Article pubs.acs.org/JPCA
Energetic and Structural Study of Bisphenols Juan Z. Dávalos,*,† Rebeca Herrero,† José C. S. Costa,‡ Luís M. N. B. F. Santos,*,‡ and Joel F. Liebman§ †
Instituto de Quı ́mica-Fı ́sica “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain Centro de Investigaçaõ em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, P-4169-007 Porto, Portugal § Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, Baltimore, Maryland 21250, United States ‡
S Supporting Information *
ABSTRACT: We have studied thermochemical, thermophysical and structural properties of bisphenols A, E, F, and AP. In particular, the standard enthalpies of sublimation and the standard enthalpies of formation in the gas phase at 298.15 K for all these species were experimentally determined. A computational study, through M05-2X density functional theory, of the various species shed light on structural effects and further confirmed, by means of the isodesmic reaction scheme, the excellent consistency of the experimental results. Our results reflect also the fact that energetic substituent effects are transferable from diphenylalkanes to bisphenols.
I. INTRODUCTION
Scheme 1. Bisphenols Studied in This Work
Bisphenols are chemical compounds with two phenol functional groups in current use as the primary raw material in the production of plastics (polycarbonate and epoxy resins), food cans (i.e., lacquer coatings), and dental composites and sealants. These compounds have received a tremendous amount of attention from research scientists, government panels, and the popular press due the potential health risks. Dodds and Lawson1 stated that diphenylmethane derivatives containing two hydroxyl groups in the para positions, among them bisphenols A, F, and S, showed estrogenic activity. Abundant data on estrogenic activity, androgenic activity, carcinogenicity and toxicity has been published since then, mostly on bisphenol A (BP-A).2−4 There is growing evidence that BP-A may adversely affect humans. It has been shown to be harmful in laboratory animal studies. These potential health implications have led to a gradually replacing of BP-A, in industrial applications, with alternative bisphenols such as bisphenol F (BP-F) and bisphenol S.5 From the ecotoxicology, human health and regulatory points of view, it is important to restrict the emissions and releases of estrogenic chemicals from the industrial processes and commercial products.6 Despite its fundamental and applied relevance, the knowledge of the thermodynamic properties of the bisphenols is still incomplete. In this work, an energetic study including thermochemical and thermophysical properties of several bisphenol derivatives (Scheme 1: bisphenol A, bisphenol E, bisphenol F, bisphenol AP) is presented. In this context, we have used a number of experimental techniques such as differential scanning calorimetry (DSC), static bomb combustion calorimetry, and Knudsen/quartz effusion technique, as well as quantum chemical calculations with the M05-2X density functional theory. © 2014 American Chemical Society
The present work is part of a systematic investigation of the energetics and structural properties of large conjugated organic molecules7 with biological, medicinal, and industrial interest.
II. EXPERIMENTAL SECTION A. Materials and Purity Control. DSC Measurements. 2,2-Bis(4-hydroxyphenyl)propane (Bisphenol A, CAS 80-057); bis(4-hydroxyphenyl)ethane (Bisphenol E, CAS 2081-085); bis(4-hydroxyphenyl)methane (Bisphenol F, CAS 620-928), and 1,1-bis(4-hydroxyphenyl)-1-phenylethane (Bisphenol AP, CAS 1571-75-1) were purchased from Sigma-Aldrich Co. (CAS Registry numbers were supplied by the author.) All the samples were carefully dried under vacuum at 353 K and used without further purification. Their purities were assessed by DSC. The mass fraction of impurities in the compounds was less than 0.005 in all cases. All samples were investigated by DSC, over the temperature range from T = 263 K to several degrees below of their melting points, with no phase transition observed in the solid state. Molar heat capacities at constant Received: April 11, 2014 Revised: April 30, 2014 Published: May 1, 2014 3705
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Table 1. Experimental Results of Standard Molar Enthalpies of Combustion, Formation (in the Solid and Gaseous States) and Sublimation at T = 298.15 K and p0 = 105 Pa (All Values in kJ·mol−1) bisphenol BP-A BP-E BP-F BP-AP a
ΔcH0m(s) −7820.0 −7163.2 −6502.4 −10194.1
± ± ± ±
ΔfH0m(s) 2.0 4.0 4.4 4.3
a
−368.6 −346.7 −328.2 −248.6
± ± ± ±
1.7 4.4 4.7 5.1
ΔgcdH0m b
± ± ± ±
137.9 137.4 140.8 162.8
ΔfH0m(g) −230.7 −209.3 −187.4 −85.8
0.7 0.7 0.6 1.2
± ± ± ±
1.8 4.5 4.7 5.2
Taken from ref 18. bTaken from ref 15.
