402
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Kim et
al.
Thermochemistry of Coal Components. 1. Xanthone? Kwang Y. Kim," Randall E. Winans,$ Ward N. Hubbard, and Carl E. Johnson Argonne National Laboratory, Chemical Engineering Division, Argonne, Illinois 60439 (Received August 16, 1977) Publication costs assisted by Argonne National Laboratoty
The energy of combustion of solid xanthone (C13HBO2)was measured by oxygen bomb calorimetry and its standard enthalpy of formation was calculated to be A"fo(c, 298.15 K) = -45.77 f 0.65 kCal,h mol-'. The heat of sublimation of xanthone was estimated, by analogues with compounds of related structure, to be 26.8 f 1.5 kcalth mol-'. A system of group additivity values was developed to permit estimation of the heat of formation in the gaseous state, and ultimately the resonance stabilization, of such compounds. Interpretations for such calculations suggest that the xanthone molecule is planar and aromatic.
I. Introduction The pyrolysis of coal yields a large number of complex aromatic materials. The relationship between these aromatic compounds and the original structure of the coal is not clear. Several investigators have oxidized coal to comparatively simple carboxylic acids which could be readily related to the original molecular architecture of the coal.'v3 These studies have resulted in the identification of certain aromatic compounds that constitute the building blocks of the coal structure. Thermochemical data for these building-block molecules would be of value in connection with the elucidation of coal structure, possible paths of decomposition, and the effects that variables such as temperature might have on processes of coal liquefaction and gasification. Because coal consists mainly of carbon, hydrogen, and oxygen,' the coal components consisting of only these elements have attracted our primary attention. Our initial studies have focused on xanthone (xanthen-9-one), which is one of the oxygen heteroaromatic units identified in studies of the oxidative degradation of coal. Even though xanthone has attracted the attention of many researchers, no thermochemical data on this compound were available, in spite of its importance in coal technology. Therefore, oxygen-bomb calorimetric combustions of xanthone were carried out and the standard enthalpy of formation was derived. These data are necessary for establishing a basic set of values whereby thermodynamic properties of more complex organic species in coal and coal-conversion processes can be estimated. 11. Experimental Section Sample. Xanthone, of unspecified purity, was purchased from the Aldrich Chemical Co., Milwaukee, Wisc. The xanthone was recrystallized twice from absolute ethanol and dried in vacuo a t 80 O C overnight, yielding needlelike crystals with a melting point of 177.5-178.0 "C (lit? mp, 173-174 "C). Analysis for volatile organic impurities in the recrystallized xanthone was performed with a coupled gas chromatograph-mass spectrometer. An OV-101 support coated open tabular column (SCOT), with the temperature programmed from 50 to 250 "C at 4"/min and with a helium flow rate of 9 mL/min, was used to analyze chloroform solutions of xanthone. No volatile impurities t Work performed under the auspices of the U.S. Energy Research and Development Administration. t ANL, Chemistry Division.
