Benzene, formation from cyclohexene

Fig. 27. Inhibition of benzene formation from cyclohexene on disordered carbonaceous...

Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5. which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cyclohexene (ring contraction)... 

There is a very significant difference between the rate of aromatization of trans- and c/i-hexatriene (Table III), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via half-hydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyelohexane and cyclohexene are produced from cw-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise Cg dehydro-cyclization. 

Kennedy and Stock reported the first use of Oxone for many common oxidation reactions such as formation of benzoic acid from toluene and of benzaldehyde, of ben-zophenone from diphenylmethane, of fratw-cyclohexanediol from cyclohexene, of acetone from 2-propanol, of hydroquinone from phenol, of s-caprolactone from cyclohexanone, of pyrocatechol from salicylaldehyde, of p-dinitrosobenzene from p-phenylenediamine, of phenylacetic acid from 2-phenethylamine, of dodecylsulfonic acid from dodecyl mercaptan, of diphenyl sulfone from diphenyl sulfide, of triphenylphosphine oxide from triphenylphosphine, of iodoxy benzene from iodobenzene, of benzyl chloride from toluene using NaCl and Oxone and bromination of 2-octene using KBr and Oxone126. Thus, they... 

Fig. 25. Inhibition of benzene (O) from cyclohexane and increasing cyclohexene formation ( ) with time on Pt(S)-[6(111) x (100)] surface. All catalysts with (111) orientation terraces behave similarly. T, 150°C 4 x 10 8 Torr reactant H2 HC, 20 1.

The last example quoted deals with dehydrogenation, this time of larger molecules, studied by UPS for cyclohexane over Pd(lll) and Pd films. Rubloff et did not study catalysis but chemisorption. At r< 120K adsorption was without decomposition, but at 300 K dehydrogenation gave benzene as the surface species. This was suggested from comparative experiments in which benzene itself was adsorbed. It appears that the 7r-orbitals of benzene are involved in bonding. They found no evidence for the formation of cyclohexene or cyclohexadiene. Their work should be compared with that of Tetenyi et al., who identified such intermediates by radiochemical methods. 

On the bases of the results and discussion stated above, we considered the role of water for the formation of cyclohexene (Fig.5). In the reaction mixture, the catalysts are in aqueous phase, and benzene and hydrogen have to pass the thick water layer surrounding the catalysts to react with each other on the active sites. When cyclohexene is produced from benzene, it is expelled by water and benzene newly coming from organic phase. High temperatures and pressures such as 170-190 °C and 60-80 kg/cm which are the optimum conditions for the production of cyclohexene are favorable for increasing the concentration of benzene in the water layer. If the desorption rate of cyclohexene from the surface is slow, cyclohexene is naturally converted to cyclohexane. But, if cyclohexene is expelled, it will be isolated from the active sites in the catalysts because of its low solubility to water. Accordingly, it could... 

Write a reasonable mechanism for the formation of cyclohexyl benzene from the reaction of benzene cyclohexene and sulfuric acid J... 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

The reduction of hydroxylamine by titanous salts in water produces the free amino radical, a reaction analogous to the formation of triphenylmethyl from the carbinol and a reducing agent.138 The amino radical will attack benzene to give diaminocyclohexadiene and di-(aminocyclohexadienyl) it converts cyclohexene into cyclohexyl-amine.139... 

In order to produce additional evidence for the above mecheuiism for aromatization over Ga203 HZSM-5 catalysts the reactions of n-hexene, 1,5 hexadiene, methylcyclopentane, methylcyclopentene, cyclohexene, cyclohexadiene at 773 K over H-2SM-5 and Ga-HZSM-5 were comparatively studied. In these exj riments low pressure and low contact were employed to observe the primary kinetic products uncomplicated by secondary reactions. The relative rates of the formation of benzene from the various hydrocarbons cited above are listed in Table 4. 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

Molybdenum trioxide (M0O3) deposited on silica was one of the first supported Mo catalysts to be prepared. In contrast to Ti/SiC>2, which is used industrially, Mo/SiC>3 did not break through commercially, mainly owing to substantial leakage of Mo under catalytic conditions. Trifiro et al. (213) showed that when M0O3 on silica is used for the epoxidation of cyclohexene with t-BuOOH in benzene at 353 K, part of the activity originates from dissolved Mo. The main reason why Mo is not entirely retained on silicas and aluminas is thought to be the formation of soluble neutral Mo-diol complexes. 

Hydrogenation of benzene over acidic catalysts or in the presence of acid results in the formation of the products resulting from alkylation by the intermediate cyclohexene such as cyclohexylbenzene, together with cyclohexane, as shown in Scheme 11.1. Slaugh and Leonard obtained cyclohexylbenzene in high selectivity in the hy-... 

The differences in liquid product composition from the two types of processes are even more pronounced. The major liquid products (see Table V) from hydropyrolysis of 2 at 550°C are C6-Ci0 cyclohexenes and cyclohexanes, and C5-C8 open-chain hydrocarbons, while in thermal cracking the main liquid product at this temperature is 1,2,3,4,5,6,7,8-octahydronaphthalene. At 600°C a much higher conversion of 2 into C5—C10 aliphatic products is observed in the hydropyrolysis Experiment 25, whereas in the thermal cracking Experiment 26 there is much higher formation of aromatic products, i.e., benzene, toluene, ethylbenzene, and... 

In 1-aryl-1,2-epoxy cyclohexenes, a significant salt effect has been observed in favor of the formation of m-diol products. A diol is obtained from hexamethyl Dewar-benzene oxide via an intermediate with carbonium ion character (Eq. 301). ... 

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