Xylene complex

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Xylenes react reversibly to form complexes with boron trifluoride (BF3) in the presence of liquid hydrogen fluoride (HF). This fact can be utilized in the separation of w-xylene from its isomers, because the m-xylene complex is much more stable than the others and also more soluble in hydrogen... 

Electron depopulation of the donor and concomitant population of the acceptor in the complex results in a lowering of the vibrational frequencies in the IR spectra of the donor and acceptor moieties. Additionally, complex formation can decrease the symmetry of the donor/ acceptor dyad and can lead to increased IR intensity or the appearance of new bands. For example, in halogen/alkylbenzene complexes, the stretching frequencies of the halogens are lowered, as seen in the shift of chlorine band from 557 cm-1 in free chlorine to 530 cm-1 in the benzene complex, to 527 cm-1 in the toluene complex, and to 524 cm-1 in the p-xylene complex. Increases in the intensity of some of the arene bands are also clearly seen [23b]. 

Carboxylic acids can also form clathrates. Xylene isomers can be separated by formation of clathrates with 1,1 -binaphthyl-2,2 -dicarboxylic acid.73 m-Xylene does not form a complex. The o-xylene complex decomposes at... 

X, and the p-xylene complex at 100-120°C. Charge transfer is a factor in the host-guest complex of 4-methyl-3,5-dinitrobenzoic acid with 2,6-dimethylnaphthalene.74 Charge transfer with tetranitrofluorenone has also been used to remove 60% of the dialkyldibenzothiophenes from petroleum that contains 1920 ppm sulfur, although no inclusion compound is involved.75 As polynitro compounds are often explosive, a better charge acceptor is needed. 

While investigating the nature of bonding in these complexes, Evans et al. reported [106] an arene exchange reaction in which a bridged xylene complex was formed by the reaction of (C5Mes)3U, KCg and p-xylene as shown in Eq. 21 ... 

Figure 211 illustrates a separation of the tricarbonylchromium complexes of isomeric arenes on a packed column. Complete resolution of the three isomeric xylene complexes, as well as of the isomeric complexes of the three trimethylbenzenes and the three tetramethyl-benzenes, was also achieved by these workers using a capillary column. 

Figure 2. Conq)uted lowest energy geometry of the p-xylene complex of cyclophane 5.

Strieter FJ, Templeton DH (1962) Crystal structure of the carbon tetrabromide-p-xylene complex. J Chem Phys 37 161-164... 

Tetrabromomethane shows two types of n-bonding with aromatic donors that are contrasted in Fig. 8a and b, showing over-the-rim coordination to the aromatic C-C bond and over-the-center coordination to the benzene ring. The over-the-rim coordination is generally similar to that observed in the dibromine complexes but the C - Br distance in the former is longer, in agreement with weaker acceptor abiUties of tetrabromomethane. Note that picryl bromide shows similar bromine coordination to the outer pyrene C-C bond with Br - C distances of 3.35 and 3.39 A [83]. A second type of coordination was reported earlier in the p-xylene complex [80] and recently in the associate with dimethylnaphtalene [53]. 

In cyclohexane, at 18°C., methyl deuteration decreases the stability of the chloranil-toluene and chloranil-m-xylene complex to virtually the same extent Kfi/K y 1.07 (AAf°/ ) = 13 cal. The greater amplitudes of the CH motions than those of CD should lead to a steric isotope effect in the reverse direction, as should the inductive effect of deuterium, so the evidence unambiguously indicates hyper-conjugative stabilization of this and similar complexes, and a hyper-conjugative isotope effect. 

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