were observed down to a detection limit of 0.015%. Therefore, no correction was made for these impurities in the sample. In combustion calorimetry of organic materials, the most significant impurities are noncombustible substances and water. Since no residual traces of material were observed after the combustions of xanthone, it was determined that the sample did not contain any noncombustible substances. Analysis for water showed that the sample contained 0.095 f 0.005% of water. Further, samples of xanthone showed no weight change after being exposed for 2 days to a laboratory atmosphere of 40% humidity. Thus it appears that water was not adsorbed by the sample during experimental handling. For all weighings, a Permas analytical balance (Fischer Scientific Co.) was used which had been calibrated to class S tolerances of NBS weights. Oxygen. The USP grade oxygen used in the combustion experiments was further purified by first passing it through a tube containing copper oxide heated to 600 "C, and then through tubes containing sodium hydroxide in asbestos, magnesium perchlorate, and phosphorus pentoxide. Calorimetric Systems. A rotating-bomb calorimeter6 and a platinum-lined bomb with an internal volume of 330 cm3 were used. The rotational feature of the bomb was not used in this investigation, The energy equivalent of the calorimetric system was determined by the combustion of NBS sample 39i benzoic acid, the certificate of which gave the specific energy of combustion as 6.3178 f 0.0006 kca1,h g-l under specified conditions.' The result of five combustion experiments was €(calor)= 3569.78 f 0.51 calth K-' (mean and standard deviation of the mean), where e(ca1or) represents the energy equivalent of the calorimetric system. Sample Combustion. For each combustion experiment, 1 mL of water was placed in the bomb. The bomb was then charged with oxygen (to 10 atm) and discharged three times to remove residual air before making the final charge to 30 atm pressure. The combustion was initiated with the aid of a cotton thread tied to an 0.08-mm platinum ignition wire attached to the electric igniter. Calorimeter temperature was monitored with a quartz thermometer (Hewlett-Packard Model 2801 A). After combustion, the bomb gases were discharged through a carbon monoxide indicator (MSA Indicating Tube, Mine Safety Appliances Co., Pittsburgh, cal& per combustion, Pa.). For a detection limit of 7 X no carbon monoxide was detected in any of the combustions. After combustion the inside of the bomb was thoroughly washed with distilled water and the wash
0022-3654/78/2082-0402$0 1.OO/O 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 403
Thermochemistry of Coal Components TABLE I: Results of Xanthone Combustion Experiments Combustion no. m'(sample)/g m"'(fuse)/g Ae,/K e (calor)(- A8,)/caltha AE(COnt)/C;?lthb AE(ign)/cala AE(HN0, )/tala A&(corr to std state)/calthc n"'AEc,"'')(fuse)/calth ~ ' A E , "'( sample)/calth AE,"'/M'(sample)/cal,h g-'
1
2
3
4
5
6
0.98976 0.99380 0.99068 0.98955 0.99293 0.98967 0.00127 0.00176 0.00136 0.00129 0.00137 0.00124 2.05421 2.04733 2.05531 2.04904 2.046 5 2 2.04669 -7333.08 -7308.5% -7337.01 -7314.62 -7305.63 -7306.23 - 9.52 - 9.50 -9.54 -9.50 -9.56 - 9.54 0.12 0.12 0.12 0.12 0.12 0.12 0.56 0.78 0.71 0.78 0.78 0.78 5.59 5.58 5.60 5.58 5.58 5.61 5.55 5.14 7.13 5.02 5.51 5.22 -7303.79 -7334.83 - 7312.32 -7303.51 -7329.06 -7306.53 -7379.35 -7380.59 -7381.11 -7380.64 -7381.25 -7382.79 Av AE,"'/M'(sample) = -7380.95 i 0.46 calth g-' Impurity correction = - 7.02 f 0.37 tala g-' AE,"'/M'(xanthone, c; 298.15)= -7387.97
a e(calor)= 3569.78 ? 0.51 c a l a K-'. Computation items taken from ref 7.
f
0.59 calth g-'
Equivalent t o ei(cont)(e'- 298.15 K ) + ef(cont)(298.15 K - O f t AOcom), Standatd deviation of the mean.
TABLE 11: Calculation of Group Additivity Values (kCdth mol-') Compound Structure' AHf" W"
Group
Benzene
19.81 ? 0.13
[C,-(H)]'
9,lO-Dihydroanthracene
3 8 . 2 f 1.1
[CHz(CB)2-(CB)416-Rb= [38.2- (8 X 3.30)]/2 = 5.9 ( i 0 . 6 )
Dibenzopyran
11.6 ?: 1.2
[O(CB)z-(CB)4]6-R = 11.6- (8 X 3.30)- 5.9=-20.7 (11.3)
9,lO-Anthraquinone
&$
-22.8
f
1.6
= 19.81/6 = 3.30 (10.02)
[ ( c o ) ( c B)z-(CB)4]6-R = [-22.8- (8 X 3.30)]/2=-24.6 (10.8)
0
a
CB represents the C atom in the benzene ring.
6-R indicates that the group is a part of a six-membered ring.
solution was titrated with 0.100 M standard NaOH to determine its acidity. The acidity of the solution was assumed to arise from the formation of "03 due to the nitrogen impurity present in the oxygen charged to the bomb.
