THEORETICAL STUDY OF CHLORINATION REACTION OF NITROBENZENE FROM DFT CALCULATIONS Council for Innovative Research

The geometric parameters of stationary points on the potential surface energy of the chlorination reaction of nitrobenzene in the presence of Aluminium chloride as catalyst were investigated theoretically by hybrid DFT (Density Functional Theory) calculations in order to determine his general reaction mechanism in gas phase and in solution. The results obtained by DFT have been compared with CCSD(T) method which is the most powerful post-Hartree Fock method in terms of inclusion of dynamic correlation. Although the electrophilic substitution reaction is widely taught in most courses in organic chemistry, the mechanism has been very few studied theoretically. The results obtained in gas phase are consistent with the traditional description of these reactions: the orientation of this substitution in meta position depends on the stability of a reaction intermediate (Wheland said). Without taking in consideration the reactants and products, six stationary points are found on the potential surface energy of this reaction. The reaction has also been studied in the presence of solvent and we’ve noted that the influence of solvent decreases the electrostatic attraction on the Wheland complex, but the general reaction mechanism remains unchanged in solution. functional method B3LYP CCSD(T) 6-31G** Gaussian optimized B3LYP obtained structure minimum or Transition for a minimum and one imaginary for a Transition State (TS).


Introduction
Many researchers are very interested to study theoretically the reactivity or the geometry of molecules with quantum chemical methods. The concept of reaction mechanism plays a major role in organic chemistry for the comprehension of reaction processes. The description of reaction mechanism associated to a given reaction is based on the calculation of the geometries and the energies of stationary points from reactants to products via transition states and possible intermediates on the potential surface energy of Born Oppenheimer [1][2][3][4][5]. The mechanism of electrophilic substitution of non substituted benzene has been studied theoretically [6][7][8][9][10][11]. A relative recent paper has confirmed the presence of many intermediates in the case of chlorination reaction of non substituted benzene catalysed by AlCl3: Two complexes Π and one σ have been characterized in this study [12]. Very recently, thermochemical parameters and IR spectra of chlorinated isomers of nitrobenzene have been calculated and showed that theoretical parameters are closed to experimental data [13]. The influence of nitrobenzene as a solvent in this reaction permits to obtain the activation barriers which are much closed to experimental values [14][15][16]. In the electrophilic substitution with benzene derivatives, the orientation of the second substitution depends on the nature of the first substituent and the stability of the intermediate σ called Wheland complex [17]. Experimental data have proved that these stable complexes σ exist and are responsible of the orientation of the substitution reaction [18][19][20][21][22][23][24][25][26][27][28].
In this work, we've propose to explore the potential surface energy of the chlorination reaction of nitrobenzene (which have an acceptor substituent of electron) in the presence of Aluminium chloride as catalyst in order to determine the geometry of all stationary points and his general reaction mechanism in gas phase and in solution before validate or invalidate the Wheland interpretation which said that the orientation of this reaction depends on the stability of a reaction intermediate.

Theoretical calculations
In the molecular orbital calculation, the chemical species participating in the chlorination reaction are assumed to be nitrobenzene (C6H5-NO2), molecular chlorine (Cl2) and Lewis acid, so that the reacting system is considered to be neutral at the starting point and to be more realistic in describing the overall reaction pathway. The aluminium chloride monomer AlCl3 is adopted as a Lewis acid catalyst in the present study. The overall reaction is formally written as follows:

C 6 H 5 -NO 2 + Cl 2 → C 6 H 4 NO 2 + (AlCl 3 + HCl)
The calculations have been carried out at the hybrid Hartree-Fock density functional method B3LYP level [29] and CCSD(T) level [30][31][32][33] with the basis 6-31G** in Gaussian 03 software [34]. The vibrational analysis has been carried out at each optimized geometry with B3LYP level in order to confirm whether the obtained structure is located at energy minimum or Transition State; all positive for a minimum and one imaginary for a Transition State (TS).

Results and discussions
The relative energies obtained for the chlorination of nitrobenzene in the presence of Aluminium Chloride as catalyst in gas phase are in Table 1

1.750
By comparing the three products of the chlorination reaction of nitrobenzene, the energy of ortho product is higher (-19.3 kcal/mol) than the meta and para products with the energies which are very closed (between -26.6 and 27.4 kcal/mol). It is the same situation with CCSD(T) calculation where the relative energies of products follows the same order of stability as in DFT calculations.
The three products have a differentiation on thermodynamic way. Geometrically, there is an important distortion towards NO2 fragment in the ortho product than meta and para product. The two distances of N-O bonds are different in ortho product; but in meta and para products, these two N-O bonds have the same distance. The distance of C-Cl bond is too small in ortho product (1.739 Å) compared to the same bond in meta and para product (1.753 and 1.750 Å respectively).

