Finally, it is interesting to emphasize the increase of the rate constants in the reaction of the CN radical in the series of alkanes: methane, ethane and propane: 14 at 298 K, respectively: 8.5.10 -13 , 2.9.10 -11 and 5.5.10 -11 cm 3 molecule -1 s -1 . This tendency is associated with the barrier height, 2.5, 0.23 and ~031 kcal mol -1 , with the exothermicity, -20.0, -22.2 and -25.9 kcal mol -1 and finally with the weakening of the C-H bond in the series methane, ethane and propane: 105.0, 101.0 and 101.4 kcal mol -1 .