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free radical halogenation > mechanisms > home      


Free radical halogenation

Click here for an animation of the free radical halogenation mechanism of methane (opens in a popup). Does not show termination steps. Requires the Flash plugin, standard with most new browsers.

Free radical halogenation is the result of chlorine or bromine added to an alkane in the presence of uv light (hv). The reaction begins with an initiation step, the separation of the halide into two radicals (atoms with a single unpaired electron) by the addition of uv light. Note the use of a single headed arrow when representing the movement of a single electron.

Initiation Step:
initiation steps


Propogation Steps:
propogation step 1

The initiation step, the formation of the chlorine radicals, is immediately followed by the propogation steps--steps directly involved in the formation of the product. As an example, isobutane will be used. The first step is the abstraction of the tertiary hydrogen atom (note that these are not protons, but actual hydrogen atoms since they each have one electron), forming the tertiary radical.

Hydrogens attached to more highly substituted carbons (ie. carbons with the most other carbons attached to them, like tertiary carbons) are kinetically more reactive because the radical they form is stabilized by neighboring alkyl groups that have the ability to donate part of their electron density inductively through the sigma framework to the electron-deficient radical carbon.

Here, the tertiary radical is stabilized by electron donation from neighboring alkyl groups.

 

A point of note about free radical processes is that the intermediates are so highly reactive and short lived that usually there is a mixture of products. This is the major downfall of radical reactions, and why they have been overlooked in industry for many years as mixtures of products are undesired, although now these radical reactions are regaining popularity with new methods to control single product formation. In our example, for instance, there would certainly be some formation of the primary radical and, ultimately, isobutyl chloride, although it would be a minor product (assisted by the fact that statistically there are nine primary hydrogens and only 1 tertiary hydrogen). Free radical chlorination is less selective than bromination, so bromination more selectively adds bromine to the more highly substituted carbons.


propogation step 2

The tertiary radical then reacts with one of the chlorine radicals formed in the initiation step to form the product. Notice that the chlorine radical is regenerated, so this reaction can, in theory, go on forever as long as there are reagents. This is called a chain reaction.


Termination Steps:
Side reactions that can stop the chain reaction are called termination steps.
termination step
termination step
termination step

Bromine reacts exactly the same way as chlorine; however, it is far more selective. If propane, for example, was the substrate, 2-bromopropane would be the dominant product, and there would be only a small amount of 1-bromopropane. Chlorine is not quite as selective, and there would be a greater amount of the chlorination of the primary carbon.

So why can't the other halogens such as fluorine or iodine be used? Iodine reacts endothermically and too slowly to be of much good, while fluorine is at the other pole--it reacts too violently and too quickly to be selective, and can, if uncontrolled, break carbon-carbon bonds. To understand why this is so, derive the DH's for the 4 reactions (flourination is highly exothermic, iodonation is endothermic).

See other mechanisms


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