Radical Stability and Selectivity

Quick answer Carbon radicals get more stable with more alkyl groups on the radical carbon: 3° > 2° > 1° > methyl, and resonance makes allylic and benzylic radicals best of all. Whichever C–H gives the most stable radical has the lowest BDE and reacts first in halogenation.

Whichever C–H gives the most stable radical is abstracted preferentially in radical halogenation — so the stability order sets the selectivity.

A carbon radical: seven valence electrons, one unpaired electron in a roughly planar p-type orbital — electron-deficient.

1. Radical Stability Runs 3° > 2° > 1° > Methyl

The radical carbon is electron-deficient, so more attached alkyl groups means more stability.

Methyl (least stable)

Primary (1°)

Secondary (2°)

Tertiary (most stable)

2. Hyperconjugation and Induction From Alkyl Groups Stabilize the Radical

Adjacent C–H bonds overlap with the half-filled p orbital (hyperconjugation), with a weak inductive assist — so more neighboring carbons lowers the radical's energy.

Primary: one adjacent carbon, little hyperconjugation

Tertiary: three adjacent carbons, maximal hyperconjugation

3. Resonance Makes Allylic and Benzylic Radicals the Most Stable of All

Next to a π system the unpaired electron delocalizes by resonance — over two carbons (allylic) or into the ring (benzylic) — which beats hyperconjugation and makes these C–H bonds the first abstracted.

Allylic radical — delocalized over two carbons

Benzylic radical — delocalized into the ring

4. Bond-Dissociation Energies Are the Experimental Proof

A lower C–H bond-dissociation energy (BDE) means an easier, more stable radical — and the numbers track the substitution order (methyl 105 → 3° 96.5 kcal/mol; allylic/benzylic ≈88–90).

Methyl · C–H BDE ≈ 105

1° ≈ 101

2° ≈ 98.5

3° ≈ 96.5 (lowest of the alkyls)

5. Radical Stability Mirrors Carbocation Stability

Radicals and carbocations are both electron-deficient at carbon, so they follow the same 3° > 2° > 1° > methyl trend — just with smaller energy gaps for the one-electron-short radical.

The tertiary radical is stabilized by the same three alkyl groups that would stabilize the tertiary carbocation.

6. Radical Stability Controls Selectivity in Halogenation

The position giving the most stable radical reacts fastest, so selective Br₂/NBS favor the more substituted C–H (2° over 1° in propane) while unselective Cl₂ gives mixtures.

Propane

2° radical — favored (Br₂)

1° radical — minor

7. Summary

3° > 2° > 1° > methyl · hyperconjugation + induction · resonance wins (allylic/benzylic) · lowest BDE = most stable radical · same trend as carbocations · sets halogenation selectivity.

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Tertiary > secondary > primary > methyl. More alkyl groups on the radical carbon means more hyperconjugation and induction stabilizing the electron-deficient center.

Resonance. The unpaired electron delocalizes across the adjacent π system — over two carbons for allylic, into the ring for benzylic — which lowers the energy far more than hyperconjugation alone. Their C–H BDEs (≈88–90 kcal/mol) are the lowest.

The 96.5 kcal/mol bond gives the more stable radical (lower BDE = easier homolysis = more stable product) and is the tertiary C–H. The 101 kcal/mol bond is primary.

Bromine's abstraction step is endothermic with a late, radical-like transition state, so the greater stability of the 2° radical is strongly felt and Br₂ is selective. Chlorine's abstraction is exothermic with an early transition state, so radical stability barely matters and Cl₂ gives mixtures weighted by the number of each type of H.

Predicting the major product

Rank the positions by resonance (allylic/benzylic) > 3° > 2° > 1°; the top one is the major product with a selective reagent (Br₂, NBS), while Cl₂ also weights by the number of each H.

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