ALANS. RODGERS
1996
Estimation of the Heats of Formation of Chlorofluorocarbons
by Alan S. Rodgersl Contract Research Laboratory, Minnesota Mining and Manufacturing Company, Saint Paul, Minnesota (Received August 10, 1966)
The nearest neighbor interaction scheme suggested initially by Zahn has been used to correlate the available heat of formation data on CnF(2,+2)-,Clz compounds with good results. The correlation so obtained may then be used to estimate the heats of formation of appropriate compounds of this class, which, combined with known reaction heats, have led to an estimation of the heats of formation of several olefins of the class CnF2,+,ClZ and of the strain energy of cyclic C4Fs-ZC1, compounds.
Introduction The failure of simple additivity rules in fluorocarbons was first noted by Lacher and co-workers2eain their studies on heats of reaction of fluoroolefins. Similar observations have been made by Patrick14 COX,^ and Skinner. As early as 1934, Zahn' pointed out that the simple additivity rules of Fajanss could be improved by including, in addition to a bond-energy term for each bond, an interaction term for each pair of adjacent bonds (adjacent in the sense that the bonds have a common polyvalent atom). LaidlerQhas presented an additivity relationship based on the subclassification of bonds, and Benson and Bussloahave developed one based on the additivity of groups. Both approaches take better account of nearest neighbor interactions than does the paired-term treatment of Zahn. However, the simpler approach of Zahn will be taken here for two reasons. First, the Zahn scheme requires only six parameters for the correlation of the compounds C,F(2,+2)-,CL while the others require ten, and second, this scheme includes the methanes (CF~-,Cl,) within its scope. This latter advantage is quite important when one realizes that the five substituted methanes comprise nearly half of the reliable data for the compounds under consideration. Before proceeding, it is of some interest to ascertain whether or not these approaches are equivalent from a practical point of view. To this end, the heats of formation of 31 hydrocarbons from C ~ H Nto CsHzo were taken from the American Petroleum Institute tables" and fitted by a least-squares procedure to the three effective Zahn parameters and the four Benson
and Buss group^.'^ The results, which are summarized in Table I, adequately demonstrate the practical equivalence of the two approaches. Table I : Statistical Comparison of the Three-Parameter Zahn Scheme and the Four-Parameter Group Scheme for 31 Hydrocarbons from C,Hlo to CPHno (kcal/mole)
Standard deviation Maximum deviationa Fraction of compounds with /deviation( > 1 kcal/mole a
Zahn
Group
scheme
scheme
0.55 -2.1
5/31
0.54 -2.0
4/31
For hexamethylethane in both cases.
(1) Department of Thermochemistry and Chemical Kinetics, Stanford Research Institute, Menlo Park, Calif. (2) (a) J. R. Lacher, J. M. McKinley, C. M. Snow, L. Michel, G. Nelson, and J. D. Park, J . Am. Chem. Soc., 71, 1330 (1949); (b) J. R. Lacher, J. J. McKinley, C. Walden, K. Lea, and J. D. Park,
ibid., 71, 1334 (1949). (3) J. R. Lacher, A. Kianpour, and J. D. Park, J . Phys. Chem., 61, 584 (1957). (4) C. R. Patrick, Advan. Fluorine Chem., 2 , 1 (1961). (5) J. D. Cox, Tetrahedron, 18, 1337 (1962). (6) H. A. Skinner, Ann. Rev. Phys. Chem., 15,449 (1964). (7) C. T. Zahn, J . Chem. Phys., 2 , 671 (1934). (8) K. Fajans, Chem. Ber., B53, 643 (1920). (9) K. J. Laidler, Can. J . Chem., 34, 626 (1956). (10) (a) 8. W. Benson and J. H. Buss, J . Chem. Phys., 29, 546 (1958); (b) see also T. L. Allen, ibid., 31, 1039 (1959). (11) American Petroleum Institute, Research Project 44, Carnegie Press, 1953. (12) NO steric repulsion parameter was included.
ESTIMATION OF THE HEATSOF FORMATION OF CHLOROFLUOROCARBONS
1997
Table 11: Heats of Formation and Heats of Atomization6 (kcal/mole) Compound
C c1
F CF4 CFaCl CFzClz CFCla CCla CCllCCl*
Qat
- AXf
Qa
by eq 4
-170.9 -29.0 -18.9 222.9 166.0 115.0 68.0 25.5 35.3
468.5 422.6 381.7 344.8 312.4 551.1
468.5 422.8 381.5 344.8 312.5 551,l
687.3
2148.2
2147.6
783.5 801.9 212.6 (192 f 4)/n
Ref
13 13 13 13 13 13 13 13 b C
2453.1
2453.7
C
2300.6 688.0 609.4/n
2300.5 688.0 612.2/n
C
d e
a The uncertainty in the heats of formation will be taken as f 2 kcal/mole although in some cases the heat of combustion data are considerably better. The standard state is that of an ideal gas a t 1atm and 25". b AHi(so1id) from L. Smith, L. Bjellerup, S. Krook, and H. Westermark, Acta Chem. S c a d . , 7, 65 (1953); AHsub from K. J. Ivin and F. 5. Dainton, Trans. Faraday Soc., 43,32 (1947). cAH,,(gas) from W. D. Good, D. R. Douslin, D. W. Scott, A. George, J. L. Lacina, J. P. Dawson, and G. Waddington, J . Phys. Chem., 63, 1133 (1959); AHf recalculated from data in ref 13. d AH~(CFFCF~)from ref 13, heat of chlorination from ref 2 corrected to 25" with ACp estimated from ref 10. e AIZ,(solid) from W. D. Good, D. W. Scott, and G. Waddington, J . Phys. Chem., 60, 1080 (1956); mr(so1id) recalculated from data in ref 13; estimated .(\H#ub from ref 4.
Discussion The heats of formation of relevant atoms and compounds have been taken from the JANAF tables1* and refer to 25". Data taken from other literature sources have been recalculated so that the heats of formatlion are consistent with the JANAF tables. The heats of formation so obtained for the compounds CnF(~n+2)--zClz are given in Table 11. The heats of a t 298.2"K by the relaThe terms in eq 1 have been labeled so as to permit the heat of atomization of a compound to be given by these simple rules: (1) a B(C-X) term for each C-X bond and X (2) a r X C Y term for each C
