ENTHALPIES OF COMBUSTION AND FORMATION OF ISOMERS OF THE AMINO-BENZOIC ACID

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1 Revue Rouaine de Chiie, 2006, 51(4), 3 7 Dedicated to Professor Dr. ALEXANDRU T. BALABAN, eber of the Rouanian Acadey on the occasion of his 75th anniversary ENTHALPIES OF COMBUSTION AND FORMATION OF ISOMERS OF THE AMINO-BENZOIC ACID Iulia CONTINEANU, a* Ştefan PERIŞANU b and Ana NEACŞU a a Institute of Physical Cheistry "I. Murgulescu", Spl. Independenţei 202, Bucharest, , Rouania b Laboratory of General Cheistry, Politehnic a University of Bucharest, Str. Polizu nr. 2, Bucharest, , Rouania Received January 24, 2006 The energies of cobustion of the o-, - and p- isoers of the aino-benzoic acid in the crystalline state were easured in a static bob adiabatic calorieter, in pure oxygen at 3040 kpa. Corrections were ade for the heats due to the ignition of saple and for the nitric acid foration. The derived enthalpies of foration for the isoers are: f H 0 cr = ±3 kj/ol for o-aino-benzoic acid, f H 0 cr= ±4.1 kj/ol for -aino-benzoic acid and f H 0 cr = 408.7±2.3 kj/ol for p-aino-benzoic acid, respectively. Their resonance energies were calculated and correlated with the corresponding olecular structures. INTRODUCTION The present paper continues our research concerning the therocheical properties of nitrogen containing organic copounds, ainly aino acids. 1-3 Although not directly involved in the protein structure, aino-benzoic acids are copounds of iportant biological activity. They are also involved in the synthesis of soe drugs, dyestuffs, aroatization agents, preservatives. In particular, p-aino-benzoic acid, part of the folic acid, is naed vitain B x and although a non-protein aino acid, it is widespread in nature. Its role in aintaining the healthy state of hair and skin is well known. o-aino-benzoic acid, known also as anthranilic acid or L vitain, is an interediary in the synthesis of tryptophan. It plays an iportant role in lactation. It is used in the production of perfues, dyestuffs, drugs, as absorbent of UV radiations and as corrosion inhibitor. The ethyl-ester of -aino-benzoic acid is used as anesthetic. Knowing the aino-acid properties is iportant for a better understanding of the protein structure, serving as odel copounds in the study of the latter. In this study we follow the influence of the reciprocal position of the two substituents, upon the therocheical properties of the three isoers enthalpies of foration, resonance energies and consequently the variation of their stability according to their structure. EXPERIMENTAL The saples of benzoic acid isoers were Merck reagents of over 98.9 % purity. Apparatus and experiental procedures The adiabatic calorieter with static bob and the basic experiental procedures used in this investigation have been already described. 2 The calorieter is of the Parr Instruents type, locally built. It is provided with an electronic syste, which allows it to * Corresponding author: icontineanu@yahoo.co

