Priority Order for Carbocation Stability
When multiple electronic effects act simultaneously, which one wins? The definitive guide.
Determining the stability of carbocations often involves conflicting electronic effects (e.g., a group shows -I but +R). To solve these problems correctly, we follow a strict hierarchy of dominance.
1. The Golden Priority Order
Memorize this sequence. The effect higher in the list overrides the effects below it.
EXCEPTION: Anti-Aromaticity leads to Least Stability (Avoid at all costs!)
2. Detailed Breakdown
A. Aromaticity
Cyclic, planar, conjugated systems with $(4n+2)\pi$ electrons are exceptionally stable.
Example: Tropylium Cation ($C_7H_7^+$) is more stable than Benzyl cation ($Ph-CH_2^+$).
B. Special Resonance Cases
- Dancing Resonance: Cyclopropylmethyl cation (CPM) is exceptionally stable due to $\sigma$-bond overlap. Stable than Benzyl.
- Back Bonding (+M by lone pair): If a heteroatom with a lone pair (O, N) is directly attached to $C^+$, it completes the octet.
$CH_3-\overset{+}{C}H-\ddot{O}CH_3$ is very stable (Octet complete).
C. Resonance (Mesomeric)
Delocalization of $\pi$ electrons. More resonating structures generally mean more stability.
Note: Resonance dominates over Hyperconjugation and Inductive effects.
Example: Allyl ($CH_2=CH-CH_2^+$) > Propyl ($CH_3-CH_2-CH_2^+$).
D. Hyperconjugation
Count the number of $\alpha$-hydrogens.
Stability $\propto$ No. of $\alpha$-H.
This explains why $3^\circ > 2^\circ > 1^\circ$ alkyl carbocations.
E. Inductive Effect
The weakest effect. Use only when other factors are absent or equal.
Stability $\propto$ +I effect.
3. Classic Comparison Examples
1. Benzyl vs Tert-Butyl Cation
Benzyl ($Ph-CH_2^+$): Resonance stabilized.
Tert-Butyl ($(CH_3)_3C^+$): 9 $\alpha$-H (Hyperconjugation).
Verdict: Usually, Resonance > Hyperconjugation. However, the massive hyperconjugation of 9 $\alpha$-H makes Tert-Butyl comparable to or slightly more stable than Benzyl in gas phase. (In solution, solvation effects matter).
2. Halogen on Benzene Ring (p-Chloro Benzyl Carbocation)
Halogens show +M (Resonance) and -I (Inductive).
Usually +M dominates -I.
Exception: For Halogens, -I is stronger than +M.
Therefore, p-Chlorobenzyl cation is less stable than unsubstituted Benzyl cation.
3. Resonance vs Inductive (Methoxy vs Nitro)
$p-OCH_3$ Benzyl: Shows +M (Strong stabilization).
$p-NO_2$ Benzyl: Shows -M and -I (Strong destabilization).
Order: $p-OCH_3 > \text{Benzyl} > p-NO_2$.
Knowledge Check
Test your application of the Priority Rule
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