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Theoretical Study of Synthesis of 1,3-dibromonaphthanlene.

Byline: Faik Gokalp and Ramazan Erenler

Summary: Naphthalene derivatives have been attracted the interest for synthesis of natural products having biological properties. Elimination reaction of tetrabromonaphthalene (1) resulted in the formation of 1,3-dibromonaphthalene (2) rather than 1,4-dibomonaphthalene (3). This phenomenon was explained by theoretical investigation. The physical properties and optimization of tetrabromonaphthalene (1), 1,3-dibromonaphthalene (2) and 1,4-dibromonaphthalene (3) were evaluated by B3lyp/6-31+G(d,p) method. Due to the HOMO-LUMO gap of 1,3dibromonaphthalene (2) was higher than that of the 1,4-dibomonaphthalene (3), the formation of 1,3dibromonaphthalene (2) was favorable. Moreover, Higher dipole moment of 1,3dibromonaphthalene (2) than 1,4-dibomonaphthalene (3) supported the synthesis of 1,3dibromonaphthalene (2) properly.

Keywords: Bromination; Naphthalene; Theoretical calculation.

Introduction

Brominations of arylhydrocarbons play a significant role in synthetic chemistry [1-3]. Brominations of naphthalene and substituted functional groups for bromine were extensively investigated [4, 5]. Many natural products consisted of naphthalene core. Hence, naphthalene gained the great importance for synthesis of natural compounds as well as medicinally and pharmaceutically valuable products [6-8]. Due to the weakness of carbonbromine bond, bromine atoms could be easily substituted by desired functional groups via metal mediated coupling reactions [9-11] as well as substitution reactions[12]. As known, thermodynamics looks the formation of any kind of product in a chemical reaction in terms of own perspective when it evaluates whether it is formed in a reaction or not [13]. The negative Gibbs free energy (IGAdeg) calculated for any product at the studied means that it can be formed spontaneously if the kinetic conditions of that product are met.

In this regard, IGAdeg value the high negative the high spontaneity for proceeding of chemical reaction to the expected side [13]. In contrast to experimental expectation, the absence of any one of the products (e.g.: 1,4-dibromonaphthalene (3)) during the reaction may be due to the fact that the required kinetic conditions are not fully attained during the reaction. These conditions can be listed as follows: (i) The number of molecule collisions during the reaction is being of enough, (ii) The collisions of molecules is not occurred at appropriate geometries, (iii) The low or high temperature of the studied experiment, (iv) The low potential energy of the activated complex containing the regarded product, (v) The unsuitable catalyst selection. Previous work, we aimed to synthesize 1,4dibromonaphthalene (3) from tetrabromonaphthalene (1) via elimination reaction. However, 1,3dibromonaphthalene (2) was formed instead of 1,4dibromonaphthalene (3) [14].

In this work, we presented the favorable formation of 1,3dibromonaphthalene (3) by way of theoretical calculation. It was presented theoretically that 1,3dibromonaphthalene (2) was formed from the elimination reaction of tetrabromonaphthalene (1).

Experimental

DFT method with Gaussian 09 program with the B3LYP functional and 6-31G(d,p) basis set was used for calculation of physical properties of compounds including stability, Gibbs free energy, dipole moment in different solvents such as EtOH, MeOH and H2O [15]. Dielectric constants were calculated for EtOH, MeOH and H2O as 24.55, 32.63 and 78.39 respectively. Dielectric constants for Tetrahydrofuran (THF) was calculated as Iu'=7.58 [16]. Dipole moments of the compounds are changed according to the phase of the medium. Moreover, the dipole moments are related to the polarity of the medium. If the dipole moments of the compounds are higher in a medium so they have high relative permittivity due to the major charge redistribution in the molecule and they will be effected by changes in the distances between the charge separations [17]. The solubility of the given compound is associated with negative values of the Gibbs free energy of solvation.

However, according to the polarized continuum (IEFPCM) model results, all conformers of compounds are found to have negative values of Gsolv in water and ethanol. A small positive value of Gsolv is obtained for toluene Thus; from the IEFPCM data calculated within this study one would conclude that all conformers of compounds should be soluble in water and ethanol and perhaps miscible with toluene. The IEFPCM data would suggest that all conformers of sarin should be better soluble in ethanol than in water. For soman the IEFPCM method leads to positive values of Gsolv for all its conformers and thus predicts its insolubility in all investigated solvents [18]. If the solvent dielectric constant of a compound increases, the Gibbs free energy value of the interaction between solvent and molecules will decrease. [19].

