J. Phys. Chem. 1992, 96, 281-290 crotononitrik
281
group of allyl cyanide. Rate of production analysis indicates that a significant reaction flux to HCN flows through reaction 77, the disproportionation reaction of two cyanomethyl radicals to form HCN and acrylonitrile. From Figure 11 it may be seen that the sensitivity of acrylonitrile toward the rate of the allyl cyanide initiation reaction H 2 C 4 H C H 2 C N -w H,C=CH + CH2CN (-1 3)
Sensitivity for Acetylene 1.1
11
exceeds unity. This implies that the reaction is slightly branching in nature. This can arise if the vinyl radical initially produced in (-13) decomposes sufficiently rapidly to C2H2 H.
+
Allylcyanide Sensitivity for Acetylene 11
0.0
8 Ja
1
--. CUfN
0.6
Figure 14. Variation with temperature of sensitivity coefficients for the
designated species. Only the most sensitive reactions are shown. abstraction reaction of cyanomethyl radicals with crotononitrile, reaction 27. From Figure 13 it may be seen that two reactions have high sensitivities for HCN production. These are the unimolecular fission of allyl cyanide into cyanomethyl and vinyl radicals (reaction -1 3) and reaction 50, the addition of H atoms to the nitrile
Conclusions The butenenitriles undergo thermal decomposition between 1200 and 1500 K at about 20 atm pressure and at residence times from 650 to 750 NS. Isomerization precedes thermal decomposition. The decomposition is free radical in nature, with a major chain mechanism involving the cyanomethyl radical, leading to the production of acetonitrile and acetylene. Other reaction pathways have been identified involving H-atom additions to the reactant isomers. Several nitrogen-containing radicals including cyanomethyl, cyanovinyl, and cyanoallyl, together with their hydrocarbon counterparts, methyl, vinyl, and allyl, play important roles in the pyrolysis mechanism. v o l y s i s of the butenenitriles can be satisfactorily modeled by a detailed kinetic mechanism involving the aforementioned radicals and hydrogen atoms. Yields of most major and minor decomposition products exhibit kinetics of formation approximately first order in the initial concentration of the starting butenenitrile isomer. The outstanding exception is acetonitrile, whose kinetics are largely influenced by cyanomethyl radical abstraction. Arrhenius parameters for initiation reactions of the butenenitriles can be obtained from experiment and kinetic modeling. Acknowledgment. We thank Drs Robin Walsh and Peter Nelson for helpful discussions. Dr. Nelson also assisted with GCMS analyses. Mr. M. Esler is thanked for research assistance. Finanical support of the ARC and CSIRO/hiversity of Sydney Collaborative Funds are gratefully acknowledged. Regis@ NO.(Z)-H$CH=CHCN, 1190-76-7; (E)-H$H
