The Electronic Structure Studies of Hydroxyl Substituted Benzoic Acids

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1 Journal of Photopolymer Science and Technology Volume 12,Number2(1999) TAPJ i The Electronic Structure Studies of Hydroxyl Substituted Benzoic Acids Wang Liyuan and Yu Shangxian Department of Chemistry Beijing Normal University, Bejing100875, China The electronic structure of various hydroxyl substituted benzoic acids is studied by using ab initio method at HF/6-31 G* level. The importance was put on the charge distribution on benzene ring and the effect of hydroxyl group. The total negative charge on benzene ring is in inverse proportion to the number of hydroxyl substituents. The net charge change on carbon atoms of benzene ring of multi-hydroxyl benzoic acids induced by hydroxyl is very close to the simple addition of that of corresponding mono-hydroxyl benzoic acids. These phenolic acids possess one to three active sites on benzene ring where the negative charges are much more than that of other sites and than that of unsubstituted benzoic acids. The relationship of electronic structure and reactivity of the compounds is discussed. The lower decarboxylation temperature is found to be well correlated with the more negative charge on the carbon atom linked to carboxyl group for most of the measured phenolic acids. Key words: ab initio, hydroxyl substituted benzoic acid, charge density, active site 1. Introduction Hydroxyl substituted benzoic acids are of special interest in molecular design of functional polymers. These compounds have the similar chemical properties as phenols because of the effect of hydroxyl substituent, such as electrophilic addition reaction and reaction with formaldehyde to form carboxyl phenolic resins, etc. It is well known that phenolic resins are fundamental material for photopolymers. Hydroxyl substituted benzoic acids also have the similar chemical properties as benzoic acid, such as decarboxylation reaction when heated. Making use of the solubility change in dilute aqueous base before and after decarboxylation reaction, these compounds have the possibility to be used in imaging materials. For example, 3,4,5-trimethoxybenzoic acid was used as thermal sensitive solving-resistant agent in C-T-P thermal imaging material by Kodak[ 1]. It is the number and relative position of hydroxyl substituents that determines the properties of these compounds. Although it is well known that hydroxyl substituent can activate ortho- and parapositions on the ring because of its conjugative effect, profound knowledge about the effect of the substituent is still lacking. There were some quantum chemical research related to substituted benzoic acids[2-3], but systematical studies on the electronic structure and its relationship with properties of hydroxyl substituted benzoic acids were not reported before. The electronic structures of various phenols have been studied by MNDO semiempirical molecular orbital method[4]. The results showed that the values of negative charge on some carbon atoms of benzene ring are much larger than that on others. These atoms with more negative charge were called active sites. Chemical reactions, e.g. electrophilic addition, will take place at these sites firstly. Among the phenols, the values of negative charge at the active sites of resorcin and 1,3,5-trihydroxy benzene are the largest. Received Accepted March 26, 1999 May 25,

2 I. Photopolym. Sci. Technol., Vol.12, No.2, 1999 same characteristics of the effect of hydroxyl substituents on the charge density of benzene ring. 1. benzoic acid 2.2- hydroxybenzoic acid 3.3-hydroxybenzoic acid 4.4-hydroxybenzoic acid 5.5- hydroxybenzoic acid 6.6- hydroxybenzoic acid 7.2,6-dihydroxybenzoic acid 8.2,4-dihydroxybenzoic acid 9.2,3- dihydroxybenzoic acid 10.2,5-dihydroxybenzoic acid 11.3,4-dihydroxybenzoic acid 12.3,5-dihydroxybenzoic acid 332

3 J. Photopolym. Sci. Technol., Vol.12, No.2, 1999 Figure 1. Geometrical structures and the charge densities on carbon atoms of hydroxyl substituted benzoic acids. la: Mulliken charge on carbon atom; b: Lowdin charge on carbon atom; C: total Mulliken charge on benzene ring; d: total Lowdin charge on benzene ring. Although the total negative charge on benzene ring is reduced by hydroxyl, the charge density at different sites of benzene ring change greatly because of the conjugative effects of hydroxyl. The charge densities at the ortho- and para-position of hydroxyl group are apparently more negative than that at other positions of benzene ring for the mono-hydroxyl benzoic acids. The charge density distribution seems more complicated for the multihydroxyl benzoic acids. However, it can be found that the effects of hydroxyl on the charge densities at different sites of benzene ring possess additivity after having investigated the calculation results carefully. For example, the charge densities on 1-carbon of benzoic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid and 5-hydroxybenzoic acid are respectively , , and for Mulliken population, , , 333

