Carbanion Stability
Carbanions are the mirror image of carbocations. Where cations have an empty orbital and a positive charge, carbanions have a filled orbital with a lone pair and a negative charge. And where the stability trend for cations is 3° > 2° > 1° > methyl, the trend for carbanions runs in reverse: methyl > 1° > 2° > 3°.
The reason is the same physics, just flipped. Alkyl groups donate electron density. A positive charge welcomes that donation (hence more substituted = more stable cation). A negative charge is repelled by additional electron density, so fewer alkyl donors = more stable anion.
The Stability Order
From most to least stable (for simple alkyl carbanions):
Methyl (CH₃⁻) > 1° (RCH₂⁻) > 2° (R₂CH⁻) > 3° (R₃C⁻)
Note: all alkyl carbanions are unstable as free species; this ranking describes their relative stability. Alkyl carbanions only exist as reactive intermediates, usually stabilized by a metal counterion (e.g., in Grignards and organolithiums).
Adjacent electron-withdrawing groups flip the trend - they stabilize carbanions rather than destabilizing them. So the most stable carbanions in practice are those next to a carbonyl (enolates) or other strong withdrawing groups.
The Two Ways to Stabilize a Carbanion
Stabilization 1: Resonance delocalization onto electronegative atoms.
This is how enolates, nitromethane anions, and malonates are so stable. A carbanion with a lone pair adjacent to a C=O pi bond can delocalize the negative charge onto the oxygen via resonance. The result is essentially an oxygen anion (alkoxide-like), which is much more stable than a carbon anion.
- Enolate of acetone (pKa alpha-H = 20): the anion has the negative charge split between the alpha-carbon and the carbonyl oxygen.
- Malonate dianion (pKa alpha-H = 13): the central carbanion delocalizes onto BOTH flanking carbonyl oxygens. The conjugate base of malonate is extremely stable.
- Nitromethane anion (pKa = 10): the carbanion delocalizes onto two oxygens of the NO₂ group.
These stabilizing groups are why alpha-carbon acidity (Section 4.3, orbital-R factor) is so pronounced.
Stabilization 2: s-character (orbital hybridization).
As discussed in Section 4.3, orbitals with more s-character sit closer to the nucleus and stabilize a lone pair (or negative charge) better:
- sp³ carbanion (25% s-character): alkyl anion - least stable.
- sp² carbanion (33% s-character): vinyl anion - more stable.
- sp carbanion (50% s-character): acetylide anion - most stable of these three.
Terminal alkynes can be deprotonated by NaNH₂ because the acetylide ion (sp) is stable enough to exist as a discrete anion. Vinyl and alkyl anions require much stronger bases (n-BuLi, tBuLi) and are usually generated in situ with no isolation.
Common Stable Carbanions in Organic Chemistry
Memorize these as the carbanion menu:
| Carbanion | Source | Approximate pKa of C-H |
|---|---|---|
| Acetylide (R-C≡C⁻) | Terminal alkyne + NaNH₂ | 25 |
| Enolate (alpha-H of ketone/aldehyde) | Ketone + strong base (LDA, NaH) | 20 (for ketones), 17 (for aldehydes) |
| Stabilized alpha-H (1,3-dicarbonyl, nitro) | Diketone/malonate + NaOEt | 10-13 |
| Grignard carbanion (R-MgX) | R-X + Mg | N/A (prepared via oxidative insertion) |
| Organolithium (R-Li) | R-X + Li metal | N/A |
| Ylides (R₃P⁺-CR₂⁻) | Phosphonium salt + base | varies |
Why Grignards Work
A Grignard reagent (R-MgX) is a carbon-magnesium bond with a strong polarization. The C-Mg bond is polarized so that the carbon carries significant partial negative charge (δ⁻ on C, δ⁺ on Mg). In effect, you have a carbanion stabilized by a magnesium counterion.
The carbanion character is strong enough that the carbon acts as a powerful nucleophile. Grignards attack carbonyls readily - too readily, in fact. They react with water, alcohols, and any acidic proton instantly, which is why Grignards must be kept bone-dry. One drop of water destroys them.
Organolithium reagents (R-Li) are even more carbanion-like because the C-Li bond is more polarized than C-Mg. n-BuLi is commonly used to deprotonate extremely weak acids (pKa > 30) that amide bases cannot touch.
Enolates: The MCAT’s Favorite Carbanions
Enolates dominate Ch 7. Quick preview:
- Base removes the alpha-H of a carbonyl (alpha-C is the carbon directly bonded to C=O).
- Resulting anion has two resonance forms: the carbanion form (charge on alpha-C) and the enolate form (charge on the oxygen, with C=C between alpha-C and carbonyl-C).
- The enolate form is the major contributor because negative charge is better on oxygen than carbon.
- Enolates attack other electrophiles: another carbonyl (aldol), an alkyl halide (alpha-alkylation), an alpha-beta unsaturated carbonyl (Michael addition).
The pKa anchors (Section 4.3) are essential here. An alpha-H of a ketone (pKa 20) is deprotonated by LDA (conjugate acid amine pKa 36) but not by ethoxide (pKa 16). A 1,3-dicarbonyl alpha-H (pKa 10-13) can be deprotonated by hydroxide or ethoxide - much easier to form.
Ylides: Neutral Carbanion Equivalents
Some species behave as carbanions without a formal negative charge. Ylides are the most common example:
- Phosphonium ylide (Wittig reagent, R₃P⁺-CR₂⁻): the adjacent positive phosphorus stabilizes the carbanion. Used to convert ketones to alkenes.
- Sulfur ylides: similar structure with sulfur as the stabilizing positive atom.
Ylides are neutral overall but have significant carbanion character at the carbon. The phosphonium Wittig reagent is a key player in making defined-geometry alkenes from carbonyls - covered in Ch 10.
Carbanion vs. Strong Base: Same Molecule, Different Role
A molecule with a carbanion is also a strong base. In fact, the stronger the carbanion, the stronger the base (because the conjugate acid - the protonated carbon - has a high pKa). This means carbanion reagents can be used as:
- Nucleophiles (attack an electrophilic carbon to form a new C-C bond).
- Bases (remove a proton from a nearby acid).
Which role wins depends on the substrate. Carbanions with no steric bulk attached (methyl lithium, phenyl lithium) tend to attack as nucleophiles. Bulky carbanions (sec-BuLi, tert-BuLi, LDA) tend to act as bases because their steric bulk prevents nucleophilic attack on hindered carbons.
Students often treat Grignards as “always nucleophiles” and organolithiums as “always bases,” but both can do both. The substrate and conditions decide.
The last two sections cover less-common but still testable topics: free radicals (odd-electron species) and the thermodynamic-vs-kinetic choice that decides which product dominates.