Overall ∆H° = – 31
i) The chain-initiating step in bromination has a very high energy of activation
(Eact = 78 kJ mol–1) ⇒ only a very tiny fraction of all of the collisions between
bromine and methane molecules will be energetically effective even at a
temperature of 300 °C.
ii) Bromine is much less reactive toward methane than chlorine even though the
net reaction is slightly exothermic.4) IODINATION:
∆H° (kJ mol–1) Eact (kJ mol–1)
Chain Initiation
I 2 2 I• + 151 + 151
Chain Propagation
I• + CH 3 –H H–I + CH 3 • + 138 + 140
CH 3 • + I–I CH 3 –I + I• – 84 Small
Overall ∆H° = + 54
i) The I–I bond is weaker than the F–F bond ⇒ the chain-initiating step is not
responsible for the observed reactivities: F 2 > Cl 2 > Br 2 > I 2.
ii) The H-abstraction step (the first chain-propagating step) determines the order
of reactivity.
iii) The energy of activation for the first chain-propagating step in iodination
reaction (140 kJ mol–1) is so large that only two collisions out of every 10^12
have sufficient energy to produce reactions at 300 °C.5) The halogenation reactions are quite similar and thus have similar entropy
changes ⇒ the relative reactivities of the halogens toward methane can be
compared on energies only.