University of Wollongong Research Online Faculty of Science - Papers (Archive) Faculty of Science, Medicine and Health 1-1-2005 Carbanions: formation, structure and thermochemistry Stephen J. Blanksby University of Wollongong, [email protected] John H. Bowie University of Adelaide Follow this and additional works at: https://ro.uow.edu.au/scipapers Part of the Life Sciences Commons, Physical Sciences and Mathematics Commons, and the Social and Behavioral Sciences Commons Recommended Citation Blanksby, Stephen J. and Bowie, John H.: Carbanions: formation, structure and thermochemistry 2005, 261-270. https://ro.uow.edu.au/scipapers/3237 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Carbanions: formation, structure and thermochemistry Abstract This chapter deals with even-electron carbanions: their formation, structure and thermochemical properties in the gas phase. There are a number of excellent reviews already available on the chemistry of carbanions: these discuss in the main, reactivity and anion molecule chemistry.1-4 In this chapter we focus primarily on the formation, structure and thermochemistry of simple hydrocarbon anions while other chapters in this encyclopaedia cover the broader aspects of carbanion chemistry (see Volume 1, “Strained Ring and Highly Basic Carbanions” and this volume, Reactions of Organic Molecules with Organic Ions: “Reactions of Anions with Carbonyl Centres: C–C Bond Forming Reactions”, and Unimolecular Dissociations of Organic Ions: “Fragmentations of Carbanions and Enolate Anions Directed From the Anionic Centre: an Aid to Structure Determination”, and “Negative Ion Mass Spectra of Underivatised Peptides: an Aid to Sequence Determination”). Keywords Carbanions, Formation, structure, thermochemistry Disciplines Life Sciences | Physical Sciences and Mathematics | Social and Behavioral Sciences Publication Details Blanksby, S. J. and Bowie, J. (2005). Carbanions: formation, structure and thermochemistry. In N. M. Nibbering (Eds.), Encyclopedia of Mass Spectrometry, Volume 4 (pp. 261-270). San Diego, CA: Elsevier. This book chapter is available at Research Online: https://ro.uow.edu.au/scipapers/3237 1 Blanksby & Bowie 22/01/13 Carbanions: Formation, Structure and Thermochemistry Stephen J. Blanksby1 and John H. Bowie2 1. Department of Chemistry, University of Wollongong, New South Wales, Australia, 2522. Phone: +61-2-4221 5484, Fax: +61-2-4221 4287, e-mail: [email protected] 2 Department of Chemistry, The University of Adelaide, South Australia, 5005 Phone: +61-8-8303 5767, Fax: +61-8-8303 4358, e-mail: [email protected] For the Encyclopaedia of Mass Spectrometry, Volume 4, Topic B19 August 2003 2 Blanksby & Bowie 22/01/13 Abstract This chapter deals with even-electron carbanions: their formation, structure and thermochemical properties in the gas phase. There are a number of excellent reviews already available on the chemistry of carbanions: these discuss in the main, reactivity and anion molecule chemistry.1-4 In this chapter we focus primarily on the formation, structure and thermochemistry of simple hydrocarbon anions while other chapters in this encyclopaedia cover the broader aspects of carbanion chemistry (see Volume 1, “Strained Ring and Highly Basic Carbanions” and this volume, Reactions of Organic Molecules with Organic Ions: “Reactions of Anions with Carbonyl Centres: C–C Bond Forming Reactions”, and Unimolecular Dissociations of Organic Ions: “Fragmentations of Carbanions and Enolate Anions Directed From the Anionic Centre: an Aid to Structure Determination”, and “Negative Ion Mass Spectra of Underivatised Peptides: an Aid to Sequence Determination”). Keywords: carbanions, hydrocarbon anions, gas phase acidity, electron affinity, anion stability 3 Blanksby & Bowie 22/01/13 1. The Formation of Carbanions The majority of organic molecules have acidic hydrogens somewhere in the molecule which may be _ removed by deprotonation to form [M-H] anions in the ion source of a mass spectrometer.2 This method of ion formation is a form of negative ion chemical ionization and usually employs strong _ _ _ anionic gas phase bases such as NH2 , HO and MeO to bring about deprotonation (eq. 1). When the proton is abstracted from a carbon centre then the resulting anion is a carbanion. _ _ RH + B → R + BH (where B = HO, NH2, MeO) (1) Numerous gas phase carbanions, such as acetylenic, allylic and benzylic anions, are readily formed by deprotonation with a gas phase base (cf. eq. 1). The carbon centres of some molecules, however, possess only poorly acidic hydrogens which cannot be removed by anionic bases of the type described above. This situation exists for all saturated hydrocarbons. For example, the deprotonation of methane _ _ by the amide anion, CH4 + NH2 → CH3 + NH3, is calculated to be endothermic by ca. 13 kcal mol-1 (see later discussion of gas phase acidity) and thus the methyl carbanion cannot be formed by