Hydroxyl Radical Kinetics by Kinetic Spectroscopy. VI. Reactions with Alkanes in the Range 300–500°K

Abstract

Flash photolysis and kinetic spectroscopy were used to study the reactivity of OH radicals toward a representative set of alkanes. Previous studies have shown that OH radicals abstract H atoms from alkanes to produce H₂O and an alkyl radical. The following formula was found to reproduce accurately all of the observed abstraction rate constants for the 10 model alkanes: $k_{\mathrm{tot}}{\text{\hspace{0.17em}=\hspace{0.17em}6.15\hspace{0.17em}×\hspace{0.17em}10}}^{11}{N}_1\exp {\text{(−\hspace{0.17em}1635\hspace{0.17em}/\hspace{0.17em}RT)\hspace{0.17em}+\hspace{0.17em}14.1\hspace{0.17em}×\hspace{0.17em}10}}^{11}{N}_2\exp {\text{(−\hspace{0.17em}850\hspace{0.17em}/\hspace{0.17em}RT)\hspace{0.17em}+\hspace{0.17em}12.6\hspace{0.17em}×\hspace{0.17em}10}}^{11}{N}_3\exp \text{(+\hspace{0.17em}190\hspace{0.17em}/\hspace{0.17em}RT)}{\mathrm{cc\; mol}}^{\text{−1}}·{\sec }^{\text{−1}}$ , where $N_1$ , $N_2$ , and $N_3$ are the respective numbers of primary, secondary, and tertiary H atoms in the alkane, and $k_{\mathrm{tot}}$ is the total (in these experiments the observed) rate constant for the abstraction of H atoms from the alkane. Thus, there is a generally applicable frequency factor and activation energy for each of the three types of H atoms found in alkanes. Methane and ethane were somewhat expected exceptions to this rule. Their rate constants are ${\text{3.31\hspace{0.17em}×\hspace{0.17em}10}}^{12}\exp \text{(−\hspace{0.17em}3772\hspace{0.17em}/\hspace{0.17em}RT)}$ and ${\text{1.12\hspace{0.17em}×\hspace{0.17em}10}}^{13}\exp \text{(−\hspace{0.17em}2447\hspace{0.17em}/\hspace{0.17em}RT)}{\mathrm{cc\; mol}}^{\text{−1}}·{\sec }^{\text{−1}}$ , respectively.