111. Results The results of the xanthone combustion experiments are summarized in Table I. The symbols of this table are as follows: m', the mass of sample; m"', the mass of fuse; Ab', the temperature rise of the calorimeter corrected for exchange of energy with surroundings; AE(cont), the energy absorbed by the contents of the bomb during the hypothetical isothermal process a t 25 "C; AE(ign), the energy input for ignition of the fuse; AE(HNOJ, the correction for the formation of nitric acid; AE(corr to std state), the sum of the remaining corrections to the standard state; n' and n'", numbers of moles of sample and fuse; AE,"' and AE,""',the molar combustion heats of sample and fuse; M', the molecular weight of xanthone. The procedural details for the above items, as well as for corrections to standard state conditions, are described by Hubbard et al.' The specific heat of xanthone was estimated to be 0.29 calth K-l g-I for the calculation of AE(cont). Any error in this estimation should not significantly affect the result. The rest of the auxiliary data were taken from ref 7. The molar energy change due to the combustion of xanthone (mol w t = 196.205) in oxygen is aE,"(xanthone, c, 298.15) = -1449.56 f 0.50 kC& mol-', for the reaction C13H80z(c) + 1 4 0 z k ) 13COz(g)+ 4H20(1) +
and the uncertainty in AE," is equal to twice the combined standard deviations contributed by the xanthone com-
bustion experiments, the calibration experiments, the uncertainty in the certificate value of benzoic acid, uncertainty of water analysis, the estimate of the heat capacity of xanthone, and the weighings. Addition of the term AnRT (An = -1) to the energy change yields the enthalpy of combustion of xanthone: AH,"(xanthone, c, 298.15) = -1450.15 f 0.50 kcalth mol-'. Subtraction of 13 times the enthalpy of formation of carbon dioxides AHfo(COz,g, 298.15) = -94.051 f 0.031 kcalth mol-', and four times the enthalpy of formation of water,* A","(H,O, 1, 298.15) = -68.315 f 0.010 kcalth mol-', from AH,'(xanthone, c, 298.15) yields the enthalpy of formation of xanthone: AHfO(xanthone,c, 298.15) = -45.77 f 0.65 k c a l ~ mol-l.
IV. Discussion Estimation of AHfo(g)and AH,,, of Xanthone. Xanthane is an important compound not only in coal conversion technology but also in structural interpretations because of the presence of two kinds of oxygen groups forming a y-pyrone ring. Even though the heat of formation has been estimatedg-I2 for some benzenoid hydrocarbons and for those ring systems containing one or two oxygen atoms, no such estimates have been reported for the y-pyrone series. The value of the standard enthalpy of formation of gaseous xanthone is needed for the interpretation of its properties. Since the required sublimation data are not available, an attempt was made to estimate AHf"(g). Table I1 gives the group additivity values needed for the estimation of the enthalpy of formation of gaseous xanthone. Also shown in Table I1 are the standard enthalpies of formation of the compounds from which the group additivity values were derived. The A H f o ( g )values are quoted from Cox and P i 1 ~ h e r . lThe ~
404
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Kim et al.
TABLE 111: Estimation of AHf"(g)
Compound
Structure
Esta
p-Benzoquinone
5,12-Tetracenequinone
Meas13
-29.2 -29.3
a \
-8.0
\
-8.1
0
a 0
I1
Anthrone
7.7
7.7b
a Estimated using group additivity values. Because anthrone has yet to be measured, we used a second estimation technique taking the average of AH^" (9,lO-dihydroanthracene) and AHf" (anthraquinone).
