Radioactive cyclohexane

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... 

FIGURE 22 Release of radioactivity from polymer discs prepared from 3,9-bis(ethylidene-2,4,8,10-tetraoxaspiro[5,5]undecane) and 35 65 mole ratio of trans-cyclohexane dimethanol and [l C]l,6-hexanediol in phosphate buffer and canine gastric juice at 37°C. Polymer conteuned 2.0% phthalic anhydride. (From Ref. 31.)... 

Figure 3. Radioactivity in products. n-Hexane- C/ Cyclohexane/H2 over Te-NaX at 450 C.

Even though cyclohexane is rapidly converted into benzene under these conditions, the results in Figure 3 clearly prove that it cannot be a gas phase intermediate in the n-hexane reaction. If it were, there would have been radioactivity in the unreacted cyclohexane when it was mixed with labeled n-hexane none was observed. This proves that the cyclization step must be further along the reaction stream and must not involve an olefin forming cyclohexane which then dehydrogenates to the aromatics. 

If cyclohexane is added as a second component to n-hexane, a similar increase of its radioactivity should be observed if it is really produced from -hexane. The appearance of radioactivity in the assumed intermediate can be observed even if its concentration in the gas phase does not correspond to sorption equilibration. 

Comparative results are shown in Table I. A considerable increase in n-hexene radioactivity is observed, whereas no radioactivity appeared in cyclohexane. 

The tritium-labelled l-(3-xenyl)cyclohexane-2-[3H], 60, has been obtained similarly (in 84.3% chemical yield and 0.89% radiochemical yield based on the total radioactivity of 3H20 used) see equation 24. 

A detailed study has been carried out of the interconversion of benzene and cyclohexane by the transfer of hydrogen atoms. This was detected by placing a radioactive label in one molecule and observing its appearance in the other this reaction occurred with Au/MgO and Au/A12C>3 between 473 and 523 K.31... 

Rzad and Schuler" studied the radiation chemistry of a solution of " C-cyclopropane in hexane over the concentration range 10 " to 10 M. The main radioactive products, which appear to result from ion molecule reactions, are propane formed by H2 transfer (50 %) and by H transfer (20 %) and mixed nonanes (30 %) formed by the addition of CaHg unit to a hexyl ion. At the lower concentrations, very pronounced dose dependence of the yields was observed. This was ascribed to a competitive formation of olefins in the radiolysis. For cyclopropane-cyclohexane solutions the chemical processes seem to be considerably more complicated. The observed yield of total radioactive products extrapolated to zero concentration of cyclopropane are 0.05 and 0.11 G units for hexane and cyclohexane, respectively. These limiting yields are of the order of magnitude of and appear to be related to, the free ion yields in these systems. Since cyclopropane was found to react with hydrocarbon ions" it is used quite often as a scavenger for positive ions, as in the work of Davids and coworkers . 

C and n-octadecane-1,2-H at a concentration of 0.35 molar in stearic acid and studied the coadsorption of stearic acid and octadecane on polished surfaces of silver, platinum, copper and iron. The films were prepared by retraction from the melt at 40 C (at room temperature the mixture was solid). The proportion of n-octadecane in the film was assayed by differential extraction with cyclohexane. The results of the investigation adequately demonstrate that n-octadecane coadsorbs with stearic acid but not necessarily as a mixed oriented monolayer. Some of the data indicate that more than a single layer is present on the surface. Thus the structure of the long-chain material on the surface may be open to conjecture, but that each constituent adsorbs and in what relative amount is directly determined by radioactive assay. 

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... 

Similar experiments have been made 91, 222) with chromia from ammonium bichromate at 450° calculation showed that here, inversely, up to 90% of the radioactive carbon of the cyclohexane passed into benzene. 

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. ...

Fig. 2. Specific molar radioactivities in the components of feed and product when reacting a mixture of [ C]-n-hexane (65%) plus inactive 1-hexene (35%) and [ C -n-hexane (62%) plus inactive cyclohexane (38%). Pulse system, catalyst Pt black, carrier gas helium.

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 ... 

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 kinetic isotope method was also applied to study whether the stepwise or direct route is valid for the dehydrogenation of the cyclohexane ring.[bi4,67] mentioned before, plotting specific radioactivities as a function of the contact time confirmed the predominance of direct pathway on Re and the stepwise dehydrogenation on chromia. 

Experiments Designed to Determine the Effect of Water Additives on Count Rates. The composition of the scintillator solution was 70 Vol % toluene and 30 Vol % Triton to which 7g/l PPO and 0.35g/l DMPOPOP were added. To each vial containing 10 ml of this solution was added the desired amount of water followed by the addition of 10 yl of the radioactive sample, H-toluene, H-H20, C-cyclohexane, C-sodlum carbonate and NaCl, the two latter compounds dissolved in a -microamount of H2O. The counting solutions prepared in this way were vigorously shaken and counted at room temperature. 

Here, diffusivities are binary values encountered earlier and can be functions of concentrations. Summing Equation 8.63 over all i, the right-hand side becomes zero and the left-hand side also becomes zero on using the Gibbs-Duhem equation. Consider now a four-component system cyclohexane (1), benzene (2), cyclohexane with a radioactive carbon tracer (1 ), and benzene with a radioactive tracer (2 ). (Radioactive and are common.) The last two are very dilnte ( infinitely dilute ) but are easy to detect quantitatively. We write out the equations for 1, r, and 2, assuming that (1) = 0 and jCj, = 0, but that their gradients... 

C]-labeled -hexane plus inactive 1-hexene, the isotopic composition of benzene and unreacted hexene in the product was nearly equal (Fig. 2), indicating that all benzene molecule should have involved a hexene intermediate. In turn, hardly any radioactivity appeared in the residual inactive cyclohexane reacted in a... 

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