Dialkylbenzenes

Olefin polymers alpha-olefin polymers (PAO), polybutenes and alkylaromatics, in particular the dialkylbenzenes (DAB). This class of compounds is the most widespread and accounted for 44% of the synthetic base market in France in 1992. 

Synthesis ofp-Ethyltoluene. j )i7n7-Ethyltoluene, the feedstock for j )-methylstyrene, is difficult to separate from the products of toluene alkylation with ethane using conventional acidic catalysts. The unique configurational diffusion effect of ZSM-5 permits -dialkylbenzenes to be produced in one step. In the alkylation of toluene with ethene over a chemically modified ZSM-5, -ethyltoluene is obtained at 97% purity (58). 

At the existing conditions, some side reactions, such as the formation of dialkylbenzene, take place ... 

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. ... 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Contrary to expectations, only an unusually small part of the diene exists in the conjugated form. Furthermore the formation of cyclohexane derivatives is noticeable. Their formation can be explained with the compounds listed on row 4 in Table 9. During the alkylation the monoolefin reacts on one side to LAB, on the other side to oligomers, and, depending on the excess of benzene, in part to the dialkylbenzenes found as byproducts in the so-called heavy alkylate (the residues of the raw alkylbenzene distillation). 

The excess benzene is distilled over a column and used as recycled benzene in the alkylation. In the bottom of the stripping section of the column the raw alkylates, consisting of LAB, heavy alkylate, and excess paraffin, are separated. This mixture is fed to a second column in which the excess paraffin is separated off. The actual purification of the LAB follows in a third column. The bottom product, heavy alkylate, consisting mainly of dialkylbenzene is also separated. Heavy alkylates are used in various applications. Both the paraffin and the LAB column are operated under vacuum. 

Bis(nitrile oxides) obtained from dialkylbenzenes have been claimed as low-temperature rubber vulcanization agents (517). Curing of poly(butadiene-co-acrylonitrile) with 2,4,6-trimethylisophthalodinitrile N-oxide produces rubbery material of good quality, however, curing of (polybutadiene) was unsuccessful (518). The solubility of dinitrile oxides and stability of their ketone solutions has been studied for their application as vulcanizing agents in the production of mbberized materials (519). 

Borodachev IV, Volkhonskaya EV, Guk YuV, Zhukov AYu, Tsypin GI, Chernysheva SYu, Antipova VF, Panova NV. Bis(nitrile oxides) of dialkylbenzenes as low-temperature rubber vulcanization agents with low unsaturation and their manufacture from dialkylbenzenes, RU Patent No. 2042664 1995 [Chem. Abstr. 1996 124 263170],... 

Wang, 1., Ay, C-L, Lee, B-J., and Chen, M-H. (1989) Para-selectivity of dialkylbenzenes over modified HZSM-5 by vapour phase deposition of silica. Appl Catal, 54, 257-266. 

The competition of the alkyl groups in p-dialkylbenzenes for ethylene provides information about the rate-controlling step in base alkylation. Early work by Pines, Vesely, and Ipatieff (19) showed that p-cymene reacted with ethylene to yield predominantly ferf-pentyltoluene. It is well known that p-cymene is almost exclusively metalated on the methyl group... 

Relative rates of reactions of dialkylbenzenes are not easily explained. The presence of p-alkyl-substitution generally causes a large decrease in rate, whereas o-xylene. which should have the same inductive effects as p-xylene, reacts very rapidly. Possibly this rapid reaction of o-xylene is caused by an intramolecular transmetalation between the carbanion adduct with ethylene and the remaining methyl group [Reaction (22)]. 

In this connection three oases may be considered 1) the strength of aprotic centres is sufficient for the activation of olefin and the following alkylation by interaction with aromatic hydrocarbon frem gaseous phase, but insufficient for aromatic ring activation. Para-dialkylbenzene formed in this case does not undergo further isomerization on these centres. The group of centres... 

In the acid-catalysed transalkylation of dialkylbenzenes, one of the alkyl groups is transferred from one molecule to another. This bimolecular reaction involves a diphenylbenzene transition state. When the transition state collapses, it can split to give either the 1,2,4-isomer or the 1,3,5-isomer, together with the monoalkylbenzene. When the catalyst used for this reaction is mordenite (Figure 7.18(c)), the transition state for the formation of the symmetrical 1,3,5-isomer is too large for the pores, and the 1,2,4-isomer is formed in almost 100% yield. (This compares with the equilibrium mixtures, in which the symmetrically substituted isomers tend to dominate.)... 

The AlCl3-catalyzed isomerization of dialkylbenzenes was studied in detail by Allen and coworkers76-79 and Olah and coworkers.80-82 The equilibrium isomer... 

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. 

It is noteworthy that the thermodynamic equilibria of the neutral dialkylbenzenes and their protonated arenium ions (a complexes) are significantly different (see Section 4.1.2). In the latter case the 1,3-dialkyl-substituted arenium ion is the most stable. 

This then readily undergoes cleavage to produce benzene and a dialkylbenzene. The initial R+ cation initiating the reaction might arise from some impurity present in the reaction mixture. Consistent with this mechanism is the observed very low reactivity of methylbenzenes due to the necessary involvement of the primary benzyl cations. At the same time 1,1-diarylalkanes undergo cleavage with great ease. 

Overberger, C. G., G. F. Endres and A. Monaci Ionic polymerization. VII. Relative reactivities of mono- and />-dialkylbenzenes as molecular terminating agents in the cationic polymerization of styrene. J. Amer. chem. Soc. 78, 1969 (1956). 

When dialkylbenzenes are passed over Nafion-H at 160°C, both isomerization and disproportionation take place [Eqs. (5.102) and (5.103)]. Monoalkylbenzenes also disproportionate under these conditions268-271 [Eq. (5.104)]. 

---