The rate constant for the intramolecular isomerization of pentyl radicals

Abstract

AbstractThe 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 isomerization equation image which 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 C5H11 radicals. Method 1 assumes that all secondary CH bonds are equally reactive towards hydrogen abstraction in which case, in addition to the value of 𝓀10, the ratio of the rate constants for abstraction from primary and secondary CH 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 𝓀10, 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 𝓀10 obtained leads to the conclusion that method 1 leads to the best value where θ = 2.303RT in kcal/mol and error limits are two standard deviations. Combination of this value with values recalculated from published lower temperature data gives which, it is concluded, is the best value in the range 438 to 923 K.