As discussed previously, alkenes normally react with HBr to give products of “Markovnikov” addition; the bromine ends up on the most substituted carbon of the alkene, and the hydrogen ends up on the least substituted carbon. However, something interesting happens when the same reaction is performed in the presence of peroxides and heat / light: the pattern of addition changes! Instead of Br ending up on the most substituted carbon of the alkene, it ends up on the least. [The stereochemistry of the reaction, however, is unchanged: it still gives a mixture of “syn” and “anti” products.]
This so-called “anti-Markovnikov” addition is intriguing. What difference could the presence of peroxides, and furthermore heat (or light) make to this reaction?
To make a long story short [not much mention of free-radical addition reactions have been made yet on this blog, so you’ll have to pardon the lack of lead-in], this reaction occurs through a free-radical process. Here are the essential details:
- Peroxides contain a weak oxygen-oxygen bond [approximately 35 kcal/mol; compare to C-H at approx 100 kcal/mol]
- Heating leads to homolytic fragmentation of this bond – that is, the bond breaks such as to leave one unpaired electron on each atom. Strong sources of light [e.g. a floodlight or other source of light radiation which reaches into the near UV] can also serve to sever this bond.
- The resulting highly reactive alkoxy radical can then abstract a hydrogen from H-Br, giving a bromine radical. This is the species that adds to the alkene.
- Addition to the alkene will preferably occur in such a way that the most stable free radical is formed [in our case, the tertiary radical]. That’s why bromine ends up on the least substituted carbon of the alkene.
- This tertiary radical then removes hydrogen from H-Br, liberating a bromine radical, and the cycle continues.
Note that only a trace [catalytic] amount of peroxide is required to get the reaction started, although of course at least one molar equivalent of HBr is required to result in full addition of HBr to the alkene.
Here it is, Chemdrawed:
This initiation step results in homolytic cleavage of O-O. The singly barbed arrows depict the movement of single electrons; two alkoxy radicals are formed. Common “peroxides” for this purpose are t-butyl peroxide or benzoyl peroxide. * [Note 1]. Alternatively other free-radical “initiators” such as AIBN can also be used.
Only a catalytic amount of peroxides are used to initate this reaction (typically 10-20 mole %, although more can be used, especially when added batchwise) , because the next step is for the peroxy radical to remove a hydrogen from H-Br:
Once formed, the bromine radical can then add to the alkene, from either face. Addition occurs in such a way as to give the most substituted radical (tertiary in this case, not secondary).
Finally, the tertiary radical then removes a hydrogen from another equivalent of H–Br, giving the final addition product. A bromine radical is generated by this process, which can then add to another equivalent of alkene.
Note that hydrogen here can attack either face of the free radical [note 2]. Therefore we obtain a mixture of syn and anti products.
This reaction pathway is most commonly observed (in Org 1 and Org 2, anyway) for addition of HBr, although a rich chemistry of radical addition reactions to alkenes exists (particularly for organostannanes).
NEXT POST: Ozonolysis of Alkenes
* Note. Benzoyl peroxide enjoys a common household use as an acne cleanser, and even makes an appearance in this classic ad.
**Note 2. The geometry of free radical carbons is that of a shallow pyramid with a low barrier for inversion, allowing for reactivity on either face. The exception is in weird cases where inversion would be highly disfavored, such as on a bridgehead.