Carbocation Stability and Structure
1. Definition & Structure:
o
Carbocations are tricoordinated, positively charged carbon atoms.
o
Hybridization: sp², leading to a planar geometry (except at bridgeheads,
where planarity is impossible, making bridgehead carbocations unstable).
2. Stability Factors:
o
o
Polar Effects:

Inductive Effect: Alkyl groups donate electrons via σ-bonds, stabilizing
the positive charge (tertiary > secondary > primary).

Field Effect: Electron donation through space (non-conjugated
substituents).
Hyperconjugation:

Delocalization of σ-electrons from adjacent C-H bonds into the empty
p-orbital.

More hyperconjugative structures = greater stability (tertiary: 6 forms;
primary: 2 forms).
3. Stability Order:
o
Tertiary carbocations (e.g., tert-butyl) are most stable due to strong
inductive effects and hyperconjugation.
o
Allylic cations: Resonance delocalizes charge over multiple atoms (e.g.,
allyl, bis-allyl), enhancing stability.
o
Unstable Cases:

Vinyl and phenyl cations lack resonance stabilization (charge
localized, very short-lived).

Bridgehead carbocations (non-planar geometry).
4. Experimental Examples:
o
Stable salts (e.g., Me₃ C⁺SbF₆ ⁻) form in superacids (e.g., SbF₅ ) at low
temperatures (< -20°C).
5. Key Contrasts:
o
Allylic vs. Non-conjugated: Allylic carbocations benefit from resonance;
alkyl carbocations rely on hyperconjugation/induction.
o
Solvent Role: Polar solvents stabilize free carbocations via solvation.
Takeaway: Stability arises from charge delocalization (hyperconjugation, resonance) and
electron-donating effects (inductive/field). Structure (planarity) and substituent type critically
determine carbocation lifetime.
Summary of Carbanion Stability and Structure
Definition & Structure:

Carbanions are negatively charged carbon species with a lone pair of electrons (sp³
hybridized, pyramidal geometry akin to amines).

Bridgehead Formation: Unlike carbocations, carbanions can form at bridgeheads
due to pyramidal geometry.

Example: Methyl anion (CH₃ ⁻) is pyramidal, observed in the gas phase.
Stability Factors:
1. Conjugate Acid Strength:
o
Inverse Relationship: Weaker conjugate acid → stronger base (less stable
carbanion).
o
Stability is inferred from the acidity of the conjugate acid (e.g., stronger acids
produce more stable carbanions).
2. Electron-Donating Effects:
o
Alkyl groups increase negative charge density, reducing stability (tertiary
< secondary < primary < methyl).
o
β-Branching further destabilizes carbanions.
3. Stabilizing Features:
o
Conjugation with Unsaturated Bonds:

o
Electron-Withdrawing Groups:

o
o
Resonance delocalizes negative charge (e.g., allylic, benzylic, and
triarylmethyl carbanions like Ph₂ CH⁻ and Ph₃ C⁻).
Adjacent carbonyl (C=O), imine (C=N), or nitro (NO₂ ) groups stabilize
via charge delocalization (e.g., nitroalkane anions are stable in water).
Increased s-Character:

Higher s-character at the carbanionic carbon enhances stability (e.g.,
sp-hybridized acetylene > sp² ethylene > sp³ ethane).

Cyclopropyl exception: Strain increases s-character, making
cyclopropyl carbanions more stable than methyl.
Aromaticity:

Aromatic systems (e.g., cyclopentadienyl anion) gain stability through
resonance and electron delocalization.
Stability Order:
Vinyl ≈ Phenyl > Cyclopropyl > Methyl > Ethyl > n-Propyl > Isopropyl > Isobutyl >
Neopentyl > Cyclobutyl > Cyclopentyl

Key Drivers:
o
s-Character: Vinyl (sp²) and phenyl (resonance) > cyclopropyl (strained sp³)
> methyl (sp³).
o
Resonance/Aromaticity: Allylic, benzylic, and aromatic carbanions are
exceptionally stable.
Experimental Examples:

Stable Carbanions:
o
Ph₂ CH⁻ and Ph₃ C⁻ (stable in dry solutions; X-ray structures confirmed).
o
Nitroalkane anions (stable in water).
o
Cyclopentadienyl anion (aromatic stabilization).
Key Contrasts with Carbocations:

Hybridization: Carbanions are sp³ (pyramidal); carbocations are sp² (planar).

Bridgehead Stability: Carbanions form at bridgeheads; carbocations cannot.

