Isomerization and Decomposition of Indole. Experimental Results and Kinetic Modeling

The thermal reactions of indole were studied behind reflected shocks in a pressurized driver single−pulse shock tube over the temperature range 1050−1650 K and densities of ∼3 × 10-5 mol/cm3. Similar to pyrrole, the main thermal reactions of indole are isomerizations. Three isomerization products are obtained under shock heating as a result of the pyrrole ring opening. These isomers are benzyl cyanide, o- and m-tolunitriles. Studies with toluene as a free-radical scavenger shows no effect on the production rates of these isomers. The decomposition products that were found in the postshock samples in decreasing order of abundance were C2H2, HCN, HC⋮CCN, C4H2, C6H5CN, CH3CN, and C6H6. Small quantities of C6H5−CH3, CH4, C5H5CN, CH2CHCN, C5H6, C6H5−C⋮CH and traces of C2H4, CH2CCH2, CH3−C⋮CH, C2H4, C6H4, and C5H5−C⋮CH were also found in the postshock mixtures. The total disappearance of indole in terms of a first-order rate constant is given by ktotal = 1015.78 exp(−83.6 × 103/RT)s-1 where R is expressed in units of cal/(K mol). At high temperatures the extent of fragmentation increases, and around 1300 K the fragmentation begins to exceeds that of the isomerizations. It is suggested that indole → benzyl cyanide isomerization starts with cleavage of the C(9)−N(1) bond followed by two 1,2 H-atom migrations. The mechanism of indole → tolunitrile isomerization involves a series of unimolecular steps which are preceded by the very fast indole ↔ indolenine tautomerism. The thermal decomposition of indole is initiated by H-atom ejection from the reactant. A reaction scheme containing 48 species 109 elementary reactions accounts for the observed product distribution. First-order Arrhenius rate parameters for the formation of the various reaction products are given, a reaction scheme is suggested, and results of computer simulation and sensitivity analysis are shown. Differences and similarities in the reactions of pyrrole and indole are discussed.