A theoretical study of the reactivity and regioselectivity of some aromatic compounds in electrophilic aromatic substitution is carried out at the B3LYP/6-31G(d) computational level. The relative reactivity of these systems is rationalized by means of the global nucleophilicity index proposed by Domingo’s group. The positional selectivity, namely o, m or p, is predicted by means of the local nucleophilicity indices [Parr fonctions]. The present study shows that the experimental trends of the relative reactivities and regioselectivities of these reactions are correctly predicted using Parr fonctions

Benharref, A.

El Hajbi, A.

Hammal, R.

ZEROUAL, Abdellah zeroual

Zoubir, M.

English

Revues Scientifiques Marocaines

Zeroual & al. / Mor. J. Chem. 3 N°2 (2015) 356-360  356   Understanding the regioselectivity and reactivity of Friedel–Crafts benzoylation using Parr functions  A. Zeroual a*, M. Zoubir a, R. Hammal a, A. Benharref b and A. El Hajbi a  a Laboratory of Physical Chemistry, Department of Chemistry, Faculty of Science Chouaib Doukkali University, El Jadida, Morocco b Laboratory of Biomolecular Chemistry, Natural Substances and Reactivity, URAC 16 Semlalia Faculty of Sciences, Cadi Ayyad University, Marrakech, Morocco   Corresponding author. E-mail address: zeroual19@yahoo.fr Received 04 Fev 2015, Revised 17 Feb 2015, Accepted 05 Mars 2015  Abstract  A theoretical study of the reactivity and regioselectivity of some aromatic compounds in electrophilic aromatic substitution was carried out using density functional theory with B3LYP/6-31G(d). The relative reactivity of these systems was rationalized by means of the global nucleophilicity index. Positional selectivity, namely o, m or p, is predicted by means of the local nucleophilicity indices (Parr functions).The present study shows that the experimental results of the relative reactivity and regioselectivity of these reactions are correctly predicted using Parr functions.  Key words: aromatic substitution, DFT, electrophilicity, nucleophilicity, regioselectivity, Parr functions  Introduction The Friedel-Crafts benzoylation of aromatics is the main route for the formation of aromatic ketones in organic synthesis. These aromatic ketones are widely used in the production of pharmaceuticals, fragrances, flavours, insecticides and other products [1]. Generally, these reactions are catalyzed by homogenous Lewis acids such as AlCl3, BF3, FeCl3, ZnCl2 or strong protonic acids (H2SO4, HF) [2]. However, these traditional catalysts have limitations such as environmental pollution hazards arising from the disposal of potential toxic wastes, reactor corrosion, and difficulties in their handling [3, 4]. To avoid these problems, in recent years, zeolites have come to be used increasingly as an environmentally friendly solid acid catalyst for Friedel–Crafts benzoylation of aromatic hydrocarbons [5], or else the reactions are carried out under solvent-free conditions, with dramatic reduction of reaction time compared to standard methods (Figure 1) [7].  R1R2R1R2MW, solvent-free+OClO Figure 1. Benzoylation of various aromatic compounds (MW: microwave) Zeroual & al. / Mor. J. Chem. 3 N°2 (2015) 356-360  357  Our aim in this work is to present a theoretical study on the reactions of electrophilic aromatic substitutions. We chose aromatic molecules as substitutes (Figure 2) and compared the results of our calculations with experimental results available in the literature [20]. OCH3 CH3 Et SMeisprOCH3OCH3CH3H3CCH3H3COOCH31 2 3 45 6 7 8123456123456 Figure 2. Aromatic molecules used as substitutes  Computational methods Density functional theory (DFT) [7, 8] computations were carried out using B3LYP [9] exchange-correlation functionals with the standard 6-31G(d) basis set [10]. Optimization was carried out using the Berny analytical gradient optimization method [11]. All computations were carried out with the Gaussian 09 suite of programs [12]. The global electrophilicity index ω [13] is given by the expression ω=μ2/2.η where μ is electronic chemical potential and η is chemical hardness. Both quantities may be considered in terms of the one-electron energies of the HOMO and LUMO frontier molecular orbitals, as εHOMO and εLUMO, μ=(εHOMO+ εLUMO)/2  and η=(εHOMO - εHOMO) respectively [14]. We introduced an empirical (relative) nucleophilicity index N [15], based on the HOMO energies obtained within the Kohn–Sham scheme [16] and defined as N= (EHOMO(Nu) – EHOMO(TCE)).  Nucleophilicity is calculated with reference to tetracyanoethylene (TCE), because this compound presents the lowest HOMO energy of a large series of molecules already investigated in the context of polar cycloadditions, which allows us to handle a nucleophilicity scale of positive values easily. The Pk+ electrophilic and Pk- nucleophilic Parr functions [17] which enable characterization of the electrophilic and nucleophilic centers of a molecule were obtained by analysis of the Mulliken atomic spin density of the radical anion and the radical cation respectively of the molecules studied.  