4 J. Photopolym. Sci. Technol., Vol.12, No.2, 1999 Figure 2. Conformation and Molecular Energy of 2-hydroxybenzoic acid Experimental results showed that resorcin and 1,3,- 5-trihydroxy benzene have highest reactivity[5-7]. It is by the help of the combined studies of experimental and theoretical aspects of phenols that we acquired further understanding for the relationship between electronic structure and properties. In this work, we try to investigate systematically the electronic structure of hydroxyl substituted benzoic acids by using ab initio molecular orbital method and to correlate the calculation results with their reactivities. We believe that the further understanding for the relationship between electronic structure and properties of hydroxyl benzoic acids will be helpful to our work in molecular design of functional polymer and other new materials related to these compounds. 2. The methods of calculations 2.1. Calculation program ab initio molecular orbital SCF calculations were carried out in the restricted Hartree-Fork formalism(rhf) for all of stationary points. All the calculations were based on HF/6-31 G* level with the PC GAMESS version 5.0 program[8], which was run on our Pentium PIT 400. The geometries of the compounds were optimized using energy gradient technique at 6-31 G* level Geometry Previous work showed that planar structure is of lower energy for substituted benzoic acids[9]. In this work we chose planar structure as stable and basic geometry for the calculation of these compounds. Several kinds of planar conformers of 2- hydroxybenzoic acid and the calculated molecular energy are presented in Figure 1. It can be seen that the molecular energy is comparatively lower for the conformers with intramolecular hydrogen bonding. Because the calculated atomic charge of benzene ring is related with the choice of conformation, in order to compare the calculation results more directly we chose I as basic geometrical structure. All the geometrical structures of the hydroxyl substituted benzoic acids used for ab initio calculation are shown in Figure 1. 5-hydroxybenzoic acid and 6- hydroxybenzoic acid are the same compounds as 2-hydroxybenzoic acid and 3-hydroxybenzoic acid separately but with different conformers. Benzoic acid and phenol are included 3. Results and discussion for comparison Effect of substituents on electronic structure The charge densities on carbon atoms of benzoic acid, hydroxyl substituted benzoic acids and phenol calculated by ab initio method are shown in Figure 2. Both calculation results for Lowdin population and Mulliken population are presented for comparison. It can be seen that the total negative charge on the ring is reduced by hydroxyl group. This is due to the strong electron-withdrawing effect of oxygen atom. This reducing effect is mainly related to the number of hydroxyl group and is only slightly influenced by the positions of the substituents. When the total charge density on benzene ring is plotted against the number of hydroxyl group, linear relationships are obtained both for Mulliken and Lowdin as shown in Figure 3. The two kinds of calculation results give different values of atomic charge but show the 334

5 J. Photopolym. Sci. Technol., Vo1.12, No.2, 1999 Figure 3. Relationship of total charge density of benzene ring and the number of hydroxyl and for Lowdin population(figure 2). Here 3-hydroxybenzoic acid and 5- hydroxybenzoic acid are the same compounds with different conformers chosen for comparison with related multi-hydroxyl benzoic acid. The changes of charge densities at 1-carbon of 3-, 4-, and 5-hydroxybenzoic acid induced by hydroxyl substituent, the value of charge of hydroxyl benzoic acid minus that of unsubstituted benzoic acid, are , and for Mulliken and , and for Lowdin. Adding these three changes and the charge density at 1-carbon of unsubstituted benzoic acid we obtain and respectively. These two values are very approaching to the ab initio calculated values of charge density on 1-carbon of 3,4,5- trihydroxybenzoic acid which are and correspondingly for Mulliken and Lowdin population. All the compounds were treated with the same manner and the results are presented in Table 1. This method can be called indirect calculation. It is shown that the two kinds of values are very approaching. This means that we can accurately estimate the values of charge densities on benzene ring of benzoic acids substituted by more than one hydroxyl group using the corresponding values of benzoic acids substituted by mono-hydroxyl without conducting large quantities of ab initio calculation. More commonly, we can easily locate the active sites(positions on benzene ring with more negative charge than others) and chose the compounds with needed molecular structure according to qualitative estimation. This is convenient and helpful in our work. Table 1. Comparison of calculation and indirect calculation value of charge density on the carbon atom linked to carboxyl group.335

J. Photopolym. Sci. Technol., Vo1.12, No.2, 1999 3.2. Effect of electronic structure on properties 3.2.1. Electrophilic addition Electrophilic addition on benzene ring is very common for aromatic compounds.

