Radical chain — anti-Markovnikov addition
Kharasch addition (peroxide-mediated HBr addition)
How the Kharasch addition (peroxide-mediated HBr addition) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: HBr, ROOR (peroxide initiator) or light, alkene. Conditions: Heat or hν, non-polar solvent. Bonds formed: C–Br (terminal); C–H; bonds broken: H–Br; C=C π; O–O. Stereochemistry: Anti-Markovnikov; radical chain. Rate and selectivity: Chain propagation; H-abstraction from HBr is fast.
At a glance
Step 1: Initiation: peroxide gives Br•
A peroxide initiator homolyzes under heat or light, and the resulting radical abstracts a hydrogen atom from H–Br, generating a bromine radical — the same initiation logic as radical halogenation.
Step 2: Br• adds to the terminal carbon
The bromine radical adds to the less hindered, terminal carbon of the alkene — not because of electronics as in the ionic HBr addition covered early in this library, but because that's where the resulting carbon radical is most stable (more substituted radical, favored just like the more substituted carbocation is in the ionic pathway).
Step 3: H-abstraction gives anti-Markovnikov product
That carbon radical abstracts a hydrogen atom from another H–Br molecule, giving the anti-Markovnikov product and regenerating a bromine radical to continue the chain — the identical starting materials as ionic HBr addition, but a radical mechanism flips the regiochemistry entirely.
Abbreviations and terms
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
- anti-Markovnikov
- Anti-Markovnikov. The opposite regiochemistry (radical HBr addition, hydroboration).
How it compares with related reactions
| Kharasch addition (peroxide-mediated HBr addition) | Thiol-ene reaction | Barton-McCombie deoxygenation | |
|---|---|---|---|
| Reagents | HBr, ROOR (peroxide initiator) or light, alkene | Thiol (R–SH), alkene, photoinitiator (e.g. DMPA) or AIBN | 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH) |
| Conditions | Heat or hν, non-polar solvent | UV (365 nm) or 60–80 °C, no oxygen, often solvent-free | Toluene, 80–110 °C, radical chain |
| Stereochemistry | Anti-Markovnikov; radical chain | Anti-Markovnikov; usually no stereocentre | Stereocentre lost (planar radical) |
| Rate / selectivity | Chain propagation; H-abstraction from HBr is fast | Propagation and chain transfer both fast; click-like | Driven by strong C=O/Sn–S bonds; chain propagation |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Kharasch addition (peroxide-mediated HBr addition)?
HBr, ROOR (peroxide initiator) or light, alkene. Conditions: Heat or hν, non-polar solvent.
Which bonds are formed and which are broken?
Formed: C–Br (terminal); C–H. Broken: H–Br; C=C π; O–O.
What is the stereochemical outcome?
Anti-Markovnikov; radical chain
What controls the rate or the selectivity?
Chain propagation; H-abstraction from HBr is fast
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make