of fluorine and methane explode.
2) Chlorine is the next most reactive –– chlorination of methane is easily controlled
by the judicious control of heat and light.
3) Bromine is much less reactive toward methane than chlorine.
4) Iodine is so unreactive that the reaction between it and methane does not take
place for all practical purposes.- The reactivity of halogens can be explained by their ∆H° and Eact for each step:
1) FLUORINATION:
∆H° (kJ mol–1) Eact (kJ mol–1)
Chain Initiation
F 2 2 F• + 159 + 159
Chain Propagation
F• + CH 3 –H H–F + CH 3 • – 134 + 5.0
CH 3 • + F–F CH 3 –F + F• – 293 Small
Overall ∆H° = – 427
i) The chain-initiating step in fluorination is highly endothermic and thus has a
high energy of activation.
ii) One initiating step is able to produce thousands of fluorination reactions ⇒ the
high activation energy for this step is not an obstacle to the reaction.
iii) Chain-propagating steps cannot afford to have high energies of activation.
iv) Both of the chain-propagating steps in fluorination have very small energies of
activation.
v) The overall heat of reaction, ∆H°, is very large ⇒ large quantity of heat is
evolved as the reaction occurs ⇒ the heat may accumulate in the mixture faster
than it dissipates to the surroundings ⇒ causing the reaction temperature to rise
⇒ a rapid increase in the frequency of additional chain-initiating steps that
would generate additional chains.
vi) The low energy of activation for the chain-propagating steps and the large