Why is the first halogenation the slowest one?

1 Answer
Feb 5, 2015

The first halogenation is the slowest because the first radical intermediate is the highest in energy.

I think you may be asking about the successive free radical halogenation of a molecule like methane:

CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄

The corresponding free radical intermediates are

·CH₃, ·CH₂Cl, ·CHCl₂, and ·CCl₃

The Cl atoms stabilize the radicals. The more Cl atoms, the more stable the radical is.

The diagram below works just as well if you replace OCH₃ with Cl.


So, breaking the C-H bond in CH₄ to form a carbocation takes the most energy. It is the slowest reaction.

Breaking the C-H bond in CH₃Cl takes less energy, because ·CH₂Cl is lower in energy. This step will be faster.

The formation of CCl₄ from CHCl₃ will be the fastest of all.