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Why is the first halogenation the slowest one?

Answer 1

Isabella Acosta

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.

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Answer 2

Ruby Allen

The first halogenation is the slowest one because it involves the breaking of a strong carbon-hydrogen bond, which requires more energy compared to subsequent halogenations where weaker carbon-halogen bonds are formed.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.