THE CONCEPT
One Unpaired Electron Instead of an Empty Orbital
A free radical is a species with a single unpaired electron sitting in an otherwise singly-occupied orbital, rather than the fully empty p-orbital found in a carbocation. Despite that structural difference, radicals are stabilized by essentially the same mechanism as carbocations: neighboring alkyl groups donate electron density into that reactive orbital through hyperconjugation, where a nearby C-H sigma bond's electrons partially delocalize into the singly-occupied orbital.
Since each additional alkyl group attached to the radical carbon provides another source of this stabilizing hyperconjugative donation, radical stability follows the same substitution-based ranking you already know: tertiary > secondary > primary > methyl. The parallel to carbocation stability is deliberate and worth leaning on directly — if you already have the carbocation ranking memorized, you already have the radical ranking too.
💡 Memory Trick
The hub's trick states the parallel and the key difference in one line: radical stability — 3° > 2° > 1° > methyl, same order as carbocations but less dramatic. The 'less dramatic' qualifier is worth taking seriously: because a radical only needs to stabilize a single unpaired electron (versus a carbocation's fully empty orbital craving electron density much more urgently), the energy gaps between tertiary, secondary, and primary radicals are meaningfully smaller than the corresponding gaps between tertiary, secondary, and primary carbocations — the ranking order is identical, but the stakes riding on that ranking are lower.
RESONANCE-STABILIZED AND UNSTABLE RADICALS
The Same Exceptions You Already Know
Exactly as with carbocations, allylic and benzylic radicals (adjacent to a C=C double bond or an aromatic ring, respectively) are considerably more stable than even a tertiary alkyl radical, because the unpaired electron can delocalize by resonance across multiple atoms rather than staying fixed on a single carbon. And exactly as with carbocations, vinyl and aryl radicals (on an sp² carbon that's part of a C=C double bond or an aromatic ring itself) are unusually unstable, since the geometry leaves the unpaired electron poorly positioned for any stabilization.
This radical stability ranking has a direct, practical consequence you already encountered in the Free Radical Halogenation of Alkanes lesson: it's exactly why bromine radical selectively abstracts a hydrogen from the most substituted carbon available — that abstraction step generates whichever radical is most stable, and the reaction preferentially proceeds through the lower-energy pathway leading to that more stable radical, just as reactions preferentially proceed through more stable carbocation intermediates.
🧪 Lab Application
You're predicting the major product of radical bromination on 2-methylbutane, which has hydrogens on primary, secondary, and tertiary carbons all available for abstraction.
1
Identify every distinct type of hydrogen available. 2-methylbutane has primary hydrogens (on the methyl groups), secondary hydrogens, and a single tertiary hydrogen.
2
Rank the radical that would form at each position. Abstraction at the tertiary carbon gives a tertiary radical; at a secondary carbon gives a secondary radical; at a primary carbon gives a primary radical — following the same 3° > 2° > 1° stability order.
3
Recall that bromine radical is highly selective. Since Br• (unlike Cl•) strongly favors forming the most stable radical, expect bromination to occur predominantly at the tertiary carbon.
4
Predict the major product. The major brominated product should be the one with bromine installed at the tertiary carbon, since that pathway proceeds through the most stable (tertiary) radical intermediate.
📌 Exam Application
Exams frequently pair radical stability directly with the Free Radical Halogenation lesson, asking you to predict major products at multiple possible positions — always rank the radical stability at each candidate position first, then apply bromine's known strong selectivity (or chlorine's comparatively weaker selectivity) to predict the product distribution.
⚠️ Most Common Radical Stability Mistakes
The most common mistake is assuming radical stability differences are as large as carbocation stability differences, leading to overconfident predictions about product ratios — remember the gaps are real but 'less dramatic.' The other frequent trap is forgetting that allylic and benzylic radicals can outrank even tertiary alkyl radicals, exactly paralleling the same exception for carbocations.
✓ Quick Self-Test
1) What is the radical stability order from most to least stable? 2) Why are radical stability differences generally smaller than carbocation stability differences? 3) Why are allylic and benzylic radicals unusually stable? 4) Why are vinyl and aryl radicals unusually unstable? 5) How does radical stability explain bromine's high selectivity in radical halogenation?
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Carbocation Rearrangements
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