Results and discussion

The frontier molecular orbital energy called as HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), Gibbs free energy, dipole moment as known the molecular structural parameters were calculated and discussed for the compounds (Fig. 1). The large HOMO- LUMO gap shows the high excitation energies that mean a good stability and a high chemical hardness [20]. If a molecule has small frontier orbital gap it will be more polarizable and generally associated with a high chemical reactivity [21], Global hardness shown as (n) can be written using Koopman's theorem indicated as n = IuLUMO - IuHOMO, Gibbs free energy of tetrabromonaphthalene (1) had a high negative value than that of the 1,3dibromonaphthalene (2) and 1,4-dibromonaphthalene (3). Hence, the process was exergonic and would have proceeded spontaneously in the forward direction to form the product. The chemical bonds formed from the reaction were stronger than those that were broken in the reactants.

In comparison of 1,3-dibromide 2 and 1,4-dibromide 3, it was observed that Gibbs free energy of 1,3-dibromide 2 was low, so reaction was favorable to form 1,3-dibromide 2 (Fig. 2).

HOMO-LUMO gap of 1,3-dibromide 2 was higher than that of the 1,4-dibromide 3 so that formation of 1,3-dibromide was favorable regioselectively. Reaction conditions for appropriate solvents were also calculated. Dipole moments of the starting compound, tetrabromonaphthalene (1) in THF, EtOH, MeOH, and water were calculated as 4.0404, 4.1954, 4.2141 and 4.2480 respectively. High dipole moment means high polarizable of the molecule to react with the substrate for formation of the product with high yield [22]. There were not considerable dipole moment differences among the solvents. Tetrabromonaphthalene (1) could be polarized the most in water to yield the highest product. However, Tetrabromonaphthalene (1) and bromine were not soluble in water properly. The HOMO-LUMO gap values for THF, EtOH, MeOH, and H2O were calculated as 0.19165, 0.19101, 0.19094, and 0.19080 respectively for Tetrabromonaphthalene (1). As a consequence, THF was a good solvent selection (Table).

Table-1: The molecule parameters of compounds in various solvents.

DFT(THF) Compounds###G###HOMO###LUMO###GAP###Dipol moment

###1###-10672.730461###-0.26586###-0.07421###0.19165###4.0404

###2###-5528.084734###-0.23442###-0.06857###0.16585###2.7665

###3###-5528.082628###-0.23264###-0.07032###0.16232###0.8083

DFT Ethanol Compounds###G###HOMO###LUMO###GAP###Dipol moment

###1###-10672.731847###-0.26523###-0.07422###0.19101###4.1954

###2###-5528.085326###-0.23457###-0.06862###0.16595###2.8614

###3###-5528.083194###-0.23295###-0.07050###0.16245###0.8222

DFT Methanol Compounds###G###HOMO###LUMO###GAP###Dipol moment

###1###-10672.732013###-0.26516###-0.07422###0.19094###4.2141

###2###-5528.085396###-0.23459###-0.06863###0.16596###2.8725

###3###-5528.083266###-0.23299###-0.07053###0.16246###0.8237

DFT Water Compounds###G###HOMO###LUMO###GAP###Dipol moment

###1###-10672.732318###-0.26503###-0.07423###0.19080###4.2480

###2###-5528.085525###-0.23463###-0.06866###0.16597###2.8926

###3###-5528.083389###-0.23307###-0.07058###0.16249###0.8263

Conclusion

The stability of 1,3-dibromonaphthalene was higher than that of the 1,4-dibomonaphthalene, the formation of 1,3-dibromonaphthalene was favorable and the dipole moment of 1,3-dibromonaphthalene,1,4-dibomonaphthalene as a result of the synthesis of 1,3-dibromonaphthalene properly.

Acknowledgements

The authors report no conflict of interests in this work. Thanks for the contribution of Kirikkale University to be used Gaussian program (BAP2016/016).

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Author:Gokalp, Faik; Erenler, Ramazan
Publication:Journal of the Chemical Society of Pakistan
Article Type:Technical report
Date:Dec 31, 2018
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