Specific radioactivity benzene

The components of the starting mixture are in rapid adsorption-desorption interaction with the surface. For example, a part of adsorbed -hexane desorbs as -hexane another part reacts to give benzene. If benzene formation involves an n-hexene surface intermediate, this hexene—the concentration of which may be eventually so small that it does not appear in the gas phase—interacts with the inactive hexene in the starting material and increases its specific radioactivity. 

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... 

Fig. 1. Specific radioactivities of individual fractions and the ratio of the corresponding values for cH /cH, as a function of benzene conversion. Adapted from Ref. 13.

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... 

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 ... 

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. 

Fig. 3. The ratio of the two pathways of benzene formation (as characterised by the ratio of the specific radioactivities of cH=, / and Bz, 7 from a mixture of [ C]-cyclohexane and inactive cyciohexene) as a function of the atomic diameter of the metal catalyst. The atomic distances in the six-membered ring are as follows (considering also possible valence angles in various conformations) are C1-C2 0.153nm C1-C3 0.2506-0.2632nm. Redrawn after Ref. 15 the point for Re taken from Ref. 14.

The three components involved in the mixtures all give benzene. A fraction of benzene should be radioactive, and its specific activity will reflect how much of this product was formed from the radioactive and how much from the inactive component of the starting mixture. 

Labeled compounds experience self-radiolysis induced by the radioactive decay. The extent of such radiation effects depends on the half-life, the decay energy, the specific activity of the sample, and the G-value for decomposition. The presence of other substances can considerably affect the amoimt of damage. Aromatic compounds such as benzene (as a solvent) can serve as a protective medium to minimize radiation self-decomposition, whereas water or oxygen enhance it. 

In experiments without reference cited products have been isolated by TLC on silica gel using (a) benzene-methanol (9 1) and (b) petroleum ether (b.p. 50—70°) diethyl ether-methanol (40 60 1, twice) for the subsequent separation of (3) and (6). On analytical scale the metabolites were quantitized at 360 or 290 nm before radioactivity measurements. On a preparative scale the metabolites were recrystallized to constant specific activity. Both methods gave comparable results. 

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