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Analysis of the Gas Phase Acidity of Substituted Benzoic Acids Using Density Functional Concepts

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Analysis of the Gas Phase Acidity of Substituted Benzoic Acids Using Density Functional Concepts molecules Article Analysis of the Gas Phase Acidity of Substituted Benzoic Acids Using Density Functional Concepts Jorge A. Amador-Balderas 1, Michael-Adán Martínez-Sánchez 2, Ramsés E. Ramírez 1,* , Francisco Méndez 2,3,4,* and Francisco J. Meléndez 5 1 Departamento de Fisicomatemáticas, Facultad de Ciencias Químicas, Av. San Claudio y 14 Sur, Col. San Manuel, Benemérita Universidad Autónoma de Puebla, C.P. 72570, Puebla, Pue., Mexico 2 Departamento de Química, División de Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana-Iztapalapa, A.P. 55-534, Mexico, D.F., 09340 Mexico 3 Le Studium Guest Research Fellow, Loire Valley Institute for Advanced Studies, 4500 Orleans & Tours, France 4 Conditions Extrêmes et Matériaux: Haute Température et Irradiation (CEMHTI), UPR3079 CNRS, CEMHTI 1, Avenue de la Recherche Scientifique, 45071 Orleans, France 5 Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Av. San Claudio y 14 Sur, Col. San Manuel, Benemérita Universidad Autónoma de Puebla, C.P. 72570, Puebla, Pue., Mexico * Correspondence: [email protected] (R.E.R.); [email protected] (F.M.); Tel.: +52-55-58044600 (ext. 3282) (F.M.); +52-22-22295500 (ext. 3974) (R.E.R.) Academic Editor: Luis R. Domingo Received: 4 February 2020; Accepted: 29 March 2020; Published: 2 April 2020 Abstract: A theoretical study of the effect of the substituent Z on the gas phase acidity of substituted benzoic acids ZC6H4COOH in terms of density functional theory descriptors (chemical potential, softness and Fukui function) is presented. The calculated gas phase DacidG◦ values obtained were close to the experimental ones reported in the literature. The good relationship between the DacidG◦ values and the electronegativity of ZC6H4COOH and its fragments, suggested a better importance of the inductive than polarizability contributions. The balance of inductive and resonance contributions of the substituent in the acidity of substituted benzoic acids showed that the highest inductive and resonance effects were for the -SO2CF3 and -NH2 substituents in the para- and ortho-position, respectively. The Fukui function confirmed that the electron-releasing substituent attached to the phenyl ring of benzoic acid decreased the acidity in the trend ortho > meta > para, and the electron-withdrawing substituent increased the acidity in the trend ortho < meta < para. Keywords: absolute gas phase acidity; Fukui function; chemical potential; softness; nucleophile; electrophile; inductive effect; resonance effect; polarizability effect 1. Introduction X H The Hammett equation (logKa /logKa = σρ) is a linear Gibbs free energy relationship (a link between equilibrium constant and reaction rates) that uses the properties of substituted benzoic acids as a reference standard to study the effect of substituent on the reactivity and properties of various molecules [1]. Although the Hammett equation is fulfilled in the meta- and para-substituted benzoic acids (the correlation of acidity with the σ scale is obtained), it is not obeyed for the ortho-isomers [2,3]. Exner et al. [3] have suggested that the main part of ortho effects in the acidity of substituted benzoic acids is due to inductive and resonance effects and steric effects are of limited importance for acidity. Verevkin et al. [4] evaluated the thermochemical properties of methylbenzoic and methoxybenzoic acids and found that the conformational peculiarities of the ortho-methoxybenzoic acid have not relevance for the enthalpy of formation of this molecule. Taft et al. [5–7], Exner et al. [8,9] and Geerlings et al. [10,11] have pointed out the usefulness of the polarizability to the analysis of structural Molecules 2020, 25, 1631; doi:10.3390/molecules25071631 www.mdpi.com/journal/molecules Molecules 2019, 23, x FOR PEER REVIEW 2 of 11 and methoxybenzoic acids and found that the conformational peculiarities of the ortho-methoxybenzoic acid have not relevance for the enthalpy of formation of this molecule. Taft et al. [5–7], Exner et al. [8,9] and Geerlings et al. [10,11] have pointed out the usefulness of the Molecules 2020, 25, 1631 2 of 11 polarizability to the analysis of structural effects in proton-transfer reactions in gas phase. Chattaraj et al. [12] found trends of changes in acidity of meta- and para-substituted benzoic acids with eelectrophilicityffects in proton-transfer based charge reactions transfer in gas index, phase. frac Chattarajtional number et al. [12 of] found electrons trends transferred of changes and in aciditygroup ofcharge.meta- andLiu paraet al.