Stability of Reaction Intermediates
Carbocations, Carbanions, and Free Radicals: Structure, Properties, and Stability Trends.
Reaction intermediates are short-lived, high-energy species formed during the conversion of reactants to products. Their stability determines the pathway (mechanism) and the major product of an organic reaction.
1. Carbocation ($C^+$)
Structure & Nature
- Definition: Organic ions containing a positively charged carbon atom with only 6 valence electrons.
- Hybridization: $sp^2$
- Geometry: Trigonal Planar (Bond angle $120^\circ$).
- Magnetic Nature: Diamagnetic (All electrons paired).
- Nature: Electron Deficient (Lewis Acid / Electrophile).
Factors Affecting Stability
Since carbocations are electron-deficient, any group that donates electrons stabilizes them by dispersing the positive charge.
- Inductive Effect (+I): Alkyl groups push electrons towards the $C^+$. More alkyl groups $\rightarrow$ More Stability.
- Hyperconjugation: More $\alpha$-hydrogens allow more delocalization of $\sigma$ electrons.
- Resonance (+R): Delocalization of positive charge over a $\pi$-system (Allyl/Benzyl) provides immense stability.
2. Carbon Free Radical ($C^\bullet$)
Structure & Nature
- Definition: Neutral species with an odd (unpaired) electron on carbon (7 valence electrons).
- Hybridization: Typically $sp^2$.
- Geometry: Planar.
- Magnetic Nature: Paramagnetic (Due to unpaired electron).
- Nature: Electron Deficient (Electrophile).
Factors Affecting Stability
Like carbocations, radicals are electron-deficient. They are stabilized by Electron Releasing Groups (ERG) via Hyperconjugation and Resonance.
The stability trend follows the same logic as carbocations:
3. Carbanion ($C^-$)
Structure & Nature
- Definition: Organic ions containing a negatively charged carbon atom with 8 valence electrons (complete octet).
- Hybridization: $sp^3$.
- Geometry: Pyramidal (Distorted Tetrahedral due to lone pair).
- Magnetic Nature: Diamagnetic.
- Nature: Electron Rich (Lewis Base / Nucleophile).
Factors Affecting Stability
Carbanions are electron-rich. Any group that withdraws electrons stabilizes them by dispersing the negative charge. Conversely, electron-donating groups destabilize them.
- Inductive Effect (-I): Electron withdrawing groups (like $-NO_2, -CN, -X$) stabilize carbanions. Alkyl groups (+I) destabilize them.
- Resonance (-R): Delocalization of the negative charge into a $\pi$-system stabilizes the ion greatly.
- s-Character: Negative charge is more stable on carbon with higher s-character ($sp > sp^2 > sp^3$). So, Alkyne anions are very stable.
*Note: This order is exactly opposite to that of carbocations and free radicals.
4. Summary Comparison
| Property | Carbocation | Free Radical | Carbanion |
|---|---|---|---|
| Charge | Positive (+) | Neutral ($\bullet$) | Negative (-) |
| Valence Electrons | 6 (Incomplete) | 7 (Odd) | 8 (Complete) |
| Hybridization | $sp^2$ | $sp^2$ | $sp^3$ |
| Shape | Planar | Planar | Pyramidal |
| Stability Order | $3^\circ > 2^\circ > 1^\circ$ | $3^\circ > 2^\circ > 1^\circ$ | $1^\circ > 2^\circ > 3^\circ$ |
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