The Addition of H-Br Across an Alkene Using Peroxides

In a previous post (you can see it here), we described the Markovnikov addition of H-Br across a double bond. In that post, you can see that the bromide atom adds to the more hindered side of the double bond, thus following the Markovnikov rule. However, if peroxides are use along with the H-Br, then something else happens.

Addition of H-Br with peroxides.

In the figure above, it's evident that there's an immediate difference compared to when H-Br was added without peroxides. The previous mechanism doesn't align with the observed product. Thus, a new mechanism is necessary to accurately explain the process.

It turns out that the mechanism involves a radical reaction. Radical reactions consist of three distinct steps: initiation, propagation, and termination. Keep in mind that radical stability is similar to carbocation stability, meaning the more substituted radical is the most stable. This radical stability determines the regiochemistry of the reaction. Let's examine the mechanism below.

Step 1 in the anti-Markovnikov addition of HBr to alkenes.

In the first step of the mechanism, we can see that heat decomposes the peroxide into two hydroxyl radicals. These radicals are high energy species that can further react to homolytically cleave the HBr molecule. Next, once we have generated the radical bromide, the propagation step can begin.

The propagation steps in the addition of HBr to an alkene with peroxides.

The propagation step of this chain reaction begins by the bromine radical attacking the alkene. This step is what drives the regiochemistry of the reaction. Here, there are two possible radicals, one on the tertiary carbon and one on the other side which would be the primary carbon. Since we know that the tertiary radical is more stable, it is the one that is formed preferentially. Finally, a new bromine radical is formed when the tertiary radical reacts with another HBr molecule and the entire process repeats itself until the reagents are used up.

The termination step could have many possible options, each of them generating a product with no new radicals. These compounds could be another molecule of product by a bromine radical and the tertiary radical reaacting together but there could also be a number of side products. The side products could be two bromine radicals reacting together to form molecular bromine, for example.

To summarize, the addition of peroxides causes the addition of HBr to an alkene to adopt a new mechanism. The mechanism is that of a radical chain reaction and therefore, we must be concerned about the stability of the generated radical leading to the anti-Markovnikov product.