overall standard deviation of the mean.13 The values for the standard molar enthalpies of formation at T = 298.15 K of H2O(l), −285.830 ± 0.042 kJ·mol−1 and CO2(g), −393.51 ± 0.13 kJ·mol−1, taken from CODATA,14 were used to derive the standard molar enthalpies of formation of the bisphenols considered. The enthalpies of sublimation were derived from the temperature dependence of the vapor pressure obtained from the Knudsen effusion technique (Clausius−Clapeyron eq 2).
pressure Cop,m were also experimentally determined by DSC. Full details are given in the Supporting Information (S1). B. Static Bomb Combustion Calorimetry. The combustion experiments for BP-E, BP-F, and BP-AP were carried out in isoperibol static micro-bomb calorimeter. Detailed description of these methods is found elsewhere.8,9 The energy equivalent of the calorimeter, ε(calor) = 2105.1 ± 0.3 J·K−1, was determined from ten combustion calibration experiments using benzoic acid (NIST standard reference sample 39j) with a massic energy of combustion of −2643 4 ± 3 J·g−1, under certificated conditions. The uncertainty quoted is the standard deviation of the mean value. Complementary details are given in S2 of Supporting Information. C. Knudsen/Quartz Effusion. The enthalpies of sublimation of BP-A, BP-E, BP-F, and BP-AP were derived from the vapor pressure measurements as a function of temperature by the mass loss combined Knudsen/quartz crystal effusion apparatus described in detail by Santos et al.10 This technique is based on the simultaneous gravimetric and quartz crystal microbalance mass loss detection. It has the advantage of requiring smaller sample sizes and results in shorter effusion times and the possibility of achieving temperatures up to 650 K.
ln(p /Pa) = −B ·(T /K)−1 + A
Equation 2 was fitted using the least-squares method to calculate the standard molar enthalpies of sublimation at the mean temperature, Tm, from B = ΔgcdH0m(Tm)/R. The molar enthalpies ΔgcdH0m at T = 298.15 K were computed using eq 3, g Δcd Hm0(T =298.15K) g = Δcd Hm0(Tm) +
∫T
298.15
m
[C po,m(g) − C po,m(s)] dT
(3)
Cop,m(g)
where = f(T) was derived from computational data at the M05/6311++G(d,p) level and Cop,m(s) = g(T) was taken from the experimental DSC results. Cop,m(g) and Cop,m(s) data are presented in the Supporting Information. The standard molar enthalpies of sublimation and formation of the bisphenols studied, in solid and gaseous states, at T = 298.15 K are listed in Table 1. We derived the enthalpy of formation in the gas phase of BP-A as ΔfH0m(g) = −230.7 ± 1.8 kJ·mol−1 (Table 1), using the experimental value of enthalpy of formation in solid state, ΔfH0m(s), reported by Pedley in his now classic archive.15 B. Structures and Thermochemistry Properties. The optimized geometries computed at level of theory used show various stable conformers, particularly BP-A and BP-F exhibit structures with C2 symmetry. Rotation of the planar phenolic moiety rings [I and II, Scheme 1] of the bisphenols considered gives rise to several rotamers, whose arrangements are described basically by α and β (also ϕ for BP-AP) dihedral angles related with rotation around the C1−CPh bonds. Rotation of the hydroxyl groups (around the CPh−OH bonds) is described by ω and θ dihedral angles. The geometry of the most stable rotamer for each bisphenol is depicted in Figure 1. The rest of the rotamers considered, including their corresponding enantiomers, display a quite low enthalpy difference relative to the corresponding most stable one (less than 1 kJ·mol−1). From energy profiles (Figure 2) we can estimate that the activation barriers of BProtamers due to the rotation of their phenolic moiety rings, are between 2 and 11 kJ·mol−1. The lowest values correspond to BP-F rotamers. In the same context, the activation barrier estimated for rotation of OH groups is almost 15 kJ·mol−1 for all BP-rotamers considered (Supporting Information).
