📘 IB Understanding
Electrophilic addition involves an electrophile (electron-pair acceptor) attacking the electron-rich C=C double bond. The π-bond breaks and two new σ-bonds form.
Mechanism: Addition of HBr to Propene
Step 1
The π-electrons of the C=C attack the Hδ⁺ of H–Br (the electrophile). The H–Br bond breaks heterolytically, forming a carbocation and Br⁻.
Step 2
The Br⁻ ion (nucleophile) attacks the carbocation, forming the haloalkane product.
Markovnikov’s Rule
When HX adds to an unsymmetrical alkene, the hydrogen atom bonds to the carbon that already has more hydrogen atoms. This produces the more stable (more substituted) carbocation intermediate.
Carbocation Stability
Tertiary > Secondary > Primary > Methyl
More alkyl groups stabilise the positive charge via the inductive effect (electron donation)
📋 Exam Tip
Always show curly arrows from the electron-rich region to the electron-poor region. In step 1, draw the arrow from the C=C bond to the H of HBr. In step 2, draw from the lone pair on Br⁻ to the C⁺.