Alkenes are electron-rich at the C=C bond (high electron density in the pi bond). This makes them susceptible to attack by electrophiles (electron-pair acceptors).
Mechanism
- The electrophile (e.g. HBr) is polarised: Hδ+−Brδ−
- The pi electrons of C=C attack Hδ+ → carbocation intermediate formed
- Br⁻ attacks the carbocation → product formed
Markovnikov's Rule
The Rule
When HX adds to an unsymmetrical alkene, the H adds to the carbon with more H atoms already attached (forming the more stable carbocation).
Rationale: the more substituted carbocation is more stable due to the inductive effect of alkyl groups.
Example: Propene + HBr
Major Product
CH₃CHBrCH₃
2-bromopropane
(Markovnikov — secondary carbocation)
Minor Product
CH₃CH₂CH₂Br
1-bromopropane
(Anti-Markovnikov — primary carbocation)
Think About It
Why is bromine water used to test for unsaturation, and what is the mechanism?
Br₂ is polarised by the electron-rich C=C bond. The electrophilic Brδ+ attacks the pi bond, forming a bromonium ion intermediate. Br⁻ then attacks from the opposite side → 1,2-dibromoalkane (colourless). The colour change from orange to colourless = positive test for alkenes.