Halogenation of Alkanes

1. The Radical Halogenation Mechanism

The chain for chlorination (bromination is identical with Br₂/Br•/HBr):

1. Initiation: Cl–Cl ⇀ 2 Cl• (heat or light homolyzes the weak Cl–Cl bond).
2. Propagation (H abstraction): Cl• + R–H → H–Cl + R•. The chlorine radical plucks a hydrogen atom (taking one C–H electron with it), leaving an alkyl radical. This is rate-determining and selects which product forms.
3. Propagation (halogen abstraction): R• + Cl₂ → R–Cl + Cl•. The radical grabs a chlorine atom from Cl₂, forming the alkyl halide and regenerating Cl•.
4. Termination: Cl• + Cl• → Cl₂; R• + R• → R–R; R• + Cl• → R–Cl.

Reactivity decides practicality: F₂ reacts explosively (too reactive), I₂ is endothermic and essentially does not react (I• + R–H is endothermic), so Cl₂ and Br₂ are the useful halogens.

2. Selectivity: Chlorine vs Bromine

Selectivity is how much a reagent discriminates among hydrogen types. By the Hammond postulate applied to H-abstraction: a more reactive halogen has an earlier, more reactant-like transition state and cares little about radical stability; a less reactive halogen has a later, product-like transition state and "feels" radical stability strongly.

Approximate relative reactivities per hydrogen:

Bromination is far more selective — it targets the 3° hydrogen almost exclusively because the 3° radical is so much more stable.

3. Predicting Product Mixtures

Amount of each product = (relative reactivity per hydrogen) × (number of equivalent hydrogens of that type). Compute each product's "rate score," then express as a ratio.

Example — monobromination of propane (CH₃CH₂CH₃): 6 primary H × 1 = 6; 2 secondary H × 82 = 164. So 1-bromo : 2-bromo = 6 : 164 ≈ 1 : 27 — almost all 2-bromopropane. For chlorination: 6 × 1 = 6 vs 2 × 4 = 8, giving ≈ 43% : 57% — a genuine mixture. Mixtures are the norm in chlorination, which is why bromination is preferred when a single product is wanted.

How it works

1. Heat or light homolytically splits X₂ into two halogen radicals.
2. The halogen radical abstracts a hydrogen (rate-determining; selects the radical), making HX and an alkyl radical.
3. The alkyl radical abstracts a halogen from X₂, making the alkyl halide and regenerating the halogen radical; the two steps repeat.
4. Termination couples radicals (R–R, R–Cl byproducts); product ratios follow (reactivity per H) × (number of equivalent H's).

Common confusions

Memory aids

"BRB — Bromine is Really a Bigot" (bromine is highly selective); chlorine is "Casual Cl" (doesn't care which hydrogen). For ratios, chant "rate per H times how many H's." The reactivity ladder F > Cl > Br > I spells "Furiously Chlorinate But Iodinate-later."

Quick review

Topic Recap

Radical halogenation converts alkanes to alkyl halides via a three-stage chain: homolysis of X₂, hydrogen abstraction (rate-determining), and halogen abstraction. Chlorine is reactive but unselective (mixtures); bromine is selective, targeting the hydrogen yielding the most stable radical. Product ratios combine relative reactivity per hydrogen with the number of equivalent hydrogens, and halogenation at a stereocenter gives a racemic mixture. Only Cl₂ and Br₂ are synthetically useful.

Knowledge Check

1. Write the two propagation steps for chlorination of methane.
2. Why is bromination more selective than chlorination?
3. For monobromination of butane (CH₃CH₂CH₂CH₃), compute the 1-bromo : 2-bromo ratio using 3° : 2° : 1° = 1600 : 82 : 1.
4. Why is fluorination impractical and iodination nonviable?
5. What is the stereochemical outcome when the abstracted hydrogen sits on a stereocenter, and why?

Answers and Rationales

1. Cl• + CH₄ → HCl + •CH₃, then •CH₃ + Cl₂ → CH₃Cl + Cl•. Rationale: the H-abstraction and halogen-abstraction steps that conserve radicals and convert CH₄ to CH₃Cl.
2. Br• is less reactive, so its transition state is later and more product-like, making it sensitive to radical stability; Cl• is more reactive with an early transition state that barely discriminates. Rationale: Hammond postulate — reactivity and selectivity trade off.
3. Butane has 6 primary H (two CH₃) and 4 secondary H (two CH₂). Primary score = 6 × 1 = 6; secondary = 4 × 82 = 328. Ratio 1-bromo : 2-bromo ≈ 6 : 328 ≈ 1 : 55. Rationale: multiply rate per H by equivalent hydrogens; bromine strongly prefers the 2° radical.
4. F₂ is too reactive — violent and unselective; I₂ is too unreactive — the I• + R–H abstraction is endothermic, so the chain cannot propagate. Rationale: only Cl₂ and Br₂ sit in the usable reactivity window.
5. Racemization (both R and S). Rationale: the radical is ~planar, so the halogen can add from either face in equal amounts.