Π 1 complex
For the three attack ways, one unique Π1 complex has been localized corresponding for a fixation on meta position. The optimized structure of this complex is in Fig. 1 below (Units of bond lengths and bond angles of all parameters are Å and degrees (° ), respectively.): Concerning the above-mentioned evidences, the complex formed by the interaction among nitrobenzene, Cl2 and AlCl3 may not be highly polarized species. Since the stabilization of the Π1 complex is mainly caused by van der Waals forces, there is no energy barrier to form such weakly bound Π-type complex in the initial stage of the reaction.This π1 complex is formed by weakness interaction between the reactants (nitrobenzene, dichlore and aluminium chloride). The optimized geometry show that the C-Cl bond increases from 2.043 Å in the reactant to 2.140 Å in the stabilized complex. It is the same situation for the Al-Cl bond which increases from 2.087 to 2.493 Å. The relative energyof st abilization of the π1 complex is -7 kcal/mol compared to isolated reactants. With CCSD (T) method, this stabilization energy becomes -6.0 kcal/mol.

Transition States TS1
The three transition states which permit the formation of σ1 complexes localized for the three ways of attack are represented in Fig. 2 below (Units of bond lengths and bond angles of all parameters are Å and degrees (° ) respectively: The relative energies obtained for TS1 in the case of meta approach is smaller (-0.9 kcal/mol) than the one obtained for ortho (+0.2 kcal/mol) and para (+2.2 kcal/mol). The energy barrier from π1 complex for ortho position is 7.2 kcal/mol, 6.1 kcal/mol for meta and 9.2 kcal/mol for para. From CCSD(T) calculations, we've obtained three TS1 with different order of stability with DFT calculations for ortho and para positions; the meta position remains the most stable of three of them: The relative energy obtained with CCSD(T) method on TS1 gives the values of +14.7 kcal/mol for ortho, +9.9 kcal/mol for meta position and +13.8 kcal/mol for para position.

Intermediates complexes σ (sigma)
The system which we call σ complexes consists of two parts: cation being the traditional σ-complex C6H5ClNO2+ and the counter anion AlCl4-part. These two parts are mutually interacting with Coulombic forces. Since this species are purely J u l y 2 9 , 2 0 1 5 ionic, the Coulombic interaction between C6H5ClNO2+ and AlCl4-causes the large stabilization of the σ-complexes intermediate whose energies are much more stable than the energy of the Π1 complex.
But in this study, the intermediates σ1 (sigma) obtained for the chlorination reaction of nitrobenzene are elucidated in Fig.  3 below (Units of bond lengths and bond angles of all parameters are Å and degrees ( °) respectively.): In these structures of σ1, the counter ion AlCl4creates a bond between one Cl atom and one C of the benzenic cycle. We've obtained then two C-Cl bonds in these complexes σ1. These two C-Cl bond are in ortho positions each others for meta and para complexes and in para positions for ortho complex. In other hand, this second C-Cl bond formed with the C of the benzenic cycle is longer than the first one (see Fig. 2). The formation of this second C-Cl bond must be caused by the electrostatic attraction between the charged entities. The differences of relative energy between the charged entities in its cationic form and the AlCl4 anion for the different positions and correspondent σ1 complexes are in table 2 below:  Table 2, we realize that in the case of non substituted benzene [12], the electrostatic attraction is -78.9 kcal/mol. In the case of aniline, we have the activation of benzenic cycle with group π donor which decreases the electrostatic attraction and increases the reactivity of benzenic cycle. The electrostatic attraction is important in the case of nitrobenzene with the group π acceptor which decreases the reactivity of benzenic cycle. In the case of chlorobenzene, we observe the values of electrostatic attraction (between -72.0 and -76.7 kcal/mol) very closed with the case of aniline. In the geometric consideration, the distance C-N is smaller in the σ1 complex meta (1.464 Å) than in the σ1 complex ortho (1.482 Å) and para (1.484 Å). It is the same situation for the distance Al-Cl which is smaller in meta complex (2.468 Å) than ortho complex (2.504 Å) and para (2.499 Å).
The formation of σ1 complex (between -9.4 kcal/mol and -11.7 kcal/mol) is less exothermic than the case of aniline; these values are smaller than the case of non substituted benzene (-20 kcal/mol). This stability is explained by the group π acceptor which is desactivant for the substitution. The relative energy of σ1 meta (-9.7 kcal/mol) and the σ1 para (-9.4 kcal/mol). With CCSD(T) method the para complex is a little more stable (-22.9 kcal/mol) than the ortho complex (-22.5 kcal/mol); meta complex remains the most stable (-24.7 kcal/mol) of three of them.
With the aim to proceed at the second step of the chlorination reaction of nitrobenzene, three σ2 complexes are been localized, see Fig. 4 below: (Units of bond lengths and bond angles of all parameters are Å and degrees (° ) , respectively.): J u l y 2 9 , 2 0 1 5 The structures of σ2 complexes are different of the ones of σ1: σ2 complex presents one C-Cl bond as in the case of non substituted benzene [12]. These σ2 complexes are more stable than σ1. The relative energies are -11.0 (ortho), -14.1 (meta) and -10.2 kcal/mol (para). TheCCSD(T) calculations on the geometry of σ2 confirm this order of stability: the relative energies obtained with this method are -15.4 (ortho ), of -17.8 (meta) and -13.7 (para) kcal/mol.

