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RadicalIntroductory
Anti-Markovnikov Addition
Also known as: Radical Addition of HBr, Kharasch Reaction, Peroxide Effect
Discovered by Morris Kharasch (1933)
General Equation
R-CH=CH₂ + HBr + ROOR → R-CH₂-CH₂Br
Description
Radical chain addition of HBr to alkenes. Peroxides generate radicals that reverse the normal Markovnikov selectivity. The bromine adds to the less substituted carbon because the more stable radical intermediate forms at the more substituted position.
Reaction Scheme
Reactants:
- • Alkene (C=C)
Products:
- • Alkyl bromide (anti-Markovnikov regiochemistry)
Reagents:
- • HBr
- • ROOR (peroxide initiator)
Conditions:
- • UV light or heat with peroxide
Mechanism (3 steps)
1
Initiation: Peroxide (ROOR) homolyzes to form alkoxy radicals (RO•). The alkoxy radical abstracts H from HBr to generate Br•.
Arrow: fishhook
Fishhook arrows — one electron at a time
2
Propagation 1: Br• radical adds to the less substituted end of the alkene, forming the more stable (more substituted) carbon radical.
Arrow: fishhookR-CH•-CH₂Br
Regioselectivity: Br adds to less substituted C
3
Propagation 2: The carbon radical abstracts H from another HBr, forming the product and regenerating Br• to continue the chain.
Arrow: fishhook
Chain propagation — one Br• generates thousands of products
Key Points to Remember
- ✓ONLY works with HBr (not HCl or HI) — thermodynamics of radical steps
- ✓Requires peroxide or UV light to initiate
- ✓Anti-Markovnikov: Br ends up on less substituted carbon
- ✓Radical chain mechanism (initiation, propagation, termination)
- ✓More stable radical = more substituted (like carbocations)
Examples
Reactant:
Propene
Reagent:
HBr, ROOR
Product:
1-Bromopropane
Yield: 85%
Reactant:
1-Hexene
Reagent:
HBr, (PhCO₂)₂
Product:
1-Bromohexane
Yield: 90%
Synthetic Utility
Access to primary alkyl bromides from terminal alkenes — complementary regiochemistry to Markovnikov addition.
Limitations
- ⚠Only works with HBr (not HCl or HI)
- ⚠Must have radical initiator (peroxides or UV)
- ⚠Radical chain can produce some dimeric byproducts
#radical#anti-Markovnikov#HBr#peroxide#chain reaction