes.183,184... 

The isomerization of naturally abundant straight-chain light alkanes is a technologically very important process for the production of branched alkanes, which are good gasoline components. Isobutane is also an important raw material for alkylation of alkenes. 

An alkane with all its carbon atoms in a single chain, with no branching or alkyl substituents, (p. 88)... 

It is evident from the data for monoalkylbenzenes on a-CD that the retention of n-alkylbenzenes is affected by the high stabilities of the complexes formed, caused by the location of the alkyl group in the cyclodextrin cavity. Branching of the side chain leads to a pronounced decrease in the retention. These results are similar those obtained for the interactions of n-alkanes and branched alkanes with a-CD (12.13). ... 

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

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