Transition States TS2
Three structures correspondent to the second transition states have been localized, see The elimination process of hydrogen atom is done in concerted manner as in the case of non substituted benzene. The relative energies correspondent to transition states are equal to -9.0 kcal/mol for ortho approach, -9.1 kcal/mol for meta and -7.0 kcal/mol for para. The energy barrier obtained from σ2 for ortho position is -2 kcal/mol, 5 kcal/mol for meta and 3.2 kcal/mol for para.

Π2 (pi2) complexes
After passing through the second transition state TS2, the system would lead to the products. The optimized structures starting from TS2 are shown in Fig. 6. These products complexes consists the donor-acceptor interaction between HCl and AlCl3 and the Π-type hydrogen bonding between HCl and chloro nitro benzene. These structures can be called as the second Π-complex. Since one of the C-C Π-bonds of the nitrobenzene ring interacts with the hydrogen atom H of HCl which was originally a hydrogen atom of reactant nitrobenzene. Π2 complexes obtained for the chlorination reaction of nitrobenzene are illustrated in Fig. 6 below (Units of bond lengths and bond angles of all parameters are Å and degrees (° ) ,respectively.): In the three cases, the limitant reaction is the transformation of π1 to σ1 via transition state TS1. In this reaction, the final product formed easily is the one that Whel and intermediate is formed rapidly (reaction with kinetic control). The meta way is favoured compared to ortho and para ways. This permit us to validate the Whel and intermediate which sai d that the orientation of chlorination reaction of nitrobenzene is governed by the stability of Whel and complex (σ1 or σ2). J u l y 2 9 , 2 0 1 5

Solvent effect on the potential surface energy
The presence of acetonitrile as solvent in the chlorination reaction of nitrobenzene with the continuum model (PCM) [41][42][43][44] don't have an important difference on the reactants: the distance C-N in the nitrobenzene decreases from 1.473 Å in gas phase to 1.468 Å in solution, N-O bond from 1.231 Å in gas phase to 1.233 Å in solution. It is the same situation on products where we did not noted a significant difference and the energies of products follows the same order of stability in gas phase and in solution for the three reactions.
In the other hands the presence of acetonitonitrile as solvent bring an important modification on the structures of wheland complexes (σ1) where the second C-Cl bond that has been formed with the benzenic cycle has disappeared in solution (see Fig. 8) but the reaction mechanism remains unchanged in solution: we've obtained the same profiles as in gas phase for the three reactions. With the influence of solvent, the differences of relative energy between the charged entities for the different positions and correspondant σ1 complexes are in Table 3 below:  Table 3 shows that in the presence of solvent, the electrostatic attraction between the charged entities decreases considerably for the three reactions (see Tables 2 and 3). This diminution of electrostatic attraction explains the disappearance of the second bond C-Cl observed in Wheland complex σ1 in gas phase. This means that this second C-Cl bond was essentially an ionic bond. There is a diminution of activation barrier on the potential surface energy in solution on the second step of the reaction (between σ2 and π2) for the three positions: for ortho attack, this activation barrier decreases from 2.5 kcal/mol in gas phase to 1.7 kcal/mol in solution, for meta approach, it decreases from 3.6 to 2.4 kcal/mol and for para approach it decreases from 2.1 to 1.1 kcal/mol.

Conclusion
At the end of this study, we've obtained the general mechanism of chlorination reaction of nitrobenzene in the presence of aluminium chloride as a model of substituted benzene with Π group acceptor. In this reaction, the limitant step is the transformation of π1 to σ1 via transition state TS1. All stationary points are been localized and the results obtained in gas phase are consistent with the traditional description of these electrophilic substitution reactions: the orientation of these reactions in meta position depends on the stability of a reaction intermediate (Wheland said). The solvent plays a major role on this potential surface energy to decrease the electrostatic attraction on Wheland complex (σ1) and to lower the activation barrier of the second step of the reaction.