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AdditionIntroductory
Markovnikov Addition
Also known as: Markownikoff Addition, HX Addition to Alkenes
General Equation
R-CH=CH₂ + HX → R-CHX-CH₃
Description
Electrophilic addition of hydrogen halides to alkenes following Markovnikov's rule: the hydrogen adds to the less substituted carbon (the one with more H's), and the halide adds to the more substituted carbon. This is because the more stable carbocation intermediate forms.
Reaction Scheme
Reactants:
- • Alkene (C=C)
Products:
- • Alkyl halide (Markovnikov regiochemistry)
Reagents:
- • HX (HCl, HBr, HI)
Conditions:
- • Room temperature or mild heating
Mechanism (2 steps)
1
Protonation: The π electrons of the alkene attack the H of HX, forming the more stable carbocation (Markovnikov intermediate). The proton adds to the less substituted carbon.
Arrow: curvedR-CH⁺-CH₃ (more stable carbocation)
Regioselectivity determined here — more stable carbocation forms
2
Halide capture: The halide anion (X⁻) attacks the carbocation to form the product.
Arrow: curved
Attack from either face — no stereocontrol
Key Points to Remember
- ✓Markovnikov's rule: H goes to C with more H's, X goes to C with fewer H's
- ✓Modern explanation: more substituted carbocation is more stable
- ✓Carbocation rearrangements possible (watch for ring expansions)
- ✓Follows electrophilic addition mechanism
- ✓No peroxides present (peroxides cause anti-Markovnikov with HBr)
- ✓Reactivity: HI > HBr > HCl (acid strength)
Examples
Reactant:
Propene (CH₃CH=CH₂)
Reagent:
HBr
Product:
2-Bromopropane (CH₃CHBrCH₃)
Yield: 90%
Reactant:
1-Methylcyclohexene
Reagent:
HBr
Product:
1-Bromo-1-methylcyclohexane
Yield: 85%
Synthetic Utility
Simple method to convert alkenes to alkyl halides with predictable regiochemistry. The halide product is useful for further substitution/elimination.
Limitations
- ⚠Carbocation rearrangements with certain substrates
- ⚠No stereocontrol (racemic at new stereocenter)
- ⚠With HBr + peroxides → anti-Markovnikov (radical mechanism)
#addition#Markovnikov#electrophilic#alkene#regiochemistry