Free Radical Bromination of n-Propane

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In organic chemistry synthesis, one of the first steps that is undertaken is to functionalize an alkane. A common method in "paper" synthesis is free-radical halogenation.

Free-radical halogenation substitutes a halogen for a hydrogen on an alkane. The two commonly used halogens are Bromine and Chlorine. Bromine is most often used because is results in only one product. Bromination, as a result of free-radical halogenation will exclusively bond at the most substituted carbon. Chlorine, on the other hand, will yield multiple products as it has no specificity for carbon substitution.

Free-radical halogenation occurs in the presence of ultraviolet light (hv). Most often, free-radical mechanisms have Initiation steps, Propagation steps and Termination steps.

The Initiation step begins with the bromine separating into two radicals with the addition of ultraviolet light.

The Propagation step begins when one of the bromine radicals abstracts a hydrogen (not a proton, the radical wants the electron) from the most substituted carbon (2) in the n-propane molecule. This results in an n-propane radical.

To keep the chain going, the n-propane radical reacts with a Br2 molecule yielding 2-bromo-propane and another bromine radical. If the propagation does not result in another radical, the reaction ends.

What can cause this chain-reaction to end? Side-reactions of two radicals. These are called Termination steps. Possible termination steps for this mechanism are:

1) Two Br radicals reacting with each other to reform Br2.

2) A Br radical reacting with an n-propane radical.

3) Two n-propane radicals reacting with each other.