Thermal F/X atomic substitution reactions with methyl halides (X = Cl, Br, I)

The thermal substitution of F for X in CH3X (X = Cl, Br, I) at 2830K has been observed by using radioactive 18F atoms moderated to thermal energies by multiple collisions in gaseous SF6 at 3000 torr. The relative rate constants for these reactions have been measured in competition with addition to C2H2 and/or abstraction from HI, and have been converted to absolute rate constants by assuming a rate constant of 1.53 x 10-10 cm3 molecule-1 s-1 for F + C2H2. The rate constants for the thermal substitution of F/X in CH3X increase as the exothermicity of the replacement reaction increases, and have the following rate constants at 2830K, in units of cm3 molecule-1 s-1: F + CH3I → CH3F + I, k6 = (8 +- 3) x 10-13; F + CH3Br → CH3F + Br, k5 = (1.7 +- 0.3) x 10-13; F + CH3Cl → CH3F + Cl, k4 = (3.7 +- 1.3) x 10-14. In each case the abstraction of H from CH3X is very much faster than the substitution reactions