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Radical Additions and Substitutions with Alkenes

Radical Addition of Hydrogen Halides to Alkenes

The radical addition of hydrogen halides to alkenes works effectively and regioselectively only in association with hydrogen bromide, with which it yields the anti-Markovnikov product. In the radical addition of both hydrogen iodide and hydrogen chloride to alkenes, a step of the chain propagation is endothermic and, therefore, the chain reaction virtually never proceeds.

Hydrogen iodide addition

Fig.1
Endothermic step in the chain propagation of HI addition.

In the case of hydrogen iodide, the addition of the iodine radical to the double bond is the endothermic step of the chain propagation.

Fig.2
Important bond energies in the first propagation step of the radical addition of hydrogen halides to alkenes.

By comparing the bond energies, the differences between the halogens chlorine, bromine, and iodine in the first propagation step become apparent. In the first propagation step, a C-C π bond is broken and a C-X bond is formed. The energy that must be applied to cleave the C-C π bond is obviously the same in all three cases. However, in contrast to chlorine and bromine, the bond energy of the C-I bond is not enough to compensate for the larger amount of bond energy that is consumed by the cleavage of the C-C π bond. Therefore, the addition of an iodine radical to a double bond is an endothermic reaction that runs only very slowly, if at all. As a result, the radical HI addition to alkenes virtually never occurs, because the chain termination reactions exceed the chain propagation. On the other hand, the addition of chlorine and bromine radicals to a double bond is rapid enough to exceed the chain termination reactions, since the C-X bond energies are comparatively more than the C-C π bond energy.

Hydrogen chloride addition

However, in the chain reaction of the radical HCl addition to alkenes, the second chain propagation step is energetically unfavourable, since the amount of energy that must be applied in order to cleave the strong H-Cl bond considerably exceeds the amount of energy that is released during the new C-H bond's formation. As a result, this step is an endothermic reaction and, much like in the first propagation step of the HI addition, the chain termination reactions exceed the chain propagation step of the HCl addition. Therefore, similar to the HI addition, the radical HCl addition to alkenes virtually never occurs.

Fig.3
Endothermic step in the chain propagation of the HCl addition.
Fig.4
Important bond energies in the second propagation step of the radical addition of hydrogen halides to alkenes.

In the final analysis, in the radical HBr addition to alkenes, both chain propagation reactions are exothermic, while in radical HCl as well as HI addition, in each case one propagation step is endothermic. Therefore, only the HBr addition actually occurs. In association with the HCl and HI addition, the polar HX addition with an ionic intermediate stage largely exceeds the radical addition, which results, contrary to the radical addition, in the Markovnikov product.

In addition to HBr, many other reagents, such as thiols and some methyl halides, may also be added to alkenes in a radical addition. Thus, various preparatively useful anti-Markovnikov products may be obtained, which are otherwise not produced by ionic additions.

Exercise: Radical addition of hydrogen halides to alkenes

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