III. COMPUTATIONAL METHODS Quantum chemical calculations were carried out using the Gaussian 09 package.11 The geometries of the compounds under investigation, and their most significant conformers, were optimized using the density functional M05-2X12 method with 6-311++G(d,p) and 6-311++G(3df,2p) basis sets without symmetry restrictions. Harmonic vibrational frequencies without scaling were calculated at the same theory level in all cases, to verify that all the stationary points are minima. The levels of theory employed in the present work provided consistent results for the considered reactions. Full computational details are given in the Supporting Information. IV. RESULTS AND DISCUSSION A. Combustion Calorimetry and Knudsen/Quartz Effusion Techniques: Standard Enthalpies of Formation in the Gas Phase, ΔfH0m(g). The results from the combustion experiments are represented by reaction 1 and shown in Table S2 of the Supporting Information. CnHmO2 (s) + (n − 1 + m /4)·O2 (g) → n·CO2 (g) + 1 2 m ·H 2O(1)
(2)
(1)
where (n = m = 14), (n = 13, m = 12), and (n = 20, m = 18) correspond to bisphenols BP-E, BP-F, and BP-AP, respectively. The standard (p0 = 0.1 MPa) molar enthalpies of combustion, ΔcH0m(s), and formation, ΔfH0m(s), in the solid state (crystalline phase) at temperature T = 298.15 K are listed in Table 1. The presented uncertainties are twice the final 3706
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(BP‐E) + 18·methane → 9· ethane + 6· ethene + 2·methanol
(5)
(BP‐A) + 19·methane → 10·ethane + 6 ·ethene + 2 ·methanol
(6)
(BP‐AP) + 26·methane → 13·ethane + 9 ·ethene + 2 ·methanol
(7)
(BP‐F) + 2·benzene → diphenylmethane + 2 ·phenol Δr Hm0(8) = −5.6 ± 5.3
(8)
(BP‐E) + methane → (BP‐F) + ethane Δr Hm0(9) = 12.7 ± 6.5
Figure 1. Molecular geometry for the most stable rotamers of bisphenols BP-A, BP-E, BP-F, and BP-AP optimized at the M052X/6311++G(3df,2p) level of theory.
(9)
(BP‐A) + methane → (BP‐E) + ethane Δr Hm0(10) = 12.3 ± 4.9
(10)
(BP‐AP) + methane → (BP‐E) + toluene Δr Hm0(11) = 1.5 ± 7.0
(11)
The enthalpy of reactions 9 and 10 indicate that the effect of the introduction of CH3 groups in BP-F (to yield BF-E) and BP-E (to yield BP-A) are practically the same. The increment in the thermodynamic stability due to the successive introduction of CH3 groups in BP-F is almost −22 kJ·mol−1 (Scheme 2). Scheme 2. Enthalpic Increments for Introduction of Methyl and Phenyl Groups in BP-F and BP-Ea
Figure 2. M05-2X/6-31G(d) calculated relative energies, ΔE, of BProtamers as a function of relative Δα dihedral angles. ΔE and Δα are evaluated with respect to the corresponding most stable BP-rotamer.
