Radical halogenation installs the first functional handle on an otherwise inert alkane, and it works through neutral radicals in a self-sustaining chain rather than polar arrows.
The whole reaction in one line: light, a halogen, and one C–H becomes one C–X. Structures drawn live.
1. It Swaps One C–H for One C–X — the Only Reaction Alkanes Reliably Do
Under heat or UV light (hν) an alkane reacts with Cl2 or Br2 to give R–H + X2 → R–X + HX; F2 is too violent and I2 unfavorable.
Ethane plus Cl2 under light gives chloroethane and HCl.
2. It Runs as a Chain: Initiation, Propagation, Termination
Initiation homolytically splits X2 into two X•; propagation is two steps that regenerate a radical (X• grabs H → R• + HX; R• grabs X → R–X + X•); termination pairs any two radicals.
3. The Hydrogen-Abstraction Step Decides Everything
Only the first propagation step — X• abstracting a hydrogen — chooses which C–H reacts, so the product is set by which carbon radical is easiest (most stable) to make.
The carbon radical produced by H-abstraction — its stability sets the selectivity.
4. Radical Stability Follows 3° > 2° > 1° > Methyl
Neighboring alkyl groups donate electron density (hyperconjugation and induction) into the radical center, so more attached carbons means a more stable radical — a 3° C–H is abstracted far more readily than a 1° one.
5. Bromination Is Selective; Chlorination Is Not
By the Hammond postulate, endothermic Br• abstraction has a late, radical-like transition state that fully feels stability (one clean product at the most substituted carbon), while exothermic Cl• abstraction has an early one that barely discriminates (mixtures).
Selective bromination of isobutane hits the single 3° C–H → tert-butyl bromide, essentially one product.
Chlorinating propane, by contrast, gives a mixture:
6. Allylic and Benzylic C–H Bonds React Fastest of All
A radical next to a C=C (allylic) or aromatic ring (benzylic) is resonance-delocalized, making it even more stable than 3°, so these C–H bonds react fastest — often brominated with NBS.
7. Summary
X2 + light swaps one C–H for C–X via a radical chain · H-abstraction is selectivity-determining · stability 3° > 2° > 1° > methyl (allylic/benzylic highest) · Br2 selective, Cl2 gives mixtures.
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The first one — X• abstracting a hydrogen from the alkane. It is the step that decides which C–H reacts and forms the carbon radical, so its transition-state energy sets both the rate and the product distribution. The second step (R• grabbing a halogen from X2) is fast and indiscriminate.
3° > 2° > 1° > methyl. More attached alkyl groups donate electron density (hyperconjugation and induction) to the electron-deficient radical center, lowering its energy — the same trend as carbocations. Resonance-stabilized allylic and benzylic radicals are even more stable than 3°.
Br2. Its endothermic H-abstraction has a late, product-like transition state (Hammond postulate) that fully "feels" radical stability, so it strongly favors the most stable radical and gives one dominant product. Cl2 has an early transition state, barely discriminates, and gives mixtures of constitutional isomers.
Because the resulting allylic radical is resonance-stabilized — its unpaired electron delocalizes across the adjacent π bond, spreading over two carbons. That extra delocalization makes it more stable (and its C–H weaker) than a tertiary radical, which is stabilized only by nearby alkyl groups. Benzylic C–H bonds react for the same reason.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.