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The radical addition of hydrogen bromide to alkenes

The addition of hydrogen bromide to alkenes has puzzled chemists for a long time. In this reaction, different products were obtained, in some cases even from the same chemist, depending on the approach. For example, freshly distilled 1-butene reacts with hydrogen bromide to form the Markovnikov product 2-bromobutane. Amazingly, the behavior is completely different if 1-butene has been exposed to the air for a while before it is converted. The reaction then proceeds considerably faster and the anti-Markovnikov product 1-bromobutane is obtained. This puzzle was solved by Kharash in the 1930s. The explanation is the formation of peroxides from the alkene and atmospheric oxygen. These peroxides easily break down into radicals by breaking the weak RO-OR bond. These radicals cause the hydrobromination to occur not ionically but according to a much faster radical chain mechanism.

Mechanism of Hydrobromination

This radical chain mechanism is initiated in the so-called chain start by the endothermic homolytic break of the labile O-O bond of the peroxides. The resulting alkoxy radicals react with the hydrobromide to form bromine radicals. This step is exothermic as the strong O-H bond is formed.

In the next step - chain growth - a bromine radical reacts with 1-butene, with the unpaired electron of the bromine radical forming a bromine-carbon bond with one of the two π electrons of 1-butene. This attack of the halogen radical occurs regioselectively, with the more stable secondary radical being formed preferentially.

This differs from an ionic addition to the double bond, in which a proton first reacts with the double bond, thereby forming the more stable secondary carbenium ion.

From a mechanistic point of view, however, the radical and ionic hydrobromination are similar in that the more stable secondary transition structure is formed in both cases. The difference is that in the radical hydrobromination the halogen attacks as a radical and in the ionic reaction the hydrogen from the HBr attacks first as a proton. As a result, the anti-Markovnikov product 1-bromobutane is formed in a radical mechanism and the Markovnikov product 2-bromobutane is formed in the ionic mechanism.