2 4 Iulia Contineanu et al. work in adiabatic conditions. The calorietric bob is a cylinder, with a volue of about 300 L, ade of stainless steel, as well as its accessories. The teperature rise was easured by eans of a Beckann theroeter; the teperature rise due to the cobustion reaction is considered as the difference between the final and initial teperature values, when the difference between two consecutive readings is about C. The initial teperature was 25.0 ± C and the initial pressure was 3040 kpa. The saples to be burned were pressed into pellets and weighed with an accuracy of 0.01 g. The pellet was placed in the bob crucible, in the proxiity of the fuse used for ignition. The cobustion was run in the presence of a cotton fuse, as an auxiliary of cobustion, if necessary. The bob was flushed with purified oxygen for three inutes, then sealed and filled with oxygen at the initial pressure entioned above. After cobustion, the gas ixture fro the bob was evacuated. The walls and fittings of the bob were washed with bidistilled water and the resulted solution was saved in a 200 L flask. Calibration The calorieter constant was deterined by eans of benzoic acid cobustion (standard reference aterial 39I fro NIST). The considered certified value of energy of cobustion of benzoic acid is ± 3.7 J/g. The calorieter constant was calculated fro the results of at least ten cobustion runs, resulting in a value of 8901 ± 2.8 J/K. Analysis of the final solution Three saples of solution of 50 L were titrated separately, with 0.1 N NaOH solutions, using ethyl orange as indicator. The ean value of three titrations was used for the calculation of the aount of nitric acid resulting fro cobustion (about 20% fro the total nitrogen). The heat due to nitric acid foration was obtained using the value of the enthalpy of foration of nitric acid - f H HNO3,aq = kj/ol. 8 RESULTS Cobustion energy of the saples The deterination of cobustion energies for the saples was done in the sae way as for the calibration of the calorieter. The results are given in Tables 1-3 for the o-, - and p-isoers. The tables entries include:, ass of the burned substance, θ, teperature rise,, total energy observed,, energy, used to ignite the saple (calculated fro ass of the fire and f H Fe 2O 3 = kj/g), energy of burned cotton (calculated fro the ass of cotton and c H cotton = J/g) 4, q HNO3, energy of nitric acid foration, u exp energy of cobustion of saple, U exp olar energy of cobustion (in J/ol), c U 0 standard olar energy of cobustion (J/ol). Table 1 Cobustion data of 2-ainobenzoic acid c u exp c U exp c U Mean value kj/ol Standard deviation ± 3.0 kj/ol In order to bring the experiental values of energy of cobustion to the standard state (T = K and P = kpa), corrections were ade with the Washburn equation 5 recoended in the case of copounds with carbon, hydrogen and oxygen of C a H b O c general forula : ( ) -0.3aP initial 1.1 b - 2c 2 %= exp U 4a P initial (1)

3 Entalpies of cobustion and isoers 5 where P stands for the initial oxygen pressure and - U exp for the experiental energy of cobustion, a, b, and c being the ato nubers of carbon, hydrogen and oxygen fro the cheical forula of the copound, respectively. Π is calculated in percents fro the experiental value. The above equation applies fairly well in the case of nitrogen copounds as well. 6 The relative error in the deterination of the heats of cobustion was better than 1%o. Table 2 Cobustion data of 3-ainobenzoic acid c u exp c U exp c U Mean value kj/ol Standard deviation ± 4.12 kj/ol Table 3 Cobustion data of 4-ainobenzoic acid , , ,3 48,9 - u exp U exp U Mean value kj/ol Standard deviation ± 2.27 kj/ol Standard enthalpies of cobustion and foration The standard cobustion enthalpies of the saples were calculated using the following equation: 0 0 ch(s) = cu(s) + n RT n = Σ ngas,products Σ ngas,reactants (2) n is the change in the gas ole nuber, in the cobustion reaction. R = J/ol K; T = K. Table 4 Cobustion and foration enthalpies of the solid state isoers of the aino-benzoic acid Isoer - c H 0 - f H 0 Literature 9,10 - c H 0 - f H 0 ortho eta para ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.4

