The reaction of propene with HOCl (formed from Cl₂ and H₂O) is an example of electrophilic addition. HOCl is a source of electrophilic chlorine (Cl⁺). The mechanism proceeds in two steps:

Step 1: Formation of the electrophile and initial attack
HOCl dissociates as: $\mathrm{HOCl}\rightleftharpoons H^{+}+{\mathrm{OCl}}^{-}$, but more commonly, Cl₂ with H₂O generates HOCl which acts as a source of Cl⁺. The electrophile (Cl⁺) attacks the electron-rich double bond of propene.

Propene is an unsymmetrical alkene. According to Markovnikov's rule, the electrophile (Cl⁺) adds to the less substituted carbon (CH₂ end), forming a more stable carbocation intermediate. The carbocation formed is secondary:

This is the carbocation: ${\mathrm{CH}}_3-{\mathrm{CH}}_2^{+}-{\mathrm{CH}}_2\mathrm{Cl}$ (or CH₃-CH⁺-CH₂-Cl)

Step 2: Nucleophilic attack
The nucleophile (OH^- from water) quickly attacks the carbocation to form the final product, propylene chlorohydrin:

Therefore, the intermediate is the secondary carbocation with chlorine on the terminal carbon: CH₃-CH⁺-CH₂-Cl.

Related Topics:
- Electrophilic Addition Reactions: These involve the attack of an electrophile on a pi bond, common in alkenes and alkynes.
- Markovnikov's Rule: In the addition of unsymmetrical reagents to unsymmetrical alkenes, the electrophile adds to the carbon with more hydrogen atoms.
- Carbocation Stability: The order of stability is tertiary > secondary > primary. The stability of the intermediate carbocation dictates the regiochemistry of the reaction.

Key Formulae/Concepts:
- General electrophilic addition: Alkene + E⁺Nu^- → Carbocation intermediate → Product
- Markovnikov's Rule: "The rich get richer" – the carbon with more hydrogens gets the hydrogen (or the electrophile adds to the less substituted carbon).
- Carbocation stability: Governed by hyperconjugation and inductive effects.