Alkylbenzenes selectivity between

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

TABLE 2-2. Selectivity Between Alkylbenzenes at Different Eluent Compositions on Reversed-Phase Column"... 

For micelles, the typical selectivities for alkylphenones mid alkylbenzenes are between 1.1 and 1.6 for SDS concentrations in the 0.06-0.5 M range, which is similar to the values observed for organic-rich eluents in aqueous-organic systems. The overall smaller a(CH2) in MLC is an indication of how closely the environment of a methylene group in the micellar mobile phase resembles that of the alkyl-bonded stationary phase. 

Hydrophobicity or hydrophobic selectivity 0-CH2 Retention factor between pentylbenzene and butylbenzene, = kp /k. This is a measure of the surface coverage of the phase as the selectivity between alkylbenzenes differentiated by one methylene group is dependent on the ligand density. 

The newer HFC refrigerants are not soluble in or miscible with mineral oils or alkylbenzenes. The leading candidates for use with HFC refrigerants are polyol ester lubricants. These lubricants are derived from a reaction between an alcohol and a normal or branched carboxyflc acid. The most common alcohols used are pentaerythritol, trimethylolpropane, neopentjlglycol, and glycerol. The acids are usually selected to give the correct viscosity and fluidity at low temperatures. 

Correlation Between Energy Characteristics of Aprotic Acid Sites in ZSM-5 Zeolites and Selectivity of Conversion of Alkylbenzenes... 

Fragmentation of alkylbenzenes over silica-alumina occurs exclusively by acid-catalyzed cracking. The reaction selectively cleaves the bond between the phenyl ring and the a-carbon of the side-chain. This occurs more than 100 times more often than the cracking of all the other bonds combined. Cracking rates of secondary alkylbenzenes are about an order of magnitude higher than those of w-alkylbenzenes. 

It has been demonstrated that visible light irradiation of the absorption band of AcrH + in the presence of organometallic compounds and alkenes and alkylbenzenes in MeCN results in efficient C-C bond formation between these electron donors and AcrH+ via photoinduced electron transfer from the donors to the singlet excited state of AcrH+ to yield the alkylated or allylated adducts selectively [89-91], The AcrH+ is also photoreduced by ethylbenzene and other alkylbenzenes to yield the corresponding 9-substituted-10-methyl-9,10-dihydroacridine [92] ... 

Using Menke s conditions, Smith et al.[29,30] have described a method for the nitration of benzene, alkylbenzenes and halogenobenzenes using zeolites with different topologies (HBeta, HY, HZSM-5 and HMordenite) as catalysts and a stoichiometric amount of nitric acid and acetic anhydride. The reactions were carried out without solvent at temperatures between -50 °C and 20 °C. For the nitration of toluene, tridirectional zeolites HBeta and HY were the most active catalysts achieving >99 % conversion in 5 min reaction time. However, HY exhibited selectivity to the p-nitrotoluene very similar to the homogeneous phase, while with HBeta, selectivities to p-nitrotoluene higher than 70% could be achieved. HBeta zeolite exhibited excellent para-selectivity for the nitration of the different monosubstituted aromatics (Table 5.1). The catalyst can be recycled and the only by-product, acetic acid, can be separated by vacuum distillation. 

Table 6.1 presents the purity specifications. The target of design is achieving over 99.9% purity. It may be seen that higher alkylbenzenes impurities are undesired. Ethyl- and butylbenzene can be prevented by avoiding olefins and butylenes in the propylene feed. N-propylbenzene appears by equilibrium between isomers and can be controlled by catalyst selectivity. 

We stated in our original paper "The method of ccmpetitive reaction rate determination can be applied only if the observed relative rates are dependent on the aromatic substrate. As our observed relative rates showed only low selectivity concerning the nature of the aromatic substrate it could have been attributed to a very fast reaction taking place when the solution of nitronium salt reagent is dropped into the solution of the mixed aromatic substrate. Thus the NO2 ion would have no possibility of reaching uniform distribution in the solution before reaction occurs with the aromatics and observed relative rates may be influenced rather by statistical factors than real competition". In order to clarify this possibility and also to determine the accuracy of the method used, we carried out experiments to establish whether real competitive reaction actually occured under our experimental conditions and whether the methods employed provided results which could establish small differences between the alkylbenzenes investigated ( ). 

