Effect of Substituents on Carbanion Stability

1. Hybridization

  In almost all areas of organometallic chemistry the primary subdivision of reactivity types is by the hybridization of the C-M carbon atom (methyl/alkyl, vinyl/aryl, alkynyl). A key second subdivision is the presence of conjugating substituents (allyl/allenyl/propargyl/benzyl).

  The fractional s-character of the C-H bonds has a major effect on the kinetic and thermodynamic acidity of the carbon acid. Only s-orbitals have electron density at the nucleus, and a lone pair with high fractional s character has its electron density closer to the nucleus, and is hence stabilized. This can be easily seen in the gas-phase acidity of the prototypical C-H types, ethane, ethylene and acetylene, as well as for cyclopropane, where the hybridization of the C-H bond is similar to that in ethylene.


2. Inductive Effects

  Electron-withdrawing substituents will inductively stabilize negative charge on nearby carbons. These effects are complex, since electronegative substituents interact with carbanions in other ways as well (e.g. O and F substituents have lone pairs, which tend to destabilize adjacent carbanion centers).


3. Conjugation - π Delocalization

  Delocalization of negative charge, especially onto electronegative atoms, provides potent stabilizations of carbanionic centers. Since almost all conjugating substituents are also more electronegative than H or CH3, there is usually a significant inductive contribution to the stabilization.


4. Second and Third Row Element Effects ("d-orbital" effects)

  All measures of acidity show that there is an unusual level of carbanion stabilization for all second row elements (Cl, S, P, Si, as well as higher elements) when these are bonded to a carbanion center.


  The origin of this stabilization has several components. Classical overlap of the lone pair with the empty d-orbitals isonly a minor contributor, since the d-orbitals are too diffuse and too high in energy. Thus both the overlap integral and the eneρgy separation are unfavorable. For the electronegative elements (Cl and S) there is an inductive component. For SR, PR2, SiR3 and higher analogs, which bear substituents on the X group, there is a major contribution of σ-hyperconjugation (delocalization of charge into X-R σ* orbitals).


  A factor comparable in size to σ-hyperconjugation is the σ bond strength effect. There is a size difference between the 3p orbitals of the S and 2p orbitals in the C-H compound. In the carbanion the C orbital increases in size, resulting in a stronger sigma bond. In an oxygen-substituted system the orbital mismatch is in the opposite direction: the p orbital at oxygen is smaller than that at carbon, and this size difference is excacerbated in the carbanion. Superimposed on these effects are possible lone pair effects (Cl, S, P).


5. Lone Pair Effects

  For the first row elements N, O, F, and perhaps also for higher elements, the presence of lone pairs has a strong destabilizing effect on a directly bonded carbanion center. This has several effects on carbanion structure: there are substantial rotational barriers around the C-X bond and the carbanion center is usually more pyramidalized.


Effect of Substituents on Carbanion Stability


pKa Table

Theory of Organolithium Reagents:
The Lithium Bond Reexamined,
  Sannigrahi, A. B.; Kar, T.; Niyogi, B. G.; Hobza, P.; Schleyer, P. v. R. Chem. Rev. 1990, 90, 1061.
Are Polar Organometallic Compounds Carbanions? The Gegenion Effect on Structure and Energies of Alkali-Metal Compounds,
  Lambert, C.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Eng. 1994, 33, 1129.

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