Results and discussion Prediction of relative reactivity The global indices obtained using DFT are a powerful tool for understanding the behavior of polar reactions. The difference in global electrophilicity between two reactants [18] can be used to predict the polar character of the process and thus the feasibility of the cycloaddition. Table 1 shows the static global properties: electronic chemical potential μ, global electrophilicity ω, nucleophilicity N and difference of global electrophilicity Δω. (Table 1)   Zeroual & al. / Mor. J. Chem. 3 N°2 (2015) 356-360  358  Table 1. Global reactivity indices μ, ω, and N for compounds 1-8 and benzoyl chloride (Figure 2) calculated using DFT/B3LYP/6-31G(d) Compound μ (ev) ω (ev) N (ev) Δω (ev) 1 2 -3.216 -3.127 0.802 0.749 2.92 2.97 1.32 1.37 3 4 5 6 -3.138 -3.277 -3.037 -2.985 0.752 0.953 0.719 0.703 2.947 3.268 3.118 3.208 1.37 1.17 1.40 1.42 7 -2.859 0.709 3.621 1.41 8 Benzoyl chloride -2.653 -4.785 0.592 2.124 3.739 1.877 1.53 ------   The electronic chemical potential of eight aromatic compounds (-3.216, -3.127, -3.138, -3.277, -3.037, -2.985, -2.859, -2.653 eV) is greater than that of benzoyl chloride (-4.785), which implies that the transfer of electrons takes place from the aromatic compounds to benzoyl chloride.  The electrophilicity index of benzoyl chloride (2.124 eV) is greater than that of all eight aromatic compounds (column 3). Consequently in this electrophilic aromatic substitution benzoyl chloride behaves as an electrophile while the eight aromatic compounds behave as nucleophiles.   The global nucleophilic indices of the reactants confirm that benzoyl chloride is an electrophile and that the eight aromatic compounds are nucleophiles.  Using local nucleophilicity indices to predict the regioselectivity of electrophilic aromatic substitution reactions According to the polar model proposed by Chattaraj [19], the local philicity indices ωk and Nk are reliable indicators for predicting the most favored interaction between two polar centers. The most favored regioisomer is that which is associated with the highest local electrophilicity index ωk of the electrophile and the highest local nucleophilicity index Nk of the nucleophile. We determined Nk for aromatic compounds 1-8 in order to predict the most likely electrophile/nucleophile interaction throughout the reaction pathway, in order to elucidate the regioselectivity of these reactions.  Table 2 summarizes the values of nucleophilic Parr functions and local nucleophilicity for the eight aromatic compounds and benzoyl chloride. The results show that the most favored interaction will take place between the para center of aromatic compounds (possessing the highest value of N‏k).  The values of Nk for atoms 4 and 6 of compounds 5 and 8 allow us to characterize the most favorable interaction. For the atoms in the ortho and meta positions in compounds 6 and 7 we see that the local nucleophilicity is condensed, indicating that these atoms are those involved in the most favorable interaction. Consequently, experimental regioselectivity is correctly predicted by Parr function [20].    Zeroual & al. / Mor. J. Chem. 3 N°2 (2015) 356-360  359  Table 2. Nucleophilic Parr functions Pk- and local nucleophilicity values Nk at the o, m and p positions of  aromatic compounds 1–8. The greatest values of local nucleophilicity indices are given in bold.  Conclusion In this work, we have theoretically examined the reaction of a series of aromatic compounds with benzoyl chloride in electrophilic aromatic substitution. Our calculations show that experimental regioselectivity is correctly reproduced. The local nucleophilicity index predicts that the some C position is more reactive than other one in all systems. Our calculations also show that the Parr function correctly reproduces the relative reactivity of the carbone position in these compounds: the interaction takes place between the carbone of benzoyl and para position of compounds 1-4 and position 6 of the compounds 5 and 8. We conclude that local nucleophilicity, as defined by Domingo’s group using the nucleophilic Parr function, reliably predicts regioselectivity in electrophilic aromatic substitution.   References [1] P. Geneste, A. Finiels, Catalysts for Fine Chemical Synthesis Microporous and Mesoporous Solid Catalysts; Derouane, E.G., Ed.; John Wiley & Sons Ltd, Chichester, 2006, 95-104. [2] G.A. Olah, Friedel-Crafts and Related Reactions, vol. 1, Wiley Interscience, New York, 1963. [3] M. Yadav, U. Sharma, J. Mater. Environ. Sci. (2011) 407-414. Compound   Pk- Nk Experimental Yield (%)  1 o m p 0.075 -0.02 0.43 0.219 -0.058 1.255 5 0 90 2 o m p 0.07 -0.01 0.46 0.207 -0.029 1.366 26 3 71 3 o m p 0.082 -0.02 0.44 0.241 -0.058 1.296 23 7 70 4 o m p 0.138 -0.071 0.309 0.450 -0.232 1.009 10 0 90 5 2 5 4=6 -0.098 -0.15 0.4 -0.305 -0.467 1.247 0 0 100 6 o m 0.42 0.01 1.347 0.032 79 21 7 o m -0.07 0.18 -0.253 0.651 0 100 8 2 5 4=6 -0.044 -0.169 0.43 -0.164 -0.631 1.60 0 0 100 Zeroual & al. / Mor. J. 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Understanding the Regioselectivity and Reactivity of Friedel–Crafts benzoylation Using Parr Functions

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Understanding the Regioselectivity and Reactivity of Friedel–Crafts benzoylation Using Parr Functions

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