Alkene Reactions: Anti-Markovnikov Bromination

Often times, when alkenes react with any reagent, there are two locations (carbons) where the functional can be added. Carbons are ranked by the number of functional groups attached: Primary, Secondary, Tertiary, and Quaternary. Most reactions will add the functional group to the higher ranked carbon (Tertiary vs Secondary for example). This is known as Markovnikov's Rule. When a reactant preferentially reacts with the lower ranked carbon (Secondary vs Tertiary),this is known as an anti-Markovnikov addition.

When hydrobromic acid (HBr) and any peroxide (R-O-O-R, ie H2O2) react with an alkene, the reaction favors the anti-Markovnikov product:

Reaction (anti-Markovnikov):

If the carbons have the same ranking (ie both are Secondary), a racemic mixture of 2 products will be formed, with the functional group being added to each carbon:

Same Carbon Rank (Secondary):

The reaction mechanism is depicted below:

Mechanism:

In the first step, the peroxide breaks apart, leading to the formation of 2 negatively charged -OR molecules (alkoxide molecules).

In the second step, the negatively charged oxygen on the alkoxide molecule pulls the hydrogen atom off the HBr molecule, leading to the formation of a negatively charged Br atom.

In the third step, the negatively charged Br atom attacks the lower ranked carbon (secondary vs tertiary in this example), sending the electrons from the alkene bond to the higher ranked carbon (tertiary carbon).

In the fourth step, the negatively charged carbanion attacks another HBr molecule, pulling the hydrogen atom off the HBr.

In the fifth and final step, Br2 is formed as a side product from the leftover Br molecules.

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