The reaction mechanism can be in-depth understood if one can determine the rate constants from different sites. Figure shows the predicted rate constants of hydrogen abstraction reactions at different sites of n-C12H26. The different sites of alkane are labeled as CnH2n+2-i, in which i is a positive integer representing the position of abstracted hydrogen atom from the end to the middle of CnH2n+2. The predicted rate constants from different sites of n-C12H26 are obviously distinct at low temperatures and the difference diminishes as the temperature rises up. In the temperature range from 300 to 1000 K, the predicted rate constants follow the order of n-C12H26-1 (the end) < n-C12H26-2 < n-C12H26-3 < n-C12H26-4 < n-C12H26-5 < n-C12H26-6 (the middle). The predicted thermal rate constants for other hydrogen abstraction reactions of alkanes with CH3/H/O are carefully checked as well. We observed that the primary alkane site always has the lowest reactivity. The reactivity of the secondary site gradually diminishes from the last but one to the middle and becomes close to each other for large alkanes. It is widely recognized that the abstraction of secondary hydrogen atoms is easier than primary ones due to their lower bond dissociation energy. Diego Troya investigated the barriers of hydrogen abstractions from primary, secondary, and tertiary sites of acyclic alkanes by ground-state oxygen atoms by high-level ab initio calculations. The calculated thermal rate constants via the transition-state theory indicate that the room-temperature relative reactivities of primary, secondary, and tertiary alkane sites are 1, 29 and 422, following the order of primary < secondary < tertiary, in agreement with our findings. This gives us more confidence on the capability of generalization.