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Organic Chemistry/Introduction to reactions/Free-radical halogenation

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Free-radical halogenation

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Free-radical halogenation is "a way to do a substitution other than by nucleophilic substitution."


RH (hydrocarbon) + X2 (halogen) -> RX (haloalkane) + HX (byproduct)

or, simplified:


RH + X2 → RX + HX

H's come off as H+ not H-, making nucleophilic substitution impossible.


CH4 Br-/-> No reaction

We must first remove a H+ to perform the reaction.

Free radicals as a rule violate the octet rule and are highly reactive, reacting even with strong bonds.


Creating a free radical

Br2 heat or light/-> 2Br° free radicals


Bromine radical reacting with (relatively unreactive) methane

Br° + CH4 -> HBR + H3C°

then


H3C° + Br2 -> CH3Br + Br° (and radical produced continues reacting with surrounding molecules)

The reaction continues happening in a chain mechanism called a free radical chain reaction.

Free radical chain reaction

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This has three steps:

  1. Initiation step: this generates radicals
  2. Propagation step: this is where the bulk of the action lies. Chain carriers generate reaction products.
  3. Termination step: anything that consumes the free radicals.


Termination step examples


Br + CH3Br -> BrCH3 + Br

Note

Fluorine gas is too reactive and does not undergo this sort of reaction as it explodes. Iodine doesn't work very well either as it is too unreactive. So the reaction works only for Cl and Br, the reaction with Cl running a bit faster than with Br.

Running free radical chain reactions with Chlorine and Bromine yield their respective end products in different proportions, a quality that can be useful in the laboratory.

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