On the stabilization of carbanions by adjacent phenyl, cyano, methoxy-carbonyl, and nitro groups in the gas phase

Abstract

Institute of Mass Spectrometry, University of Amsterdam, Amsterdam, The Netherlands By using the method of Fourier transform ion cyclotron resonance mass spectrometry, substituent stabilization energies of homologous series of cycloalkyl carbanions, Ξ-c-CnH2n-2 (n = 3, 4, 5, 6, 7) with π-accepting substituents (Ξ = Ph, CN, COOMe, NO2) have been determined experimentally in the gas phase as the difference between the proton affinity of the substituted and corresponding unsubstituted (Ξ = H) cycloalkyl carbanions.The stabilization energy data have been analyzed in terms of Taft's parametrization of polarizability, field/inductive, and resonance effects. The linear regression analyses show excellent correlations within the ΞCH2 (-) Ξ-c-CnH2n-2 (-) (n = 4, 5, 6, 7), and Ξ-c-C3H4 (-) carbanion series, from which it appears that the contributions of polarizability effects are independent of the above type of carbanions and only depend on the nature of the substituent.Further, it follows that inductive stabilization is more effective in the substituted methyl, ΞCH2 (-), than in the substituted cycloalkyl, Ξ-c-CnH2n-2 (-) (n = 4, 5, 6, 7) carbanions. This result suggests that inductive stabilization is counteracted by the electron releasing effect of alkyl groups.Resonance stabilization is significantly more effective in the substituted cycloalkyl, Ξ-c-CnH2n-2 (-) (n = 4, 5, 6, 7), than in the substituted methyl, ΞCH2 (-), carbanions, which suggests that m contrast to inductive stabilization, resonance stabilization is assisted by the electron releasing effect of alkyl groups.Finally, it appears that substitutent stabilization in the geometrically restricted substituted cyclopropyl carbanions, Ξ-c-C3H4 (-), is dramatically less effective than in the corresponding geometrically unrestricted larger substituted cycloalkyl carbanions, Ξ-c-CnH2n-2 (-) (n = 4, 5, 6, 7). The linear regression analyses of the substituted cycloalkyl carbanions indicate that reduction of the stabilization energy is caused not exclusively by a geometrically hindered resonance stabilization, but also to a smaller extent by a less efficient inductive stabilization in the substituted cyclopropyl carbanions.

Four decades of joy in mass spectrometry

“…The ratio of the abundances of the (CH 3 ) 2 RSiO − and (CH 3 ) 3 SiO − ions then reflect the acidity difference between methane and the alkane RH. The proton affinities measured for alkyl and cycloalkyl anions in my group by use of the desilylation method and the home‐built FT‐ICR instrument agreed very well with those obtained by use of the FA‐SIFT method (Peerboom et al, 1992) and have been extended to cycloalkyl anions with various adjacent π‐accepting substituents, the results being analyzed in terms of polarizability, field/inductive, and resonance effects (Peerboom, de Koning, & Nibbering, 1994).…”

Gas-Phase Ion–Molecule Reactions of Small Nitroalkanes and Their Deprotonated Anions

“…Bimolecular reactions of nitromethane, nitroethane, 2nitropropane and nitrocyclopropane [M -H] anions were investigated with the gas-phase triatomic molecules, extending our early study 19 on deprotonated nitromethane. In addition to the rearranged products formed in the ICR studies (from deprotonated nitromethane 18 and nitrocyclopropane anions 4,5 ), we observed formation of ion-neutral adducts [RNO 2 •X]…”

“…3 With the ground state multiplicity problem resolved theoretically, the gas-phase acidity of a-substituted cyclic and acyclic alkanes continues to be a subject of extensive study. [4][5][6] In general, acidity is the result of trade-off between inductive, resonance, polarizability and lone-pair electron repulsion effects and is a measure of stabilization for the deprotonated carbanions. The most acidic hydrogen in nitroalkanes is the one on the a-carbon because of the contribution of the inductive effect of the nitro group on the neutral 1 and the resonance structures 2 and 3 which stabilize the anion formed by deprotonation (Scheme 1).…”

“…6 While these arguments apply to 4 in comparison with acyclic nitroalkanes, a detailed study including the ring-size effect and parametric analyses has provided a comprehensive picture of a-carbanion stabilization. 5 It is interesting to observe how these effects are manifested in the gas-phase ion-molecule reactions involving nitroalkanes and their deprotonated anions.…”

“…Apart from acidity determinations, 15 reactivity studies of nitroalkanes and deprotonated anions are relatively few in the gas phase. 4,5,[16][17][18][19] Some intriguing ipso-substitution processes have been observed for ion-molecule reactions involving nitrobenzene and its deprotonated anion, 20 and a detailed study has been reported on the reactions of methyl nitrate with negative ions (S N 2, E CO 2 and alkyl nitrate nitration) as an important gas-phase counterpart of solution chemistry. 21 We also explore a possibility that negative ion chemistry may be applied to the mass spectrometric detection of explosive nitro compounds.…”