Markovnikov’s rule is a principle in organic chemistry that predicts the regioselectivity of certain reactions involving unsymmetrical alkenes, especially electrophilic addition reactions.
The rule states:
When a protic acid (HX) adds to an unsymmetrical alkene, the hydrogen (H) from HX attaches to the carbon with more hydrogens already bonded, and the halide (X) or other group attaches to the carbon with fewer hydrogens.
In simpler terms: “The rich get richer”—the carbon that already has more hydrogens gets the new hydrogen.
Example: Addition of HBr to propene (CH₃-CH=CH₂):
- The double bond is between CH (middle carbon) and CH₂ (end carbon).
- H adds to the CH₂ carbon (which has 2 hydrogens, more than CH which has 1).
- Br attaches to the CH carbon (which had fewer hydrogens).
Product: CH₃-CHBr-CH₃ (2-bromopropane)
Why it happens:
The more stable carbocation forms during the reaction intermediate. The carbon that ends up with the positive charge is the more substituted carbon, which is more stable due to hyperconjugation and inductive effects.