Radical Addition of HBr

Radical Addition of $\text{HBr}$ (The Peroxide Effect)

While standard addition of hydrogen halides ($\text{HX}$) to alkenes follows ionic, Markovnikov pathways, the addition of hydrogen bromide takes a fascinating mechanistic turn in the presence of peroxides. Known historically as the peroxide effect, introducing peroxides reverses the regiochemical outcome entirely, yielding the anti-Markovnikov alkyl halide.

1. Why Only $\text{HBr}$? (Thermodynamics of $\text{HCl}$ and $\text{HI}$)

You might wonder why this radical addition works for $\text{HBr}$ but not for other hydrogen halides like $\text{HCl}$ or $\text{HI}$:

• $\text{HCl}$ Addition: The propagation step where a carbon radical abstracts a hydrogen from $\text{HCl}$ is strongly endothermic and energetically unfavorable, so the chain reaction fails to propagate.
• $\text{HI}$ Addition: The initial addition of an iodine radical ($\text{I}^\bullet$) to the alkene is endothermic and reversible, meaning iodine tends to fall off rather than add across the double bond.
• $\text{HBr}$ Addition: Both propagation steps (bromine radical addition to the alkene, and hydrogen abstraction from $\text{HBr}$) are energetically favorable (exothermic), allowing the radical chain to run smoothly.

2. Mechanism: Initiation and Propagation

The reaction proceeds via a radical chain mechanism catalyzed by peroxides ($\text{ROOR}$):

1. Initiation: Heat or light homolytically cleaves the weak oxygen-oxygen bond of the peroxide to form two alkoxy radicals ($\text{RO}^\bullet$). These radicals abstract a hydrogen from $\text{HBr}$ to generate the active bromine radical ($\text{Br}^\bullet$).
2. Propagation Step 1 (Regioselective Addition): The bromine radical adds to the alkene pi bond. $\text{Br}^\bullet$ adds first, attaching to the less substituted carbon because doing so leaves the unpaired electron behind on the more substituted, stable carbon radical ($\text{3}^\circ$ or $\text{2}^\circ$).
3. Propagation Step 2 (Hydrogen Abstraction): The resulting carbon radical abstracts a hydrogen atom from another molecule of $\text{HBr}$, forming the anti-Markovnikov alkyl bromide product and generating a fresh $\text{Br}^\bullet$ to continue the chain.

3. Contrasting Ionic vs. Radical Regiochemistry

The difference in regiochemistry comes down to which species attacks the alkene first:

• Ionic Addition (without peroxides): A proton ($\text{H}^+$) adds first to form a carbocation intermediate. The proton goes to the less substituted carbon to place the positive charge on the more substituted carbon, yielding Markovnikov products.
• Radical Addition (with peroxides): A bromine radical ($\text{Br}^\bullet$) adds first. It goes to the less substituted carbon to place the unpaired electron on the more substituted carbon radical. When the carbon radical captures a hydrogen in the final step, the bromine ends up on the less substituted carbon (anti-Markovnikov).

4. Predicting Products: With vs. Without Peroxides

When given an unsymmetrical alkene, always check the reaction conditions for peroxides:

• $\text{HBr}$ alone (Ionic): Follows Markovnikov's rule ($\text{H}$ goes to less substituted carbon, $\text{Br}$ goes to more substituted carbon). Watch out for carbocation rearrangements.
• $\text{HBr}$ with $\text{ROOR}$ (Radical): Follows anti-Markovnikov's rule ($\text{H}$ goes to more substituted carbon, $\text{Br}$ goes to less substituted carbon). No carbocation rearrangements occur because the intermediate is a carbon radical.

5. Where This Reaction Sits Among Alkyl Halide Syntheses

In our broader toolkit for making alkyl halides from alkenes, radical $\text{HBr}$ addition fills a very specific niche:

• To make a Markovnikov alkyl halide without rearrangements, use standard $\text{HX}$ (though substitution of alcohols via $\text{PBr}_3$ or $\text{SOCl}_2$ is often cleaner in synthesis).
• To make an anti-Markovnikov alkyl halide directly from an alkene, the peroxide-catalyzed addition of $\text{HBr}$ is the primary and most reliable method available.