estimation of AHfo(xanthone, g, 298.15) involved the addition of eight [C,-(H)] bond values, one [(CO)(cB)2--(cB)&E bond value, and one [o(cB)2-(cB),]&R bond value. Thus, AHfo(xanthone, g, 298.15) = (8 X 3.3)
+
(-24.6) + (-20.7) = -18.9 kcalth mol-'; the possible error was calculated to be h1.6 kcal,h mol-I. From the group values given in Table I1 and selected values from B e n ~ o n the , ~ AHfo(g)'s were estimated for anthrone and several other quinones. These values are presented in Table 111, along with experimental values for comparison. The heat of sublimation of a compound can be estimated if sublimation data are available for related compounds having a similar molecular weights and structures. The sublimation heat of xanthone (mol wt = 196.2) is estimated to be 26.8 kcalth mol-' from the average of dibenzopyran (mol wt = 182.2, AHHsub= 26.8)14and anthraquinone (mol wt = 208.2, A H s u b = 26.8).15 The heat of sublimation may also be estimated by taking the difference of the estimated AHfo(g)and the measured AH,O(c) to be 26.9 kcalth mol-l for xanthone. Both estimates are in a good agreement with each other. Resonance Energy. Resonance always has the effect of increasing the stability of any molecule in which it is an important feature. Since xanthone is known to be one of the stable oxygen heteroaromatics in c ~ a lits, ~resonance ~ ~ energy was calculated to compare it with those of other aromatic compounds with similar molecular structures. The heat of combustion of xanthone(g) was calculated using the values of the bond contributions and their corrections as reported by Klages16 and Wheland.17 The difference between the calculated value for the molar heat of combustion and the experimental value is regarded as the resonance energy. The resonance energies are presented in Table IV. Biswas and Senla assumed that the xanthone molecule was planar in an x-ray study of its crystal and molecular structure. In addition, x-ray crystallographic studies of anthrone, which is similar to xanthone in molecular
TABLE TV: Heats of Combustion of Resonance Hybrids (kcal,h mol-l) Heat of combustion Obsd13
Calcd
Resonance energy
Anthracene
1713.5
1796.3
82.8
Planar
Anthraquinone
1567.2
1648.6
8 1.4
Planar
Xanthone (xanthen-9-one)
1477.0a
1556.5
79.5
(Planar)
Anthrone
1666.0b
1743.1
77.1
Planar
D ibenz opyran (xanthene)
1575.8
1651.0
75.2
Nonplanar
9,lO-Dihydroanthracene
1764.8
1837.6
72.8
Nonplanar
Diphenyl ether
1482.1
1555.4
73.3
Nonplanar
Benzophenone
1578.7
1647.5
68.8
Nonplanar
Diphenylmethane
1673.7
1742.0
68.3
Nonplanar
Compound
Structure
a Calculated from observed AH; (c) and estimated AH^^,,. and A H f " (anthraquinone).
Remark
Estimated taking the average of AHf"(dihydr0anthracene)
Studies of Solvent Effects
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 405
structure, led Srivastavalgto assign a planar structure to anthrone. Therefore, the enthalpy of formation (Table 111) and resonance energy (Table IV) of anthrone were estimated and compared with those of xanthone. The resonance stabilization energy of xanthone falls between those of anthraquinone and anthrone, both of which are known to be planar molecules. These findings suggest that xanthone may also be planar. The somewhat greater resonance energy of xanthone, when compared with that of anthrone, may be due to the contribution of the unshared electrons of the ether oxygen atom to the resonance conjugation. This is consistent with the reported results20-22that the xanthone molecules, to which the y-pyrone ring is coupled, are aromatic.
References and Notes (1) Throughout this paper 1 calm = 4.184 J and 1 atm = 101.325 kPa. (2) R. Hayatsu, R. G. Scott, L. P. Moore, and M. H. Studier, Nature (London), 257, 378 (1975). (3) R. Hayatsu, R. E. Winans, R. G. Scott, L. P. Moore, and M. H. Studier, Fuel, submitted for publication. (4) D. W. Van Krevelen, “Coal”, Elsevier, Amsterdam, 1961. (5) A. F. Holleman, “Organic Synthesis”, Collect Vol. I, Wiley, New York, N.Y., 1941, p 552.