4 6 Iulia Contineanu et al. The cobustion experiental data are given for the following reaction, the sae for all three isoers: C 7 H 7 NO 2 (s) O 2 7 CO H 2 O(l) N 2 n = For calculating the foration enthalpies of the investigated copounds, the following values were considered: 0 f Η = ± 0.13 kj/ol, 0 Η (l) CO f = ± kj/ol. 7 H O 2 2 DISCUSSION The enthalpies of cobustion and foration of the three isoers of aino-benzoic acid were reported long tie ago, by Nabavian and co-workers 9 and by Lebedeva and co-workers 10. Our experiental values for the enthalpies of cobustion are larger in absolute value than those of the French group 9, but the order of the values for the three isoers is the sae (decreasing values of c H 0 in the order o-, p-, -). However the differences between our values and those of reference 9 are lower than the cuulated uncertainties. Obviously, our enthalpies of foration are less negative than those reported by the above entioned authors. The calculation of resonance energies The stabilization of the investigated olecules by resonance was evaluated by eans of their resonance energies. The resonance energy, defined as the energy difference between the systes of delocalized and localized π electrons was calculated referring to contributions of the bonds contained by the olecules. 8 Consider the reaction of dissociation into atos of the copound (atoization): (1) C 7 H 7 O 2 N 7 C + 7 H + 2 O + N ( H at ) The energy of atoization H at is obtained fro the enthalpy of foration reaction and the theral effects of the following reactions: (2) C(graphite) C (715.8) (3) 1/2 H 2 H (217.9) (4) 1/2 O 2 O (249.3) (5) 1/2 N 2 N (472.8) (all values in kj/ol) Cobination of cheical equations 1-5 yields: H at (exp) = f H 0 kj/ol (3) On the other hand H at, the atoization energy, is expressed as the su of the bond energies fro the olecule: H at (calc) = - Σ D i (A B) (4) The bond energies refer to olecules in the gaseous state, so that the energies H at (exp) and H at (calc) ust refer to the sae phase. If the substance is solid or liquid, the phase transition enthalpy (subliation or vaporization) has to be taken into account. f H 0 = f H 0 (s) + H 0 (phase transition) (5) Consequently, the resonance energy will be: E resonance = H at (exp) - H at (calc) (6) All values are referred to the standard teperature ( K). The atoization energy of an aino-benzoic acid calculated fro bond energies 8 (eq. 4, see Table 5, for bond energy values) is kj/ol if the specific value of the benzenoid carbon-carbon bond is taken

5 Entalpies of cobustion and isoers 7 into account and 72.8 kj/ol, with single and double bond energies. The gas phase standard enthalpies of foration of the investigated copounds were calculated (eq. 5), by eans of our experiental values of foration enthalpies in the solid state and of the subliation enthalpies taken fro the literature. 11 Table 5 The bond energies Bond C-H C-C C a -C a C-N N-H C-O C d -O d O-H Energy The resulting resonance energies (eq. 6) are shown in Table 6. In the last colun of this table the overall resonance energies of the isoeric aino-benzoic acids are shown, while the values of the precedent colun exclude the resonance energy of the benzenoid ring. It ay be seen that the lowest value of the atoization energy, and consequently of the resonance energy is et for the eta isoer, followed by para and ortho. The sae order was reported by Nabavian et al. 9, including for resonance energies calculated with the schees of Klages and Franklin. This is the reversed order with respect to that observed in the crystalline state and this is due to very different values of the subliation energies. In the crystalline state the eta isoer is building a zwitterion as was proved by Sasonowicz et al. 12, by eans of quantu cheical coputations and by eans of IR and NMR spectra. In the gaseous state the structure of the ortho isoer is stabilized by the interaction between the neighbor carboxyl and the aino groups. Isoer ainobenzoic acid ortho eta para Table 6 Resonance energies of the isoeric aino-benzoic acids subl H ± ± ± H at (exp) E rez a E rez b a values of the resonance energies without the resonance energy of the benzenoid ring b overall resonance energies of the isoeric aino-benzoic acids REFERENCES 1. I. Contineanu and D.I. Marchidan, Rev. Rou. Chi., 1994, 39, I. Contineanu and D.I. Marchidan, Rev. Rou. Chi., 1997, 42, I. Contineanu and S. Predescu, Rev. Rou. Chi., 2001, 46, W. N. Hubbard, D. W. Scott and G. Wadington, in: F. D. Rossini Ed., Experiental Therocheistry, Interscience Publ. Inc(Ed), New York, 1956, chap. V, pp E.W. Washburn, J. Res. Nat. Bureau Standards, 1933, 10, Cox, J. D., Pilcher G., Therocheistry of Organic and Organoetallic copounds, Acadeic Press, London, 1970, p CODATA Bulletin NR. (April 1978), Recoended Key Values for Therodynaics, First law" 9. P.M. Nabavian, R. Sabbah, R. Chastel, M., Laffitte, J. Chi. Phys., 1977, 74, N.D. Lebedeva, N.M. Gutner, V.L. Ryadnenko, Russ. J. Phys. Che., 1971, 45, R. Sabbah, R. Chastel, M. Laffitte, Can. J. Che., 1974,, M.Sasonowicz, T. Hrynaaszkiewicz, R. Swislocka, E. Regulska, W. Lewandowski, J. Mol. Str., 2005, , 345-3