Stability Orders of Carbocations, Carbanions, Radicals

Stability orders tell you which intermediate forms fastest and survives longest. Carbocations: tertiary > secondary > primary > methyl. Free radicals: the same. Carbanions: the exact reverse, methyl > primary > secondary > tertiary.

The intermediate's stability decides the rate of the slowest step: a reaction forming a tertiary carbocation runs far faster than one forced to form a methyl cation.

Key Concepts

• Hyperconjugation: the main stabilizer of carbocations and radicals. A neighbouring C−H sigma bond overlaps with the empty or half-filled p orbital, spreading the charge or unpaired electron over more atoms.
• Inductive effect (+I): alkyl groups push electron density toward the positive carbon through sigma bonds, further stabilizing carbocations. Both effects grow with the number of alkyl groups attached.
• Why carbanions reverse: a carbanion already has excess electrons, so electron-donating alkyl groups push even more negative charge onto it and destabilize it. The methyl anion, with no alkyl groups, is the most stable simple carbanion.
• Resonance beats everything: an allyl or benzyl cation, with the charge delocalized over several atoms by resonance, is more stable than even a tertiary alkyl cation. The same holds for allyl and benzyl radicals and anions.

How It Works

• Step 1 — count the alkyl groups on the charged carbon. Tertiary means three carbons attached, secondary two, primary one, methyl none.
• Step 2 — count the alpha hydrogens. Each C−H bond on a neighbouring carbon can hyperconjugate. The tert-butyl cation (CH3)3C+ has 9 alpha hydrogens, the isopropyl cation (CH3)2CH+ has 6, the ethyl cation CH3CH2+ has 3, and the methyl cation CH3+ has 0.
• Step 3 — rank by that count. Nine hyperconjugating bonds beat six, which beat three, which beat zero — giving exactly the order 3° > 2° > 1° > CH3+ for carbocations.
• Step 4 — check for resonance. If the charge or unpaired electron sits next to a double bond or benzene ring, resonance delocalization overrides the alkyl-group count. The benzyl cation C6H5CH2+ outranks the tert-butyl cation.
• Step 5 — flip the logic for carbanions. More alkyl groups mean more donated electron density onto an already negative carbon, so the order reverses: CH3- > 1° > 2° > 3°.

Worked Example

Rank the carbocations (CH3)3C+, (CH3)2CH+, CH3CH2+, and CH3+ by stability, using alpha-hydrogen counting.

1. (CH3)3C+ (tert-butyl): the positive carbon is attached to 3 methyl groups, each with 3 hydrogens → 3 × 3 = 9 alpha hydrogens.
2. (CH3)2CH+ (isopropyl): attached to 2 methyl groups → 2 × 3 = 6 alpha hydrogens.
3. CH3CH2+ (ethyl): attached to 1 methyl group → 1 × 3 = 3 alpha hydrogens.
4. CH3+ (methyl): no neighbouring carbons → 0 alpha hydrogens.
5. More alpha hydrogens means more hyperconjugation: (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+, the 9:6:3:0 fingerprint of the 3° > 2° > 1° > methyl order.

Common Misconceptions

• Carbanions follow the same order as carbocations. Wrong — it is the exact reverse. Students who memorize one order and apply it to both lose marks on every carbanion question.
• More alkyl groups always stabilize. Only for electron-deficient species (cations, radicals). For electron-rich carbanions, alkyl groups destabilize.
• The phenyl group stabilizes a cation by the inductive effect. Wrong mechanism. Phenyl stabilizes by resonance delocalization of the charge into the ring; its inductive effect is actually electron-withdrawing.

Real-World Applications

• Drug metabolism: the body often oxidizes drugs at tertiary C−H positions first, because the radical intermediate formed there is the most stable.

Count the alpha hydrogens: carbocations and radicals rank 3° > 2° > 1° > methyl by hyperconjugation; carbanions rank the reverse. Resonance can override both rules, so always check for a neighbouring pi system first.