O-Xylene hydrogenation

Figure 14 Cross-section of a DCA host channel sliced parallel to the direction of the channel (carbon, hydrogen and oxygen atoms are represented by gray, white and black, respectively) with arrays of o-xylene (hydrogen atoms are omitted for clarity, and carbon atoms are represented by open circles).

The H2-chemisorption data indicated fliat the average platinum particle size was not affected by the metal uptakeAoading and all three catalysts had a platinum particle size of ca. 1.5 nm in diameter. The catalyst exhibited high activities in o-xylene hydrogenation. Cis and trans 1, 2-dimethylcyclohexane were the only reaction products. The hydrogenation rate was found to increase by increased hydrogen partial pressure. Coirplete hydrogenation of o-xylene could be achieved at 460 K at H2 to o-xylene molar ratio of 12. 

Figure 1.33. Kinetics of o-xylene hydrogenation The apparent activation energy defined as...

It was previously shown (15) that the selectivity for O-xylene hydrogenation (cis and trans dimethylcyclohexane) is sensitive to electronic density modification. In order to estimate electronic transfer on Pt catalysts induced by Au addition, Table 4 shows that it was not observed any change in selectivity for the 0-xylene reaction on Pt-Au catalysts. 

Such change in selectivity could be explained by assuming that gold, by a ligand effect, could change the electronic properties of platinum. The selectivity of o-xylene hydrogenation reaction (cis and trans dimethylcyclohexane) has shown to be sensitive to electronic density modifications... 

To hydrogenate an aromatic ring, it s necessary either to use a platinum catalyst with hydrogen gas at several hundred atmospheres pressure or to use a more effective catalyst such as rhodium on carbon. Under these conditions, aromatic rings are converted into cyclohexanes. For example, o-xylene yields 1,2-dimethylcvclohexane, and 4-terf-butylphenol gives 4-terf-butyl-cyclohexanol. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

Subsequently, rate coefficients were determined for the zinc chloride-catalysed bromination of benzene, toluene, i-propyl-benzene, r-butylbenzene, xylenes, p-di-f-butylbenzene, mesitylene, 1,2,4-trimethyl-, sym-triethyl-, sym-tri-f-butyl-, 1,2,3,5-and 1,2,4,5-tetramethyl- and pentamethylbenzenes, all at 25.4 °C and in acetic acid, and it was shown that the reaction was inhibited by HBr.ZnCl2 which accumulates during the bromination and was considered to cause the first step of the reaction (formation of ArHBr2) to reverse320. The second-order coefficients for bromination of o-xylene at 25.0 °C were shown to be inversely dependent upon the hydrogen bromide concentration and the reversal of equilibrium (155)... 

Two structures are possible for the interaction of aromatic hydrocarbons with acids.270 In the a-structures a covalent bond is established between the acidic reagent and a particular carbon atom of the benzene ring. The a-structures are essentially classical carbonium ions. In the -structures a non-classical bond is established, not to any particular atom, but to the -electron cloud in general. It is quite likely that both types of structure are represented by actual examples. Thus m-xylene interacts more strongly with hydrogen chloride than does o-xylene, but the difference between the two hydrocarbons is much more pronounced when their interactions with a boron trifluoride-hydrogen fluoride mixture are compared. This is readily understandable... 

Recently195, the hydrogen bond basicity scale (p.K hb as logarithm of the formation constant of 4-fluorophenol/base complexes in carbon tetrachloride, equilibrium 21) has been measured for several nitro derivatives (nitromethane, nitrobenzene, IV-nitrocamphorimine, 2-nitropropane, 4-nitro-o-xylene, 4-nitroanisole, lV,./V-diethyl-4-nitroaniline, l-dimethylamino-2-nitroethylene, l-piperidino-2-nitroethylene) ... 

Fig. 18. Comparison of ciajtrans ratios obtained from o-xylene and its tetrahydro derivatives 1,2- and 2,3-dimethyIcycIohexene as a function of hydrogen pressure PtOj (97).

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

Davis 69) found no considerable variation in the o-xylene versus ethylbenzene ratio as a function of hydrogen pressure. He also observed that the relative amount of o-xylene from n-octane increased (a) with decreasing Pt loading of the catalyst (70) b) with increasing tin addition 69, 7J) (c) with the poisoning of the catalyst with thiophene (77) and d) if octenes or octynes... 

Phthalazine and its homologs and derivatives are easily hydrogenolyzed. Electroreduction in alkaline medium gave 1,2-dihydrophthalazines [495], while in acidic media [495] and on catalytic hydrogenation [496], the ring was cleaved to yield o-xylene-a,a -diamine. 

There are no examples of nucleophilic substitution of hydrogen on thianthrene. Methylmagnesium iodide, in the presence of [1,3-bis (diphenylphosphino)-propyl]nickel dichloride caused ring opening, probably initially via 48 o-xylene, from a second organometallic attack, and, by... 

This paper records the results of a similar study of the bond structure of tetrahydronaphthalene and a comparison of these results with our earlier work on indan and o-xylene [lc], thus permitting a rather comprehensive evaluation of the Mills-Nixon effect. What is believed to be a novel steric effect on intramolecular hydrogen bonding also will be described. 

---