(6) W. N. Hubbard, C . Katz, and G. Waddington, J. Phys. Chem., 58, 142 (1954). (7) W. N. Hubbard, D.W. Scott, and G. Waddington in “Experimental Thermochemistry”, F. D. Rossini, Ed., Interscience, New York, N.Y., 1956, Chapter 5, p 75. (8) CODATA Bulletin No. 17, ICSU CODATA Paris, 1976. (9) S. W. Benson, “Thermochemical Kinetics”, 2nd ed, Wiley, New York, N.Y., 1976. (10) S. E. Stein, D. M. Golden, and S. W. Benson, J. Phys. Chem., 81, 314 (1977). (1 1) S. Stein, D. M. Golden, and S. W. Benson, Predictive Scheme for Thermochemical Properties of Polycyclic Aromatic Hydrocarbons, Appendix C, Report No. FE-2202-2, Stanford Research Institute, May 1976. (12) W. C. Herndon, Thermochim. Acta, 8, 225 (1974). (13) J. D.Cox and G. Pilcher, “Thermochemistry of Organic and Organometallic Compounds”, Academic Press, London, 1970. (14) R. C. C a s , S. E. Fletcher, C. T. Mortimer, H. D. Springall, and T. R. White, J. Chem. Soc., 1406 (1958). (15) A. Magnus, 2. Phys. Chem. (Frankfurt am Main), 9, 141 (1956). (16) F. Klages, Chem. Ber., 82, 358 (1949). (17) G. W. Whehnd, “Resonance in Organic Chemistry”, Wiley, New York, N.Y., 1955. (18) S. C. Biswas and R. K. Sen, Indian J. Pure Appl. phys., 7,408 (1969). (19) S. N. Srivastava, Acta Crystallogr., 17, 851 (1964). (20) J. Gayoso, H. Bouanani, and A. Boucekkine, Bull. SOC.Chim. Fr., 3-4, 538 (1974). (21) H. Bouanani and J. Gayoso, Bull. SOC.Chim. Fr., 3-4, 545 (1974). (22) G. H. Stout, T. S. Lin, and I. Singh, Tetrahedron, 25, 1975 (1969).
Studies of Solvent Effects. 1. Discrete, Continuum, and Discrete-Continuum Models and Their Comparison for Some Simple Cases: NH,’, CH30H, and Substituted NH4’ P. Claverie,” J. P. Daudey, J.
Langlet, B. Pullman, D. Piazzola,
Institut de Biologie Physico-Chimique, Laboratoire de Biochimie Thgorique associh au CNRS, 75005 Paris, France
and M. J. Huron Institut Francais du P6tro/e, 92, Rueil-Malmaison,France (Received March 28, 1977)
In order to study solute-solvent interactions, three different models are considered: (1) a “discrete” model, according to which a finite number of solvent molecules are placed around the solute molecule, and the total interaction energy is calculated by simplified formulas; (2) a “continuum” model, according to which the solvent surrounding the solute is simulated by a continuum medium (the method of calculation takes into account the actual shape and charge distribution of the solute molecule and is an extension of the solute-solvent problem of the method previously used by Huron and Claverie for pure liquids); (3) a “discrete-continuum” combined model, according to which a small number of solvent molecules (corresponding to so-called solvation sites) interacting strongly with the solute are treated as discrete, while the remaining solvent is simulated by a continuous medium. Then these models are applied to the study of simple cases: solvation of NH4+in water and ammonia, solvation of methanol in water. It appears that some caution is necessary, because the various theoretical steps (into which the solvation process is decomposed) do not always lead to similar values in different models, but a satisfactory agreement (between the various methods and previous available results) may nevertheless be obtained concerning the total values corresponding to the complete solvation process. Since such a comparison of the various methods reveals their respective shortcomings, it may be expected that the simultaneous use of such different models will finally lead to an optimum methodology and thus allow us to deal sucessfully with various problems involving solvent effects.
Introduction In recent years there has been considerable interest in the interpretation of solvent effects on the properties of molecules in particular in relation to conformation^^-^^ and electronic ~pectra.l~-~O The first purpose of this paper is t o present simple methods for calculating the molecular interactions in liquids and to correlate the calculated energies with thermodynamic properties. In liquids, the molecules are close to each other and we have to consider the interactions of a given molecule with all the surrounding ones. The large number of molecules 0022-3654/78/2082-0405$01 .OO/O
is one of the major difficulties of the treatment of liquids which leads to the use of approximate models: (1)We may consider the molecules of the solvent around the solute as individual entities, in which case, we use a microscopic representation of the solvent, and the simplification concerns the number of molecules of the solvent around the solute taken into account. Generally in these representations, only the solvent molecules near the solute are retained.1J-26 (2) We may represent the solvent as a continuous medium, in that case we consider all the solvent molecules
0 1978 American
Chemical Society
