Radioactive cyclohexene

Cyclohexadiene was converted totally on a Ni catalyst when the benzene conversion was - 5-6%. From this point onwards, the concentration of cH dropped dramatically with a simultaneous increase in its specific radioactivity exceeding that of cH, when the Bz conversion reached 10% as shown in Fig. 3. The formation of radioactive cyclohexene from [ C]-benzene supplied further evidence of the existence of stepwise hydrogenation, even if it is not the exclusive route. 

Experiments were made by us (91) on dehydrogenation of cyclohexane over a rhenium catalyst at 336° with the addition of cyclohexene It was found that 77% of the radioactive carbon passed into cyclohexane, since not more than 17% passed into benzene. The total quantity of cyclohexene varied in this case from 0.9% in the initial mixture to 1.7% in the catalysate at the time of contact t = 61.7 seconds. Thus, cyclohexene is generally hydrogenated in the presence of cyclohexane. It is quite evident that hydrogen is necessary to this end... 

A new case of flat orientation upon dehydrogenation of cyclohexane on a sample of chromia has been observed recently (228). Thechromia is distinguished from speciments studied before in that it was crystalline, as the X-ray analysis showed. The energy of activation was low here (13 kcal/mole) and cyclohexene was not found. It was shown by means of the radiochemical method that cyclohexene was not an intermediate product, because radioactive cyclohexane previously added did not enter the reaction. The model of the reaction is a variant of the sextet model, considered by the author in the paper of 1929 (6), without the central atom of metal [variant IV, Fig. 18 or Fig. 2 of the author s paper... 

The specific radioactivities of the products cyclohexene and benzene were identical in the case of Ni. However, with Pt and Rh the specific activity of benzene was higher than that of cyclohexene, suggesting that some other factor was important for these metals. Two possibilities were considered. Either a direct pathway, not involving a cyclohexene intermediate, was also operative, or cyclohexene was formed on the surface but its rate of desorption was very low compared with its rate of further dehydrogenation. 

Cyclohexane dehydrogenation represents another classical example for isotopic studies. Balandin s sextet mechanism predicted direct dehydrogenation of cyclohexane over several metals, assuming a planar reactive chemisorption of the reactant. Cyclohexene is also readily dehydrogenated to benzene. The use of hydrocarbons labelled with established the true reaction pathway. T6tenyi and co-workers[ °di] reacted a mixture of [ 0]-cyclohexane and inactive cyclohexene on different metals and measured the specific radioactivity of the fractions (cyclohexane, cH, cyclohexene, cH= and benzene, Bz) in the product at low conversion values (Table The... 

Table 1. Relative molar radioactivities (r%/m%) in the products after reacting a mixture of [ C]-Cyclohexane and Cyclohexene. ...

The reaction hexenes —> hexadienes was demonstrated without using radiotracers both on oxide and metal catalysts, Nil 1 and Ptj l Mixtures containing [ " CJ-hexene contributed to the clarification of the further reaction pathway. These studies showed that neither the hexene cyclohexane nor the hexene —> cyclohexene ring closure pathway took place.Table 2 indicates that radioactivity appeared in both the hexatriene and 1,3-cyclohexadiene fractions when their inactive form was admixed to radioactive hexene. The aromatisation of both inactive components was much more rapid than that of hexene, therefore their specific radioactivities showed very low absolute values, however, these were still higher than that of benzene produced mainly from these non-radioactive precursors. The true precursor of ring closure should have been cis-cis-1,3,5-hexatriene. Its ring closure takes place without any catalyst from 513 The stepwise dehydrogenation of open-chain hydrocarbons produces cis- and trans-isomers of alkenes and alkadienes. Any c s-c s-triene... 

C]-hexene, 1% cyclohexene, 75% cyclohexadiene. Due to the incomplete separation of hexenes and cyclohexene resulted in the appearance of radioactivity as an artifact in the cyclohexene impurity. After Ref. 20. 

The loss of the hydroxyl group from either the starting cyclohexanol (to give cyclohexane), or from phenol (to give benzene) can also take place. Dehydration of cyclohexanol to cyclohexene is also possible as summarised in Scheme 3. These reactions were studied over various metal catalysts, by using different mixtures containing one labelled component. Typical results obtained on Cu, Ni and Pt catalysts are summarised in Table 5. The specific radioactivities decreased in the sequence cH-ol > cH-one > phenol on Cu and Ni catalysts, while a different order cH-ol > phenol > cH-one was observed on Pt, as well as on Pd. Thus, the sequential reaction 1 2 leads to phenol in the former two catalysts and the direct route to phenol lA is possible on Pt and Pd. The following relative rates were determined for Ni catalyst ... 

Hot-atom chemistry of a number of fluorocarbons [perfluoro-n-hexane, -cyclohexane, -(methylcyclohexane), -cyclohexene, -benzene, and -toluene] has been studied, the substrates being activated by means of the nuclear reactions F(n,2n) F and F(y,n) F a large number of F-labelled compounds were obtained, but, except in the case of perfluoro-n-hexane (suggested displacement reactions C,Fi4 + F -+ CjFjs F + F- C,Fi4 + > F- -> CsF i F + CF,- C,Fi4 + F- - CjF. F + CsF,-), most of the radioactive products were not recognized. 

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