-substituted [13] employed benzoic molecular acids with electr electrophilicityostatic potential based and charge the transfervalence index, natural fractional atomic numberorbitals ofto electronspredict the transferred acidity of and 196 group singly, charge. doubly, Liu etand al. triply [13] employed substituted molecular benzoic electrostatic acids. (the potentialortho-benzoic and theacids valence were naturalexcluded). atomic Vianello orbitals and to predictMaksic the [14] acidity found of that 196 the singly, acidity doubly, in a and series triply of substitutedpara-substituted benzoic benzoic acids. (theacidsortho increased-benzoic with acids the were stability excluded). of the Vianello corresponding and Maksic conjugate [14] found bases. that theHollingsworth acidity in a serieset al. of[15]para examined-substituted the benzoic effects acidsof substituents increased withon the the p stabilityKas of a of set the of corresponding 16 meta- and conjugatepara-substituted bases. benzoic Hollingsworth acids using et al. density [15] examined functional the etheoryffects of[B3LYP/6-311G(d, substituents on thep)] pcalculations,Kas of a set ofthey 16 metafound- and a goodpara -substitutedcorrelations benzoicwith the acids Löwdin, using Mulliken, density functional AIM, and theorynatural [B3LYP population/6-311G(d, analysis p)] calculations,charges on atoms they found of the a gooddissociatin correlationsg carboxylic with the acid Löwdin, group. Mulliken, Méndez AIM, et al. and studied natural the population effect of analysissubstituent charges on the on acidity atoms ofand the reactivity dissociating of para carboxylic-substituted acid group.phenols M [16–18],éndez et ethanol al. studied derivatives the effect [19] of substituentand their anions on the and acidity found and that reactivity the extension of para-substituted of the charge phenols dispersal [16 –in18 the], ethanol anions derivatives was the key [19 to] andunderstanding their anions the and gas-phase found that acidity the extension[17,20]. In ofaddition the charge to the dispersal analytical in equations, the anions developed was the key from to understandingthe density functional the gas-phase theory acidity (DFT) [17[21],20 and]. In the addition hard toand the soft analytical acids and equations, bases (HSAB) developed principle from the[22,23] density showed functional the mathematical theory (DFT) [relationship21] and the hard between and soft the acids gas andphase bases acidity (HSAB) and principle the hydrogen [22,23] showedcharge [16]. the mathematical relationship between the gas phase acidity and the hydrogen charge [16]. MostMost ofof the the previous previous studies studies have have evaluated evaluated the aciditythe acidity of the ofmeta the- and metapara- and-substituted para-substituted benzoic acids,benzoic while acids, in while general in thegeneralortho -substitutedthe ortho-substituted has been has excluded. been excluded. Therefore, Therefore, in this paper in this we paper focused we ourfocused attention our attention in the eff inect the of effect the substituent of the substituent Z on the Z on gas the phase gas phase acidity acidity of a set of ofa setortho of -,orthometa-, -meta and- and para-substituted benzoic acids ZC6H4COOH (1a–6c, Figure 1) in terms of the density functional para-substituted benzoic acids ZC6H4COOH (1a–6c, Figure1) in terms of the density functional theory descriptorstheory descriptors electronegativity electronegativityχ [21] and softnessχ [21] Sand(related softness with theS (related inductive with and polarizabilitythe inductive eff ects,and polarizability effects, respectively [24–28]) of substituent (Z) and fragments (ZC6H4, ZC6H4COO). We respectively [24–28]) of substituent (Z) and fragments (ZC6H4, ZC6H4COO). We found that there is a found that there is a good relationship between the acidity of substituted benzoic acids and the good relationship between the acidity of substituted benzoic acids and the electronegativity of ZC6H4, electronegativity of ZC6H4, ZC6H4COO and ZC6H4COOH indicating the importance of the inductive ZC6H4COO and ZC6H4COOH indicating the importance of the inductive contributions. While for the contributions. While for the softness of ZC6H4COOH and its fragments there is not a relationship softness of ZC6H4COOH and its fragments there is not a relationship with the acidity, suggesting less importancewith the acidity, of the suggesting polarizability less contributions.importance of Thethe polarizability Fukui function, contributions. a density functional The Fukui theory function, (DFT) a descriptordensity functional related withtheory the (DFT) resonance descriptor effect related [20,29 ],with showed the resonance that the electron-releasingeffect [20,29], showed substituent that the attachedelectron-releasing to the phenyl substituent ring of benzoic attached acid to (1a the–3c )phenyl decreases ring the of acidity benzoic in the acid trend (1aorth–3c)o >decreasesmeta > para the, andacidity the in electron-withdrawing the trend ortho > meta substituent > para, and (4a –the6c) electron-withdrawing increases the acidity in substituent the trend ortho (4a–<6cmeta) increases< para. the acidity in the trend ortho < meta < para. H O O O2 O1 1 Z =Z =Z =H C1 a b c 1a Za=CH3, Zb=Zc=H 4a Za=CF3, Zb=Zc=H C2 1b Z =CH , Z =Z =H 4b Z =CF , Z =Z =H Za b 3 a c b 3 a c C7 1c Zc=CH3, Za=Zb=H 4c Zc=CF3, Za=Zb=H C3 2a Za=OCH3, Zb=Zc=H 5a Za=NO2, Zb=Zc=H 2b Zb=OCH3, Za=Zc=H 5b Zb=NO2, Za=Zc=H C 6 C4 2c Zc=OCH3, Za=Zb=H 5c Zc=NO2, Za=Zb=H C 5 Zb 3a Za=NH2, Zb=Zc=H 6a Za=SO2CF3, Zb=Zc=H 3b Zb=NH2, Za=Zc=H 6b Zb=SO2CF3, Za=Zc=H Zc 3c Zc=NH2, Za=Zb=H 6c Zc=SO2CF3, Za=Zb=H Figure 1.
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