There are significant deviations between theoretical and experimental (from XRD available data) α, β, ω, and θ dihedral angles of BP-A and BP-F. The observed deviations can be attributed to the fact that the theoretical calculations refer to the gas phase, whereas the experimental structures are determined in the solid state, where packing and intermolecular interactions can override the small, intramolecular, rotameric effects. The computed energies and enthalpies for the stable conformations of the molecules studied are described in detail in the Supporting Information. The consistency of the enthalpies of formation in the gas phase, ΔfHm0(g), of bisphenols investigated were checked using “bond separation isodesmic reaction (BSI)”16 (eqs 4−7) and other isodesmic reactions (eqs 8−11). The ΔfH0m(g) values deduced from these reactions are rather close to experimental values (deviations less than 2.7 kJ·mol−1). In the case of the BSI method, it has been necessary to use a high basis set such as 6-311++G(3df,2p). For reference compounds, we have taken the following experimental ΔfH0m(g) values, in kJ·mol−1: methane (−74.87 ± 0.34), ethane (−84.0 ± 0.4), and ethene (52.47 ± 0.30) from Chase;17 methanol (−201.5 ± 0.2), benzene (82.6 ± 0.7), phenol (−96.4 ± 0.9), and toluene (50.5 ± 0.5) from Pedley;15 diphenylmethane (164.9 ± 2.2) from NIST.18
a
This trend is comparable to the increments in the stability of the corresponding alkyl substitutions in aliphatic hydrocarbons. However, the destabilizing effect by enthalpic increment for introduction of phenyl group in BP-E molecule to yield BP-AP is between 9 and 13 kJ·mol−1 higher than the corresponding enthalpic increments for the successive introduction of phenyl groups in hydrocarbons such as toluene to yield diphenylmethane and triphenylmethane. On the other hand, Figure 3 shows a good linear correlation between the standard molar enthalpies of formation, expressed in kJ·mol−1, of bisphenols BP studied and the corresponding diphenylalkanes (diPhalkane), where the OH groups are replaced by hydrogens). The linear correlation obeys eq 12 with R2 = 0.996: Δf H 0 m(diPh‐alkane,g) = 1.019 ·Δf Hm0(BP,g) + 353.2 (12)
(Reciprocally: ΔfH0m(BP,g) = 0.978·ΔfH0m(diPh-alkane,g) − 346.1, with the same R2) ΔfH0m(g), in kJ·mol−1, of 1,1-diphenylethane (134.5 ± 1.1) and 1,1,1-triphenylethane (265.8 ± 2.3) were taken from refs 19 and 20, respectively, whereas for 2,2-diphenylpropane 121.0 ± 2.3 was estimated in this work by means isodesmic reactions method (Supporting Information). The slope of relationship 12, close to 1, indicates that a pure thermodynamic-additivity
(BP‐F) + 17·methane → 8· ethane + 6· ethene + 2·methanol
All values consigned are in kJ·mol−1.
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European Social Fund for financial support to Centro de ́ Investigaçaõ em Quimica, University of Porto (strategic project PEst C/QUI/UI0081/2013). José C. S. Costa acknowledges FCT and the European Socia Fund (ESF) under the third Community Support Framework (CSF) for the award of a Ph.D. Research Grant SFRH/BD/74367/2010.
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(1) Dodds, E. C.; Lawson, W. Synthetic Oestrogenic Agents without the Phenanthrene Nucleus. Nature 1936, 137, 996. (2) Vandenberg, L. N.; Maffini, M. V.; Sonnenschein, C.; Rubin, B. S.; Soto, A. M. Bisphenol-A and the Great Divide: A Review of Controversies in the Field of Endocrine Disruption. Endocrine Rev. 2009, 30, 75−95. (3) Rochester, J. R. Bisphenol A and Human Health: A Review of the Literature. Reprod. Toxicol. 2013, 42, 132−155. (4) Tsai, W.-T. Human Health Risk on Environmental Exposure to Bisphenol-A: A Review. J. Environm. Sci. Health Part C 2006, 24, 225− 255. (5) Molina, J. M.; Amaya, E.; Grimaldi, M.; Sáenz, J. M.; Real, M.; Fernández, M. F.; Balaguer, P.; Olea, N. In Vitro Study on the Agonistic and Antagonistic Activities of Bisphenol-S and other Bisphenol-A Congeners and Derivatives via Nuclear Receptors. Toxicol. Appl. Pharmacol. 2013, 272, 127−136. (6) Vandenberg, L. N.; Hauser, R.; Marcus, M.; Olea, N.; Welshons, W. V. Human Exposure to Bisphenol A (BPA). Reprod. Toxicol. 