The aci-nitro equilibrium of picrylacetone: a kinetic and thermodynamic study in 50∶50 and 30∶70 (v/v) H2O–Me2SO mixtures †

Moutiers, Gilles; Peignieux, Audrey; Terrier, François

doi:10.1039/a804087g

Cited by 8 publications

(7 citation statements)

References 32 publications

(26 reference statements)

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“…So far, the investigation of protonation/deprotonation dynamics at carbon has been limited to molecules where acidity is boosted by introduction of an electron-withdrawing group − or by removal of an electron. − These restrictions can be removed by application of the laser flash electron photoinjection technique. In a preceding paper, we have described the procedures by which the protonation kinetics of carbanions not bearing electron-withdrawing substituents or not modified by removal of an electron can be measured by this technique under a wide variety of conditions, up to values close to the diffusion limit.…”

Section: Introductionmentioning

confidence: 99%

Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

et al. 2003

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The diphenylmethane-diphenylmethyl anion acid/base couple in N,N-dimethylformamide is taken as an example for investigating the dynamics of proton transfer at carbon in a system where the acid is not activated by an electron-withdrawing group or by removal of an electron. The laser flash electron photoinjection technique is applied to the determination of the rate constant for the protonation of diphenylmethyl anion by an extended series of acids that offers a range of driving forces encompassing over 1.2 eV. The plot of the rate constant versus the pK(a) difference between diphenylmethane and the acids or of the activation free energy versus the standard free energy of the reaction exhibits clear "inverted region" behavior (by a factor of 80 in terms of rate constants). While such behaviors have been predicted and observed for outersphere electron-transfer reactions, previous evidence for proton-transfer reactions was scarce. Entropic factors, derived from an investigation of the temperature dependence of the experimental rate constants, are also discussed.

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Section: Introductionmentioning

confidence: 99%

Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

et al. 2003

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“…neutral or positively charged organometallic residues, in governing the reactivity of carbon acid sites. 5,6 In this communication we report our finding that the ionization of the P-(formylmethyl)triphenylphosphonium cation 1 is associated with the lowest intrinsic reactivity known to date for a carbon acid in aqueous solution. As will be shown, this result rules out classical carbon acid behaviour according to eqn.…”

mentioning

confidence: 80%

The extremely low intrinsic reactivity of the P-(formylmethyl)triphenylphosphonium cation: an artefact due to strong covalent hydration of the carbonyl group

Moutiers¹,

Peng²,

Peignieux³

et al. 1999

J. Chem. Soc., Perkin Trans. 2

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“…A first, difficulty encountered in the deciphering of the reasons that underlie this intrinsic slowness is the fact that, until recently, the experimental data on which the discussions were based concerned two families of rather peculiar acid−base couples. One of these is constituted by carbon acids that bear an electron-withdrawing group directly attached to the carbon atom or is located in a conjugated position to it on unsaturated substituents, such as ketones or nitroalkanes. ,− …”

Section: Introductionmentioning

confidence: 99%

Why Are Proton Transfers at Carbon Slow? Self-Exchange Reactions

and

Savéant

2004

J. Am. Chem. Soc.

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When the quantum character of proton transfer is taken into account, the intrinsic slowness of self-exchange proton transfer at carbon appears as a result of its nonadiabatic character as opposed to the adiabatic character of proton transfer at oxygen and nitrogen. This difference is caused by the lesser polarity of C-H bonds as compared to that of O-H and N-H bonds. Besides solvent and heavy-atom intramolecular reorganizations, the kinetics of the reaction are consequently governed at the level of a pre-exponential term by proton tunneling through the barrier. These contrasting behaviors are illustrated by an analysis of the CH(3)H + (-)CH(3), H(2)O + OH(-), and (+)NH(4) + NH(3) self-exchange reactions. The effect of electron-withdrawing substituents and the case of cation radicals are discussed within the same framework taking the O(2)NCH(2)H + CH(2)=NO(2)(-) and (+.)H(2)NCH(2)H + (.)CH(2)NH(2) as examples. Illustrated by the CH(2)=CH-CH(2)H + (-)CH(2)-CH=CH(2) couple, it is shown that the "imbalanced character of the transition state" is related to heavy-atom intramolecular reorganization. Combination of these various effects is finally analyzed, taking the O(2)N-CH(2)=CH-CH(2)H + CH(2)=CH-CH=NO(2)(-) and (+.)H(2)N-CH(2)=CH-CH(2)H + (.)CH(2)-CH=CH(2)-NH(2) couples as examples.

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The aci-nitro equilibrium of picrylacetone: a kinetic and thermodynamic study in 50∶50 and 30∶70 (v/v) H2O–Me2SO mixtures †

Cited by 8 publications

References 32 publications

Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

Evidence for Inverted Region Behavior in Proton Transfer to Carbanions

The extremely low intrinsic reactivity of the P-(formylmethyl)triphenylphosphonium cation: an artefact due to strong covalent hydration of the carbonyl group

Why Are Proton Transfers at Carbon Slow? Self-Exchange Reactions

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