A most reasonable interpretation of these results would be that the reactive entity is some species, perhaps radical in nature, which shows a very different intramolecular selectivity from that of the nitronium ion. The efficacy of nitrogen dioxide itself appears ruled out, however, by the low conversions. Study of the intermolecular selectivity of the reagent by competition (Table III) indicates that the reagent is about as selective as the nitronium ion between alkylbenzenes. The relative rates observed are compared with two systems (, , 11,22) where the nitronium ion is well established as the electrophilic species. For toluene the unusual isomer ratios were observed in these competition runs. 

The variations in selectivity along an homologous series implies that a larger number of compounds are eluted per unit time with micellar mobile phases, as compared to the traditional aqueous-organic mobile phases. It can be observed in Fig. 9.2, that between ca. 30 and 120 min, three peaks are eluted with a 75 25 methanol-water mobile phase, as compared to six or more with an aqueous 6% Brij 35 micellar mobile phase, although the ability to resolve solutes is less for the micellar phase. The observed selectivity difference is not attributable to a difference in solvent strength, as the alkylbenzenes begin to elute earlier with the methanol-water mobile... 

Allqrl groups attached to the benzene ring are known to direct entering substituents into the ortho and para positions. Substitution in the meta position, therefore, was an unexpected finding when alkylbenzenes were monoalkylated (or benzene was dialkylated). It was subsequently established that such meta substitution is generally not caused by a kinetic aU lation of low selectivity, but thermodynamically controlled isomerization. The relative amovmts of the three isomeric dialkylbenzenes that are produced depend on the aromatic hydrocarbon, the alkylating agent, the catalyst, and the reaction conditions. In mechanistic interpretations, therefore, it is important to differentiate between direct alkylation controlled by the kinetics of the reaction and subsequent or concurrent isomerization of the alkylated product controlled by thermodynamic factors. 

Alkylbenzenes and alkylnaphthalenes were studied on a silica colunui using hexane, hexane/1-chlorobutane, hexane/l-bromobutane, or hexane/IPA as the mobile phase [619]. Modifier levels ranged fiom 0.005% to 10%. Capacity fiictors versus carbon number were plotted for each solvent mixture. Selectivity decreased for all solvent modifiers except 1-chlorobutane, for which selectivity increased as the level increased from 2% to 8%. The authors attribute diis to the formation of n-complexes between the 1-chlorobutane and the PAH solutes. Selectivity decreased, as expected, for the alkylnaphthalenes when I-bromobutane was used. Selectivity was lost rapidly as the level of IPA increased from 0.01%0.05% indicating that at low IPA concentrations IPA (or the water contained in the IPA) readily modifies or deactivates the silica support. 

This effect was well illustrated in a case study reported by Wu and Clausen (34). Here five alkylbenzenes were separated by an HPLC method. When this method was directly scaled to UHPLC, the separation was adequate, but the column efficiencies were lower than would be predicted. It is likely that the lost efficiency was due to the effects of extra-column band broadening. Pressure drops also did not scale as predicted. This is likely due to physical differences between the HPLC and UHPLC particles. In a similar manner to that described, it was noted that selectivity factors did not remain constant as particle size changed. For these reasons, it is emphasized that the theoretical scaling factors given earlier are approximations, and the observed relationships will likely vary from theory because of additional uncontrolled factors. Again, it is imperative that the stationary phase chemistry remain constant as the method is scaled to an alternate platform. 

In 2007, Cassol et al., found that the selectivity on the extraction of a specific aromatic compound is influenced by anion volume, hydrogen bond strength between the anion and the imidazohum cation and the length of the l-methyl-3-alkylimidazolium alkyl side chain. The interaction of alkylbenzenes and sulfur heterocyles with the IL is preferentially through CH-tt hydrogen bonds and the quantity of these aromatics in the IL phase decreases with the... 

---