2007, 24, 139−177. (7) Dávalos, J. Z.; Guerrero, A.; Herrero, R.; Jiménez, P.; Chana, A.; Abboud, J. L. M.; Lima, C. F. R. A.; Santos, L. M. N. B. F.; Lago, A. F. Neutral, Ion Gas-Phase Energetics and Structural Properties of Hydroxybenzophenones. J. Org. Chem. 2010, 75, 2564−2571. (8) Dávalos, J. Z.; Roux, M. V. The Design, Construction and Testing of a Microcombustion Calorimeter Suitable for Organic Compounds Containing C, H and O. Meas. Sci. Technol. 2000, 11, 1421−1425. (9) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry; Rossini, F. D., Ed.; Interscience: New York, 1956; Chapter 5. (10) Santos, L. M. N. B. F.; Lima, L. M. S. S.; Lima, C. F. A. C.; Magalhães, F. D.; Torres, M. C.; Schröder, B.; Ribeiro da Silva, M. A. V. New Knudsen Effusion Apparatus with Simultaneous Gravimetric and Quartz Crystal Microbalance Mass Loss Detection. J. Chem. Thermodyn. 2011, 43, 834−843. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (12) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2006, 2, 364−382. (13) Olofsson, G. Assignment of Uncertainties. In Combustion Calorimetry; Sunner, S., Mänson, M., Eds.; Pergamon Press: Oxford, U.K., 1979; Chapter 6. (14) CODATA. Recommended key values for thermodynamics. 1975. J. Chem. Thermodyn. 1976, 8, 603−605. (15) Pedley, J. B. Thermochemical Data and Structures of Organic Compounds; Thermodynamics Research Center: College Station, TX, 1994. (16) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (17) Chase, M. W., Jr. NIST-JANAF Thermochemical Tables, 4th ed. J. Phys. Chem. Ref. Data, Monograph 9, 1998, 1−1951. (18) NIST Chemistry Webbook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, 20899 (retrieved May 7, 2014). (19) Verevkin, S. P. Thermochemical Properties of Diphenylalkanes. J. Chem. Eng. Data 1999, 44, 175−179.
Figure 3. Linear correlation of the enthalpies of formation for diphenylalkanes and bisphenols.
model works, accordingly the substituent effects in phenylalkanes and bisphenols are the same. So, this result reflects also the fact that substituent effects are transferable from diphenylalkanes to bisphenols.
V. SUMMARY AND CONCLUSIONS Consistent experimental and theoretical investigation of structures and thermodynamic stability of bisphenols A, E, F, and AP are reported and discussed in this work. Combustion calorimetry, Knudsen effusion technique, and DSC experimental techniques as well as quantum chemical calculations, with the M05-2X (density functional theory) method, were employed. The theoretical calculations, by means of isodesmic reactions, provide interesting results that serve (i) to confirm the excellent consistency of the experimental ΔfH0m(g) results and (ii) to determine that the stabilizing effects by successive introduction of CH3 groups in bisphenols and aliphatic hydrocarbons are similar. In addition, we have determined a linear correlation between ΔfH0m(g) for diphenylalkanes and the corresponding bisphenols. It shows that the variation of the thermodynamic stability for the introduction of alkyl or phenyl substituent is the same in both series.
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ASSOCIATED CONTENT
S Supporting Information *
S1. DSC measurements: Molar heat capacities at constant pressure. S2. Combustion calorimetry. S3. Sublimation method: Knudsen effusion/quartz crystal microbalance. S4. Computational results (including geometrical data, energy profiles, coordinates, and enthalpies). S5. Standard enthalpy of formation, ΔfH0m(g), of 2,2-diphenylpropane. This material is available free of charge via the Internet at http://pubs.acs.org
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*J. Z. Dávalos: phone, 34-915619400; e-mail, jdavalos@iqfr. csic.es. *L. M. N. B. F. Santos: phone, 34-915619400; e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The support of the Spanish MICINN Project CTQ2009-13652 is gratefully acknowledged. Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and to the 3708
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(20) Verevkin, S. P. Thermochemical Properties of Triphenylalkanes and Tetraphenylmethane. Strain in Phenyl Substituted Alkanes. J. Chem. Eng. Data 1999, 44, 557−562.
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