6 J. Photopolym. Sci. Technol., Vo1.12, No.2, Effect of electronic structure on properties Electrophilic addition Electrophilic addition on benzene ring is very common for aromatic compounds. The reactivity is closely related to charge density of the ring. The electronic structure and reactivity of phenolic compounds have been reported before[4-7]. Experimental results showed that those phenols, such as resorcin and 1,3,5-trihydroxy benzene, with the most negative charge on some carbon atoms of benzene ring, have the highest reactivity. It was also found that electrophilic addition took place at the active sites that were identified by quantum chemical method. Hydroxyl substituted benzoic acids can take the same reactions as phenols do, such as electrophilic addition and reaction with formaldehyde to form phenolic resins. From Figure 2 it can be found that 2,6-DHBA, 2,4- DHBA, 3,5-THBA, 2,4,5- THBA, 2,3,5- THBA, and 2,4,6-THBA have more negative charge on some carbon atoms of the rings than that of other phenolic acids. It can be predicted that these compounds have higher electrophilic reactivity. Meanwhile, all the phenolic acids have one to three sites on benzene rings, signed with asterisk in Figure 2, at which there is more negative charge than that at other sites or the same sites of unsubstituted benzoic acids. It can be predicted that electrophilic addition will probably take place at these sites. Taking 2,4-dihydroxybenzoic acid as an example, 3- and 5-position are active sites where more negative charge is populated. The electrophilic addition will probably take place at these two sites and the phenolic resin formed by the compound and formaldehyde following structure: Decarboxylation reactivity will have the The decarboxylation of hydroxyl substituted benzoic acids is of special significance to our experiments for thermal imaging polymers. The decarboxylation temperatures of these compounds vary with the number and position of hydroxyl. The charge density on the carbon atom linked to carboxyl group and the decarboxylation temperature[10]of some hydroxyl substituted benzoic acids are listed in Table 2. An apparent trend can be investigated that the higher is the charge density, the lower the decarboxylation temperature will be. The only exception is 2,3-dihydroxybenzoic acid. This means that more negative charge on the carbon atom linked to carboxyl group is beneficial to decarboxylation at lower temperature. It is apparent that the negative charge will be increased by the existence of more hydroxyl groups at orthoand para-position of carboxyl group. Table 2 Charge on carbon atoms linked to carboxyl 4. Conclusion group and decarboxylation 1. The total negative charge on benzene ring of hydroxyl substituted benzoic acid is reduced by hydroxyl and is in inverse proportion to the number of hydroxyl group. Both Mulliken and Lowdin population reflect the same characteristics but give different absolute values of charge densities. 2. The effects of hydroxyl groups on the charge density of benzene ring possess additivity. This means that we can accurately estimate the charge distribution on benzene ring of multi-hydroxyl substituted benzoic acids by that of mono-hydroxyl substituted benzoic acids and this will help us to avoid some complex calculation. temperature 336

7 J. Photopolym. Sci. Technol., Vol.12, No.2, There are one to three active sites on the rings of the phenolic acids where electrophilic additions will probably take place because more negative charge are there than other sites. 4. A lower decarboxylation temperature can be well correlated to a higher charge density on carbon atom linked to carboxyl group for most of the measured compounds. 5. Quantum chemistry calculation can not only give the best explanation to the relationship of structure and properties but also can be used to predict the structure and properties of unknown compounds of the same kinds. This will help us to find the suitable compounds for our special interests in molecular design of functional polymers. Acknowledgment The authors would like to thank Professor Decai FANG for his kind help in ab initio calculation. Reference 1. Kodak Co., USP5,491,046(1996) 2. S. P. Gupta & P. Singh, Indian J. Biochem. Biophys., Vol.14, pp89-92, March(1977) 3. M.H.Palmer, et al. J. Molecular Structure, 52(1979), pp He Shaoren, et al. Journal of Beijing Normal University(Natural Science), No.4, pp67-71(1989) 5. Yu Shangxian, et al. J. Photopolym. Sci. Technol., Vol.l, No.2, pp (1988) 6. Yu Shangxian, et al. J. Photopolym. Sci. Technol., Vol.2, No.l, pp51-56(1989) 7. Gu Jiangnan, et al. Reactivity Determination of Carbon Atoms on Benzene Rings of Phenols, Proceedings of SPIE, Vol.3678(1999) 8. MW. Schmidt, K.K. Baldridge, J.A. Boatz, ST. Elbert, MS. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S.J. Su, L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem., 14, (1993); Alex A. Granovsky, Moscow State University, PC GANESS version 5.0, Kenneth S. Alexander, et al. Journal of Pharmaceutical Science, V65, No.6, Zhou Yingquan, et al. Studies about Thermal Decarboxylation of Hydroxyl Acids Used in Thermal Imaging, J. Photopolym. Sci. Technol., Vol. 12(1999) 337

Chapter 23 Phenols CH. 23. Nomenclature. The OH group takes precedence as the parent phenol.

Chapter 23 Phenols CH. 23. Nomenclature. The OH group takes precedence as the parent phenol. CH. 23 Chapter 23 Phenols Nomenclature The OH group takes precedence as the parent phenol. Carboxyl and acyl groups take precedence over the OH group. The OH group is a strong electron-donating group through