M6. Thermal Decomposition and Isomerization Processes of Alkyl and Allyl Radicals

A. Miyoshi, N. Yamauchi, T. Harada, K. Kosaka, M. Koshi, and H. Matsui

1. Thermal Decomposition and Isomerization Processes of Alkyl Radicals

The thermal decomposition process of alkyl radicals plays an important role in the high-temperature combustion of alkanes. However, few direct measurement of this process has been reported at elevated temperatures. For long-chain alkyl radicals, the isomerization process is also important since the isomerized radicals give different products, but no direct investigation on this process has been reported. In the present study, the thermal decomposition processes of two isomers of propyl radicals (n-C₃H₇ and i-C₃H₇), three isomers of butyl radicals (n-C₄H₉, s-C₄H₉, and i-C₄H₉), and n-C₆H₁₃ radical have been investigated by using shock-tube apparatus coupled with atomic resonance absorption spectrometry (ARAS). Isomeric alkyl radicals were generated by the thermal decomposition of respective alkyl iodides. Since the molecular decomposition processes (® alkene + HI) are possible in the thermal decomposition of alkyl iodides, initial concentration of alkyl radicals was determined by following the concentration of iodine atoms, the counterpart products to the alkyl radicals, by ARAS.

Branching fractions for the competitive decomposition pathways (C-C bond cleavage versus C-H bond cleavage) have been determined by following the hydrogen atom yield by ARAS. In the investigated temperature range (900-1400 K), for all investigated alkyl radicals, the energetically favored C-C bond cleavage was found to dominate over the C-H cleavage except for i-C₃H₇ radical, for which the C-C bond cleavage is impossible without the isomerization process to n-C₃H₇ radical. The distinct results for n-C₃H₇ versus i-C₃H₇, that is the H-atom yield is < 6% for n-C₃H₇ while it is 100% for i-C₃H₇, shows that the isomerization does not occur between these radicals in the present temperature range. Also, the similar results for n-C₄H₉ versus s-C₄H₉ shows the isomerization barrier is still very high for 1,3-hydrogen shift. For longer chain alkyl radical, n-C₆H₁₃, preliminary results strongly suggest the 1,5-hydrogen shift. The yield of hydrogen atom decreases as temperature decreases in 1050–1350 K. Since the thermal decomposition of n-C₆H₁₃ radical is expected to produce hydrogen atom with 100% yield (1a or 1b), the observed smaller yield should be ascribed to the occurrence of the isomerization to 2-C₆H₁₃ radical (2) followed by its decomposition (3).

•CH₂CH₂CH₂CH₂CH₂CH₃ + M ® C₂H₄ + •CH₂CH₂CH₂CH₃ + M (1a)

[followed by •CH₂CH₂CH₂CH₃ + M ® C₂H₄ + C₂H₅ + M, C₂H₅ + M ® C₂H₄ + H + M]

•CH₂CH₂CH₂CH₂CH₂CH₃ + M ® CH₂=CHCH₂CH₂CH₂CH₃ + H + M (1b)

•CH₂CH₂CH₂CH₂CH₂CH₃ + M ® CH₃CH₂CH₂CH₂C(•)HCH₃ + M (2)

CH₃CH₂CH₂CH₂C(•)HCH₃ + M ® •CH₂CH₂CH₃ + CH₂=CHCH₃ + M (3)

[followed by •CH₂CH₂CH₃ + M ® C₂H₄ + CH₃]

2. Thermal Decomposition Process of Allyl Radicals

Allyl radical is an important intermediate in the combustion of alkanes and alkenes. Because of its resonance stabilization energy (RSE), the thermal decomposition and oxidation processes are expected to be different from, and slower than, the other hydrocarbon radicals. No direct measurement of the thermal decomposition process has been reported. In the present study, the thermal decomposition of allyl radical has been investigated directly with a shock-tube. The allyl radicals were generated in the thermal decomposition of the allyl iodide.

C₃H₅I + M ® C₃H₅ + I + M D H₂₉₈ = 170.3 kJ mol^–1 (4a)

The iodine atom yield was measured to be 100% within the experimental error limit and the molecular dissociation process,

C₃H₅I + M ® C₃H₄ + HI + M (4b)

was found to be very minor above 900K.

Since the enthalpy of the reaction of interest,

C₃H₅ + M ® C₃H₄ + H + M D H₂₉₈ = 245.6 kJ mol^–1 (5)

is larger than that for (4a), reaction (5) can be followed directly by observing the H atoms after the rapid completion of reaction (4a).

Fig. 1 shows the observed H-atom concentration. The ultimate yield of H-atom is apparently smaller than unity especially at lower temperatures. This can be explained by the equilibrium between allyl radicals and dissociation products.

C₃H₅ + M « C₃H₄ + H + M (5, –5)

The observed H-atom concentration is expressed by,

[H] = [C₃H₅]₀, (6)

k’ = k₅(2 – c _e) / c _e ,

where c _e denotes the fraction of dissociation of allyl radical at the equilibrium. The thermal decomposition rate constants were derived by the least squares fitting to equation (6). No pressure dependence was found in the experimental density range (~ 1 ´ 10¹⁹ molecules cm^–3). An Arrhenius plot give the rate expression as,

k₅ = 1.9´ 10¹² exp(–205[kJ mol^–1] / RT) s^–1. (1100–1350 K)