Electrophilic Addition of HBr to Propene (Markovnikov)

Step 1: Protonation of double bond

SMILES: CC=C  +  $H^{+}$  →  $CC^{+}$C  (secondary carbocation at C-2)

The π electrons of propene (CC=C) attack $H^{+}$. H adds to C-1 (more H atoms per Markovnikov), placing the positive charge on C-2 (secondary carbocation → more stable than primary carbocation at C-1).

$\text{C-1 carbocation (1°): CH}_3\text{–CH}_2\text{–}\overset{+}{\text{CH}}_2 \quad \text{(LESS stable)}$

$\text{C-2 carbocation (2°): CH}_3\text{–}\overset{+}{\text{CH}}\text{–CH}_3 \quad \text{(MORE stable)} \checkmark$

Step 2: Nucleophilic capture of carbocation

$CC^{+}$C  +  $Br^{-}$  →  CC(Br)C  (2-bromopropane — Markovnikov product)

$Br^{-}$ (nucleophile) attacks the $sp^{2}$-hybridized (planar) carbocation at C-2 from either face.

Free Radical Addition of HBr to Propene (Anti-Markovnikov — Kharasch)

Initiation:

ROOR  →[hν/$\Delta$]→  2 RO•  →  RO• + HBr  →  ROH + Br•

Propagation Step 1 — Br• addition to C-1:

CC=C  +  Br•  →  CC•(Br... wait: Br adds to C-1)
SMILES result: Br-$CH_{2}$-•CH-$CH_{3}$  (2° radical at C-2 — more stable)

Propagation Step 2 — H• transfer:

Br$CH_{2}$-•CH$CH_{3}$  +  HBr  →  Br$CH_{2}$$CH_{2}$$CH_{3}$ + Br•
SMILES product: CCCBr  (1-bromopropane — Anti-Markovnikov)

Why C-1 and not C-2 for Br•? Br• adds to C-1 because this generates the more stable SECONDARY radical at C-2. The more stable the radical intermediate, the lower the activation energy.