Substituent effects in α-substituted carbanions

Abstract

The para¹³C shifts of benzylcarbanions PhCH^–X (VI) in Me₂SO, acidities of toluene carbon acids PhCH₂X (I) in Me₂SO, and gas-phase acidities of methane carbon acids CH₃X (VIII) are submitted to mono- and bi-parametric substituent treatment using previously defined sets of σ_c^–, σ_IB, and σ_R^– constants. It is confirmed that these scales are appropriate for describing interactions between contiguous functionalities, as opposed to literature values which account for remote interactions. Gas-phase acidities respond excellently to substituent effect treatment, while solution data show deviations for the SO₂R and CN substituents. Such deviances are accounted for as a manifestation of SSSAR (Stabilization due to Specific Solvent Assisted Resonance) for groups which exhibit extensive charge transfer from the carbanionic carbon to the π-accepting substituent (COR, CO₂R, CONMe₂, NO₂, etc.). The mesomeric component of substituent effects is about three times more important than the polar–inductive one in governing the acidities of carbon acids in Me₂SO solution and in affecting the chemical shift of the para monitor in PhCH^–X. Relative to the gas phase, the solvent enhances by a factor of 2 the sensitivity of acidities of carbon acids to the mesomeric components of substituent effects. The sensitivity of the para monitor in PhCH^–X to polar–inductive components of substituent effects shows, relative to the neutral precursor PhCH₂X (I), a three-fold increase and, relative to the PhNHX family, a two-fold increase. The sensitivity of the para monitor in PhCH^–X to mesomeric components of substituent effects shows a three-fold increase relative to the PhNHX family.