Premium
The rate constant for the intramolecular isomerization of pentyl radicals
Author(s) -
Marshall Roger M.
Publication year - 1990
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550220905
Subject(s) - chemistry , isomerization , radical , pentane , reaction rate constant , intramolecular force , hydrogen atom abstraction , thermochemistry , thermal decomposition , butane , propene , thermodynamics , transition state theory , computational chemistry , atmospheric temperature range , organic chemistry , kinetics , quantum mechanics , physics , catalysis
The thermal decomposition of n ‐pentane has been investigated in the temperature range 737 to 923 K. Making various assumptions, the detailed distribution of major products (methane, ethane, ethene, propene, and 1‐butene) is used to evaluate the rate constant for the unimolecular isomerizationwhich proceeds via a five‐membered, cyclic transition state. Two alternative sets of assumptions are used. Common to both of them are assumptions concerning the thermochemistry and rate constants for decomposition of the C 5 H 11 radicals. Method 1 assumes that all secondary CH bonds are equally reactive towards hydrogen abstraction in which case, in addition to the value of 10 , the ratio of the rate constants for abstraction from primary and secondary CH bonds is evaluated. Values about a factor of two higher than published values for similar molecules are obtained. The alternative, method 2, assumes that the ratio of abstraction from the 1‐ and 2‐ positions of n −pentane is the same as that published for n −butane, in which case, in addition to the value of 10 , the ratio of the rates of abstraction from the 3‐ and 2‐ positions of n ‐pentane is obtained. The value obtained is 0.401 which is to be compared with the statistically expected (and assumed in method 1) 0.5. Detailed discussions of the values of 10 obtained leads to the conclusion that method 1 leads to the best value\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log(}k_{10} /{\rm S}^{ - 1} ) = 11.96 \pm 0.77 - (23.4 \pm 2.0)/\theta $$\end{document}where θ = 2.303 RT in kcal/mol and error limits are two standard deviations. Combination of this value with values recalculated from published lower temperature data gives\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log(}k_{10} /{\rm S}^{ - 1} ) = 11.08 - 20.04/\theta $$\end{document}which, it is concluded, is the best value in the range 438 to 923 K.