Reaction Mechanisms: Alkenes and Halogenoalkanes

Reaction mechanisms

A mechanism shows how bonds break and form using curly arrows — each arrow shows the movement of an electron pair (from a bond or lone pair to where a new bond forms). Bonds break either homolytically (one electron to each atom → radicals) or heterolytically (both electrons to one atom → ions).

Alkanes — free-radical substitution

Alkanes react with halogens in UV light by free-radical substitution:

• Initiation: UV breaks the halogen bond homolytically → two radicals (Cl•).
• Propagation: radicals react and regenerate radicals (chain reaction).
• Termination: two radicals combine, ending the chain.

Alkenes — electrophilic addition

The C=C double bond is a region of high electron density that attracts electrophiles (electron-pair acceptors).

• With HBr, Br₂ or H₂O(steam)/H₂SO₄: the electrophile adds across the double bond.
• Mechanism: the π electrons attack the electrophile, forming a carbocation intermediate; a nucleophile then bonds to the carbocation.
• Markovnikov's rule: the major product forms via the more stable carbocation (tertiary > secondary > primary).
• Bromine water test: alkenes decolourise orange bromine water (electrophilic addition).

Halogenoalkanes — nucleophilic substitution

The C–halogen bond is polar (Cᵟ⁺), so it's attacked by nucleophiles (electron-pair donors, e.g. OH⁻, CN⁻, NH₃).

• The nucleophile donates a lone pair to the δ+ carbon; the halogen leaves as a halide ion.
• Reactivity: C–I is weakest, so iodoalkanes react fastest; C–F is strongest (slowest).
• Products depend on the nucleophile: OH⁻ → alcohol; CN⁻ → nitrile (extends the chain); NH₃ → amine.

Elimination

Halogenoalkanes can also undergo elimination (with hot ethanolic KOH) to form alkenes (losing HX). Substitution vs elimination depends on conditions (aqueous vs ethanolic KOH; temperature).

Worked example

Predict the major product when HBr adds to propene, and explain using Markovnikov's rule.

• The H adds to give the more stable (secondary) carbocation, so Br bonds to the middle carbon → 2-bromopropane is the major product. ✓

Common mistakes

• Drawing curly arrows from atoms instead of from bonds or lone pairs (electron pairs).
• Confusing electrophilic addition (alkenes) with nucleophilic substitution (halogenoalkanes).
• Forgetting Markovnikov's rule / the more stable carbocation for the major product.

Exam tips

• Learn each mechanism with correct curly arrows and intermediates (radical, carbocation).
• Match reagent + conditions to the product (substitution vs elimination; nucleophile type).
• Use carbocation stability to predict major products.

Key facts to remember

• Curly arrows show electron-pair movement; bonds break homolytically (radicals) or heterolytically (ions).
• Alkanes → free-radical substitution (UV); alkenes → electrophilic addition (via carbocation, Markovnikov); halogenoalkanes → nucleophilic substitution (iodo fastest) or elimination.
• Nucleophiles donate electron pairs to δ+ carbon; electrophiles accept electron pairs from the C=C.