Dodecene reaction paths

The relative contributions of the three proposed reaction paths remain to be determined. The products from both the retro-ene and the addition reactions can be predicted with some quantitative certainty. For dodecene, the former produces only 1-nonene and propylene. The product distributions from the addition paths should be identical to those from the cracking of the corresponding paraffins. These product distributions can be predicted using the method of Rice and Kossiakoff (16). This approach parallels excellently with experiments (16,17). (From available data we estimate its accuracy as d=10%.) For dodecene, the addition of hydrogen atoms, methyl radicals, and ethyl radicals will produce Ci2H25, Ci3H27, and Ci4H29 parent radicals. The product distributions predicted by the Rice-Kossiakoff method for the decomposition of these radicals at 525°C are shown in Table IV. 

Table V. Relative Contributions of Reaction Paths in Dodecene Cracking...

The measured reaction orders support the proposed mechanism. Path A is certainly first order in reactant. Paths B and C may be 1/2, first, or 3/2 order, depending on the termination reaction (15). Most likely, termination involves combination or disproportionation of small chain carrying radicals (CH3, C2H5 ). With first-order initiation, this would result in 3/2-order kinetics. The overall reaction order would then be somewhere between first (Mechanism A) and 3/2 (Mechanisms B and C). The measured order of 1.33 for dodecene agrees with this prediction. The fact that propylene and nonene are formed with reaction orders of 1.10 and 1.16 with respect to dodecene (Table III) supports the hypothesis that they are formed largely by a first-order decomposition. 

The estimated relative contributions of the three paths for dodecene cracking under a variety of conditions are shown in Table V. Two trends are visible in the data. First, at constant temperature and increasing dodecene partial pressure, the relative importance of the retro-ene path decreases while that of the abstraction path increases. The contribution of the addition path remains constant. The average estimated relative contributions of the retro-ene reaction at 525°C taken from Table V correspond to a reaction order of 0.95 0.4 in dodecene this satisfies the requirement that retro-ene reaction be first order. 

The reason for the increase in the importance of the H abstraction path with increasing pressure is more subtle. While the reaction order for the radical addition path is 1.33 (the same as the overall reaction), the abstraction path is 1.40 order in dodecene (from the data in Table V). This may be attributable to a decrease in [H ] with increasing pressure because more ethyl radicals are stabilized to ethane and fewer are decomposed to ethylene and H- as the pressure is increased (see Figure 7). If H atoms are the most efficient addition species, a decrease in their relative concentration would result in a relative decrease in the contribution of the addition path and a corresponding increase in the role of the H abstraction route. 

The second trend is that at constant dodecene partial pressure as the temperature is increased, the relative importance of the retro-ene reaction decreases slightly, while the H abstraction path and the radical addition path show much more marked temperature dependences. The former increases while the latter decreases in relative importance. This suggests that the activation energy for hydrogen abstraction is significantly higher... 

In this case, the endothermicities of Reactions A-3 and A-4 are not equal since Reaction A-3 produces an allyl ladical. Therefore, one would expect Reaction A-3 to be somewhat faster than Reaction A—4. If Radicals I and II are present in equal concentrations, then the yield of nonene from the hydrogen-abstraction path is between one and two times the yield of octene. The nonene in excess of this is formed by the retro-ene reaction. This defines a range of importance for the molecular path and by difference the contribution of the hydrogen abstraction path. This procedure is illustrated for the dodecene cracking yields from column two of Table II. 

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