overall heat of reaction ⇒ high reactivity of fluorine toward methane..
vii) Fluorination reactions can be controlled by diluting both the hydrocarbon and
the fluorine with an inert gas such as helium or the reaction can be carried out
in a reactor packed with copper shot to absorb the heat produced.2) CHLORINATION:
∆H° (kJ mol–1) Eact (kJ mol–1)
Chain Initiation
Cl 2 2 Cl• + 243 + 243
Chain Propagation
Cl• + CH 3 –H H–Cl + CH 3 • + 4 + 16
CH 3 • + Cl–Cl CH 3 –Cl + Cl• – 106 Small
Overall ∆H° = – 102
i) The higher energy of activation of the first chain-propagating step in
chlorination (16 kJ mol–1), versus the lower energy of activation (5.0 kJ mol–1)
in fluorination, partly explains the lower reactivity of chlorine.
ii) The greater energy required to break the Cl–Cl bond in the initiating step (243
kJ mol–1 for Cl 2 versus 159 kJ mol–1 for F 2 ) has some effect, too.
iii) The much greater overall heat of reaction in fluorination probably plays the
greatest role in accounting for the much greater reactivity of fluorine.3) BROMINATION:
∆H° (kJ mol–1) Eact (kJ mol–1)
Chain Initiation
Br 2 2 Br• + 192 + 192
Chain Propagation
Br• + CH 3 –H H–Br + CH 3 • + 69 + 78
CH 3 • + Br–Br CH 3 –Br + Br• – 100 Small