HBr + Propene — Markovnikov

Propene (CH₂=CH-CH₃) + HBr (no peroxide) → 2-Bromopropane

π attack on H⁺ → 3° carbocation

The C=C π electrons attack H of HBr. H bonds to the less substituted CH₂ end (Markovnikov). Br⁻ departs. More stable secondary/tertiary carbocation forms on the other carbon.

π electrons attack H⁺ → H goes to CH₂ end → carbocation on CH (more stable)

Conditions

No peroxide

Room temp, polar solvent (CHCl₃)

Ionic mechanism

Key Notes

H adds to CH₂= end (more H) → carbocation on more substituted C

Markovnikov: H to more hydrogen-bearing carbon

Rearrangements possible (2° → 3° shift)

Exceptions

With ROOR peroxide → anti-Markovnikov (free radical, HBr only, not HCl/HI)

JEE Key Points

Carbocation stability: 3° > 2° > 1° > methyl

Rate = k[HBr][alkene]

Only HBr gives anti-Markovnikov with peroxide