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

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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: ktot = 6.15 × 1011 N1exp(− 1635 / RT) + 14.1 × 1011 N2exp(− 850 / RT) + 12.6 × 1011 N3exp(+ 190 / RT) cc mol−1·sec−1, where N1, N2, and N3 are the respective numbers of primary, secondary, and tertiary H atoms in the alkane, and ktot 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 3.31 × 1012exp(− 3772 / RT) and 1.12 × 1013exp(− 2447 / RT) cc mol−1·sec−1, respectively.