deprotonation. A further disadvantage of the deprotonation method is observed for molecules with more than one potentially acidic centre. In such instances, deprotonation can lead to a mixture of isomeric anions or, in cases where there exists sufficient differences in acidity between the centres, only a single isomer which does not possess the desired regiochemistry may be formed. For example, the benzylic -1 protons of ethyl benzene (PhCH2CH3) are substantially more acidic (> 20 kcal mol ) than either the _ homobenzylic protons (PhCH2CH3) or the ring protons and thus only the benzylic anion (PhCHCH3 ) can be formed by deprotonation.5 Clearly, alternate methods of carbanion formation are required which (a) form carbanions from organic molecules which lack suitably acidic hydrogens and (b) are regioselective. The most common alternatives to deprotonation for the generation of carbanions include; (i) the collision induced decarboxylation of a carboxylate anion (eq. 2), where the precursor carboxylate anion is generated by deprotonation of a carboxylic acid (RCO2H), or by an SN2(C) reaction between _ _ _ a suitable nucleophile (NH2 , HO or MeO ) and an ester (RCO2R', where R' is usually Me); (ii) the 4 Blanksby & Bowie 22/01/13 collision induced elimination of formaldehyde from a primary alkoxide anion (eq. 3), where the precursor anion is formed by deprotonation of a suitable primary alcohol; and (iii) the gas phase 6,7 SN2(Si) reaction first used in the gas phase by DePuy and outlined in eq. 4 (Nu = F, RO). These three methods can be used to generate carbanions which cannot be formed by direct deprotonation and generally produce a single regioisomer with the charge site occupying the position of the displaced substituent. _ _ RCO2 → R + CO2 (2) _ _ RCH2O → R + CH2O (3) _ _ Me3SiR + Nu → R + Me3SiNu (where Nu = F, RO) (4) 2. Thermochemistry of carbanions The gas phase acidity of a molecule RH is a particularly important thermochemical property because, among other things, it provides vital information concerning the reactivity of the corresponding anion _ R (see General Concepts: “Gas-phase Acidities, Experiment and Theory”). The gas phase acidity of a molecule RH is, by definition, the molar Gibbs free energy (ΔacidG298) necessary to heterolytically dissociate it into a proton and an anion at 298 K according to eq. 5. In the case where heterolytic cleavage is of a C–H bond, a carbanion is produced. The enthalpy of eq. 5 is known as the enthalpy of 8,9 deprotonation (ΔacidH298) and is related to the gas phase acidity via ΔacidG298 = ΔacidH298 – TΔacidS298. The enthalpy of deprotonation can also be considered as the proton affinity (PA) of the anionic base _ R (i.e., the enthalpy of the reverse reaction in eq. 5). A large number of gas phase acidities have been measured and the values for a few selected hydrocarbons are listed in Table 1 (in units of kcal mol-1). A regularly updated and readily searchable compilation of gas phase acidity data may be found on the NIST database.10 _ RH → R + H+ (5) The measurement of the gas phase acidity of a molecule provides a convenient pathway for the determination of the heat of formation of the corresponding anion, which is critical for predicting the 5 Blanksby & Bowie 22/01/13 thermochemically favoured outcomes of anionic reactions. In the case of eq. 5, if the gas phase acidity _ is measured, then ΔacidH298[RH] can be determined, which is related to the heat of formation of the R _ + anion via the relation, ΔfH298[R ] = ΔacidH298[RH] + ΔfH298[RH] – ΔfH298[H ]. In this expression, + ΔfH298[RH] is the heat of formation of RH, which is usually available from calorimetry and ΔfH298[H ] = 365.7 kcal mol-1 is the precisely known heat of formation of H+. _ _ R → R• + e (6) Another important thermochemical property of carbanions (and anions generally) is the electron binding energy, which corresponds to the energy required to remove an electron from an anion. For an even-electron anion, such as that depicted in eq. 6, removal of an electron results in a neutral radical and a free electron. The enthalpy of this reaction is the electron binding energy, which is more commonly referred to as the electron affinity (EA) of the corresponding radical and is usually reported in units of electron volts. The electron affinity of a neutral is often used as an indicator of the “stability” of the corresponding anion. That is, if the electron affinity of R is positive, then the anion _ R is said to be “bound” or stable with respect to electron detachment, whereas if the electron affinity _ of R is negative, then the anion R is “unbound” or unstable with respect to electron detachment. In the latter case, a nascent anion formed by one of the methodologies discussed above (eq. 1-4) will spontaneously lose its electron to generate the neutral radical. The electron affinities of numerous carbon centred radicals are listed in Table 1 (in units of eV).