Isomeric alkylbenzenes

Problem 10.17 Give the structures of the isomeric alkylbenzenes C H,u that can have one, two, or three isomeric monoiodo derivatives. 

Aromatization of n-octane yielded ethylbenzene and o-xylene, the products of direct 1,6 closure, but m- and p-xylenes were also formed.205 Isomeric dimethyl-hexanes, expected to give only one aromatic compound each according to 1,6 carbon-carbon closure, produced all isomeric alkylbenzenes.206 Alkanes that cannot undergo dehydrogenation did not yield aromatic compounds, which pointed to the importance of alkene intermediates in aromatization. 

A survey of the literature reveals that the majority of the nonbiodegradable detergents are alkylbenzene sulfonates (ABS). Theoretically, there are over 80,000 isomeric alkylbenzenes in the range of C10 to C15 for the alkyl side chain. Costs, however, generally favor the use of dodecene (propylene tetramer) as the starting material for ABS. 

Jinno, K. Kawasaki, K. Correlations between retention data of isomeric alkylbenzenes and physical parameters in reversed-phase micro high-performance liquid chromatography. Chromatographia 1983, 17 (6), 337-340. 

Smith, R.M. Reversed-phase liquid chromatography of isomeric alkylbenzenes. J. Chromatogr. 1981, 209, 1-6. 

In isomeric alkylbenzene and alkylphenol derivatives, degradation decreases as the phenyl group is moved from a position near the terminal end of a linear alkyl group to a more central position. 

Because of isomerization, alkylation of benzene with tertiary alkyl haUdes can also yield secondary alkylbenzenes rather than only tertiary alkylbenzenes (20). For example, the / fAhexylbenzene, which is first formed by the reaction of benzene with 2-chloro-2,3-dimethylbutane and AlCl isomerizes largely to 2,2-dimethyl-3-phenylbutane by a 1,2-CH2 shift. With ferric chloride as the catalyst, / fAhexylbenzene does not undergo isomerization and is isolated as such. 

Isomerization is promoted by either acids or bases. Higher alkylbenzenes are isomerized in the presence of AlCb/HCl or BF3/HF olefins with most mineral acids, acid salts and silica-alumina saturated hydrocarbons with AlCb or AlBr,3 promoted by 0.1 percent of olefins. 

A good deal of experimental care is often required to ensure that the product mixture at the end of a Friedel-Crafts reaction is determined by kinetic control. The strong Lewis acid catalysts can catalyze the isomerization of alkylbenzenes, and if isomerization takes place, the product composition is not informative about the position selectivity of electrophilic attack. Isomerization increases the amount of the meta isomer in the case of dialkylbenzenes, because this isomer is thermodynamically the most stable. ... 

Not only the linear Cl0-Cl8 a-olefins but also the linear C10-Cl8 olefins with internal double bonds, the so-called -v /-olefins, are of great importance in surfactant chemistry, n-a-Olefins and n-y-olefins have the same suitability for the manufacture of linear alkylbenzenes, the most important synthetic anionic surfactants, by alkylation of benzene. Nowadays medium molecular weight n- /-olefins are industrially produced by two processes the catalytic dehydrogenation of the corresponding n-alkanes [4,28] and the cometathesis of low and high molecular weight n-v /-olefins, obtained by double-bond isomerization of the isomeric n-a-olefins [29]. 

The CMC of commercial AOS and other surfactants at 40°C has been determined by Gafa and Lattanzi [6] who plotted the surface tension of aqueous surfactant solutions against concentration. The surface tensions were determined with the ring method according to du Nouy. Table 5 gives their CMC values in mmol/L and the surface tension at the CMC in mN/m. Table 5 also contains CMC values of isomerically pure sodium alkyl sulfates, sodium alkylbenzene-sulfonates, sodium hydroxyalkanesulfonate, and sodium alkenesulfonates at 40°C, taken from the literature [39 and references cited therein]. 

It has been found that cyclohexa-2,4- and -2,5-dienones undergo a light-induced valence isomerization reaction in strong acid analogous to the alkylbenzenes, to yield 2-hydroxy-bicyclo[3,l,0]hexenyl cations. The hydroxybenzenium ion (34), for example, underwent a clean photoisomerization to 35 at temperatures below — 60° (Paxrington and Childs, 1970). Cation 35 was also produced upon similar irradiation of 36. 

For the reason given above and for other reasons, it is unlikely that the encounter complex is a n complex, but just what kind of attraction exists between Y+ and ArH is not known, other than the presumption that they are together within a solvent cage (see also p. 694). There is evidence (from isomerizations occurring in the alkyl group, as well as other observations) that n complexes are present on the pathway from substrate to arenium ion in the gas-phase protonation of alkylbenzenes. ... 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

When discussing isomerization of alkylbenzenes, it is useful to recollect that the alkylbenzenium ion (a complex or arenium ion) intermediates involved are identical to those of the alkylation reaction of aromatics ... 

As pointed out (see Section 5.1.4) alkyl group migration (i.e., isomerization) in these carbocationic intermediates can take place with ease, even under conditions where the product dialkylbenzenes themselves do not undergo further isomerization. Kinetically controlled alkylation free of thermodynamically controlled alkyl group migration therefore cannot be judged on absence of product alkylbenzene isomerization. 

Superacids were shown to have the ability to effect the protolytic ionization of a bonds to form carbocations even in the presence of benzene.190 The formed car-bocations then alkylate benzene to form alkylbenzenes. The alkylation reaction of benzene with Ci—C5 alkanes (methane, ethane, propane, butane, isobutane, isopentane) are accompanied by the usual acid-catalyzed side reactions (isomerization, disproportionation). Oxidative removal of hydrogen by SbF5 is the driving force of the reaction ... 

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

The reaction of alkylbenzenes with ethylene in the presence of sodium occurs with successive replacement of one or two benzylic hydrogens to produce mono-and diethylated compounds.235 Propylene reacts to form isomeric products with the preponderance of the branched isomer236 [Eq. (5.61)]. When potassium is used as catalyst, indan derivatives can also be formed237 [Eq. (5.62)] ... 

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