This section introduces the first reactive intermediate that isn't an ion — the free radical — and the reaction that makes alkyl halides from alkanes. Alkanes are famously unreactive, but under heat or UV light (hν), halogens (Cl₂, Br₂) will substitute a hydrogen for a halogen through a radical chain mechanism with three phases:

1. Initiation: light homolytically cleaves the halogen (Cl₂ → 2 Cl•), producing radicals. Homolysis gives each atom one electron (drawn with single-barbed "fishhook" arrows), unlike the heterolytic cleavage of ionic chemistry.

2. Propagation (two repeating steps): a halogen radical abstracts a hydrogen from the alkane to give an alkyl radical + HX; the alkyl radical then reacts with X₂ to give the alkyl halide + a new halogen radical, which continues the chain.

3. Termination: two radicals combine, ending the chain.

The pre-health payoff is selectivity, which rests on radical stability: tertiary > secondary > primary > methyl (the same order as carbocations, and for the same reason — hyperconjugation and induction stabilize the electron-deficient center). This produces one of the most-tested contrasts in the course:

Bromination is highly selective. Br• is unreactive and picky, so it abstracts the hydrogen that gives the most stable (usually tertiary) radical, yielding a nearly single major product. This reflects the Hammond postulate: bromination's H-abstraction is endothermic with a late, product-like transition state that "feels" the stability of the radical.
Chlorination is unselective. Cl• is highly reactive and abstracts hydrogens almost statistically, giving mixtures. Its H-abstraction is exothermic with an early transition state.

A stereochemical consequence: radical halogenation at a new stereocenter proceeds through a planar radical, so it produces a racemic mixture of the two configurations. Alkyl halides made here become the substrates for the substitution and elimination chemistry (SN1/SN2/E1/E2) covered in later tracks.

"Break It with Light"