N. E(. SMITH,G. GORIN,W. D. GOOD,AND J. P. MCCULLOUGH
940
The Heats of Combustion, Sublimation, and Formation of Four Dihalobiphenyls’
by N. K. Smith, G. Gorin, Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma
W. D. Good, and J. P. McCullough Contribution S o . 191 f r o m the Thermodynamics Laboratory of the Bartlesville Petroleum Research Center, Bureau of M i n e s , U . S. Department of the Interior, Bartlesville, Oklahoma (Received ,Vooember 1 , 1969)
The structure and energetics of dihalobiphenyls were investigated by thermochemical methods. Heats of combustion of four dihalobipheiiyls were determined by rotatingbomb combustion Calorimetry, and the vapor pressure of each was measured by the Knudsen effusion technique. The following values, in kcal. mole-’, are reported for the heat of formation in the vapor state, AHfoZg8 15, from graphite and gaseous hydrogen, fluorine, and chlorine: 2,2’-difluorobipheiiyl, -45.4; 4,4‘-difluorobiphenyl, -46.5; 2,2’-dichlorobiphenyl, f30.4; and 4,4’-dichlorobipheiiyl, +28.8. The results are shown to be consistent with known values of interplanar angles of the molecules and earlier calculations of delocalization energy.
Introduction Electron diffraction, dipole moment, and other kinds of studies have shown that biphenyl and its derivatives exist in the gas state with the phenyl rings in a nonooplanar configuration. The angle between the rings, measured with respect to rotation about the central C-C’ bond, increases with increasing size of substituent groups in the 2,2’-positions. Resonance conjugation between the rings would be greatest when the rings are coplanar and also should vary with the size of substituents in the 2,2’-positions. The development of precision methods for combustion calorimetry of organic halogen compounds3 has made possible an experimental study of the effects of 2,2’-substituents on the extra delocalization energy resulting from conjugation between the rings of biphenyl. I n this research, the heats of combustion and formation of 2,2’- and 4,4’-difluorobiphenyl and of the corresponding dichlorobiphenyls were determined by rotating-bomb calorimetry and vapor pressure nieasurements. The angle between the rings in each 4,4’derivative is approximately the same as in biphenyl; thus, the observed differences in energy between the The Journal of Physical Chemistry
isomers should be greater for the chloro compounds because the greater size of the substituents forces a greater deviation from coplanarity. The experimental results confirm this supposition and provide an experimental verification of some theoretical calculations of delocalization energy in biphenyls. Experimental Calorimetric A p p a r a t u s and Procedures. The rotating-bomb calorimeter BNR-2 has been de~cribed.~ Platinum-lined bomb Pt-3b,0 internal volume 0.353 l., was used for the experiments with the difluorobiphenyls. Tantalum-lined bomb Ta-l,* internal vol(1) Abstracted from the Ph.D. thesis submitted by N. K. Smith to the Graduate School of Oklahoma State University. (2) G. H. Beaven and D. M. Hall, J . Chem. Soc., 4637 (1956). (3) W. D. Good and D. W. Scott, “Experimental Thermochemistry,” Vol. 11, H. A. Skinner, Ed.. Interscience Publishers, Inc., New York, N . Y., 1962, Chapter 2, pp. 15-39. (4) N. K. Smith, D. W. Scott, and J. P. McCullough, J . P h y s . Chem., 6 8 , 934 (1964). ( 5 ) W. D. Good, D. W. Scott, and G. Waddington, ibid., 60, 1080 (1956). (6) 1%‘. D . Good, D. R. Douslin, D. W. Scott, A. George, J. Id. Lacina, J. P. Dawson, and G. Waddington, ibid., 6 3 , 1133 (1959).
HEATSOFCOMBUSTION, SUBLIMATION, AND FORMATION OF DIHALOBIPHEITYLS
ume 0.341 l., was used for the experiments with the dichlorobiphenyls. The basic procedures used in this investigation for the combustion calorimetry of organic fluorine compounds have been d e ~ c r i b e d . ~The volatility of the difluorobiphen;yls was judged to be of borderline significance; therefore, pellets of these compounds were sealed in polyester bags to prevent loss of sample.’ In all experiments with the difluorobiphenyls, the bomb initially contained 10 ml. of water. For the comparison experirment~,~ a solution of H F was put into the bomb initially in such amount and concentration that the solution obtained after combustion was nearly the same as that produced in the combustion of the difluorobiphenyls. The procedures used for the coinbustion calorimetry of the dichlorobiphenyls are described in the preceding papers4 Sample confinement was not necessary. In all experiments with the dichlorobiphenyls, the bomb initially contained 9.97tj ml. of 0.1156;j M hydrazine dihydrochloride solution. For the comparison experiments, the bomb inihially contained a solution of hydrazine dihydrochloride and hydrochloric acid, which upon dilution with the water produced by combustion of the sample, produced a solution of nearly the same amount and concentration as that obtained from combustion of the dichlorobiphenyls. Combustion calorimetry of the dichlorobiphenyls was done before the work reported in the preceding: paper4 showed that arsenious oxide is superior to hydrazine dihydrochloride as a reducing agent. The uncertainty about the thermochemistry of hydrazine dihydrochloride discussed in that paper affects the accuracy of the present results; however, they are reported in enough detail that they can be revised, if‘ necessary, whenever better thermochemical data for hydrazine dihydrochloride become available. The results for the 2,2‘- and 4,4’-compound are affected nearly the same, so the difference between the beak of formation reported here would be unchanged by such a revision. V a p o r Pressure A p p a r a t u s and Procedures. The vapor pressures of the four dihalobiphenyls were measured by the Knudsen effusion m e t h ~ d . ~The ,~ effusion cells, imade of aluminum with brass lids and polytetrafluoroethylene gaskets, weighed about 8 g. The areas of the two orifices used were computed from measurements of their diameters and the assumption that the orifices were circular. The diameters were measured with a microscope-stage micrometer, with a traveling microscope, and with an optical comparator; an average of the values so obtained was used in the calculations. The rate of effusion was
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determined by weighing the cells before and after evacuation for a known period of time. Timing of the experiments (using an electric timer) was started when a Pirani gage indicated a pressure of 10-8 mm. and stopped when air was admitted into the system through the same three-way stopcock thr