Stabilizing Groups: Carbanions require electron-withdrawing groups;
carbocations favor electron-donating groups.
Takeaway: Carbanion stability hinges on charge delocalization (resonance, aromaticity,
electron-withdrawing groups) and hybridization (s-character). Substituents and geometry
critically influence reactivity and lifetime.
Free Radical Stability and Structure
Definition & Structure:

Free Radicals: Species with one or more unpaired electrons.

Hybridization:
o
Planar (sp²): Simple alkyl radicals (e.g., CH₃ •) adopt a planar geometry with
the unpaired electron in a p orbital.
o
Pyramidal (sp³): Radicals bonded to electronegative atoms (e.g., CF₃ •) or in
strained systems (e.g., cyclopropyl) become pyramidal, with the electron in
an sp³ orbital.
o
Flexibility: Energy difference between planar and pyramidal forms is small,
allowing near-planar geometries.
Stability Factors:
1. Hyperconjugation:
o
Stabilizes radicals by delocalizing the unpaired electron via σ-bonds (tertiary
> secondary > primary).
2. Resonance Delocalization:
o
Allylic and benzylic radicals: Enhanced stability due to electron
delocalization across π-systems.
o
Pentadienyl radicals: Exceptionally stable due to extended conjugation.
3. Steric Hindrance:
o
Triphenylmethyl radicals: Primarily stabilized by steric protection (hindered
recombination), with minor resonance contributions.
4. Substituent Effects:
o
Electronegative groups (e.g., F in CF₃ •) promote pyramidal geometry but do
not inherently stabilize the radical.
Stability Order:
Tertiary > Secondary > Primary

Key Examples:
o
Stable Radicals: Allylic, benzylic, pentadienyl, and triphenylmethyl radicals
(some persist indefinitely).
o
Unstable Radicals: Simple alkyl radicals (e.g., CH₃ •) are short-lived in
solution but stabilize in solid matrices.
Key Structural Insights:

Cyclopropyl Radicals: Pyramidal due to ring strain, deviating from typical planar
alkyl radicals.

Environmental Impact: Radicals in crystal lattices or rigid matrices exhibit longer
lifetimes compared to solution.
Takeaway: Radical stability depends on hyperconjugation, resonance delocalization,
and steric protection. Geometry (planar vs. pyramidal) is influenced by hybridization and
substituents, but stability is primarily governed by electronic factors.
Electron Delocalization and Aromaticity
1. Electron Delocalization:

Localized electrons: Restricted to a single atom or bond (e.g., σ bonds in alkanes).

Delocalized electrons: Shared across three or more atoms (e.g., π electrons in
benzene).

Significance: Explains the unique stability and reactivity of benzene, which classical
localized models could not.
2. Concept of Aromaticity:


Definition: Aromaticity arises from cyclic electron delocalization, conferring:
o
Unusual stability (resonance stabilization).
o
Chemical inertness (prefers substitution over addition, unlike alkenes).
o
Magnetic properties (due to mobile π electrons).
Historical Context:
o
Term "aromatic" originally described fragrant compounds (e.g., benzene,
toluene, benzaldehyde) but now refers to stabilized cyclic conjugated
systems.
3. Benzene’s Structure (Kekulé’s Model):


Key Evidence:
o
X-ray diffraction (1930s) confirmed benzene is planar, with sp²-hybridized
carbons and equal C–C bond lengths (1.39 Å).
o
Bond lengths are intermediate between single (1.54 Å) and double bonds
(1.34 Å).
Resonance Hybrid:
o
Benzene is a superposition of two Kekulé structures (resonance forms).
o
The actual structure is a resonance hybrid with delocalized π electrons
forming a continuous "doughnut-shaped" cloud above and below the ring.
4. Resonance Energy:

Definition: The extra stability gained from electron delocalization, quantified as the
energy difference between the real molecule and its hypothetical localized form.

Significance:
o
Explains why aromatic compounds are more stable than non-aromatic
analogs.
o
Benzene’s resonance energy is ~36 kcal/mol, reflecting its exceptional
stability.
5. Criteria for Aromaticity:
Aromatic compounds must exhibit:
1. Cyclic Conjugation: A planar ring with overlapping p orbitals.
2. π-Electron Delocalization: Continuous cloud of electrons.
3. Hückel’s Rule: (4n + 2) π electrons (e.g., benzene has 6 π electrons, n=1).
Key Takeaways:

Delocalization stabilizes molecules by spreading electron density.

Aromaticity is a cornerstone of organic chemistry, explaining the stability/reactivity of
benzene and related compounds.

Resonance energy quantifies stabilization from delocalization, critical for predicting
chemical behavior.
Examples:

Stable aromatic systems: Benzene, naphthalene, cyclopentadienyl anion.

Non-aromatic: Cyclooctatetraene (non-planar, 8 π electrons).