6701
Stabilizing Interactions between Substituents Attached to the Same Saturated Carbon Atom. Enthalpies of Addition of Methanol, Pyrrolidine, and 1-Butanethiol to 1,l-Dichloro-2,2-difluoroethy1ene1 Jack Hine and F. E. Rogers
Contribution from the Evans Laboratory of Chemistry, The Ohio State University, Columbus, Ohio 43210. Received March 11, 1968 Abstract: Enthalpies of addition of methanol, pyrrolidine, and 1-butanethiol to 1,l-dichloro-2,2-difluoroethylene to give 1,l-difluoro-2,2-dichloroethyl derivatives have been measured in solution. From determinations and estimates of enthalpies of solution and of vaporization, it is calculated that the gas phase additions are exothermic by 37.2, 36.3, and 30.0 kcal/mol, respectively. From these and literature data it is estimated that the additions of methanol, pyrrolidine, butanethiol, hydrogen bromide, and chlorine to 1,l-dichloro-2,2-difluoroethyleneare more exothermic than their additions to ethylene by 21.1, 18.9, 8.2, 4.3, and -2.7 kcal/mol, respectively. These results show that compounds with fluorine atoms attached to the same saturated carbon atom as oxygen, nitrogen, and, to a lesser extent, sulfur atoms are stabilized relative to the corresponding hydrogen compounds. The significance of these observations is discussed.
he disproportionation of methyl fluoride to methane and carbon tetrafluoride is exothermic by about 36 kcal/mol of carbon tetrafluoride. Stabilization of a one-carbon compound by an amount equal to the resonance energy of benzene (the disproportionation of cyclohexene to cyclohexane and benzene is exothermic by 36 kcal/mol of benzene) shows that stabilizing interactions of groups attached to the same saturated carbon atom are large enough to merit further investigation. Suggestions that the tendency of fluorine atoms to cluster around the same saturated carbon atom is due to stabilization by double-bond-no-bond (db-nb) resonance2s3
T
I
I
were reviewed recently4 and it was pointed out that attachment of two oxygen atoms to the same saturated carbon atom leads to an increase in thermodynamic stability that is about as large as that associated with the clustering of two fluorine atoms around a saturated carbon atom. In order to learn whether this “clustering effect” is best considered in terms of db-nb resonance or of some other explanation, it seemed desirable to study cases of mixed interactions between two different atoms attached to the same saturated carbon atom. Cases in which fluorine, the most electronegative of the elements, was attached to the same saturated carbon atom as alkoxy and amino groups, which have strong resonance electron-donating ability, were of particular interest. Compounds containing these structural features appear, from molecular structure evidence, to be stabilized, relative to the (1) Part XI11 in the series “Polar Effects on Rates and Equilibria.” For the preceding part see J. Hine, L. G . Mahone, and C. L. Liotta, J . Am. Chem. SOC.,89, 5911 (1967). This work was supported in part by grant GP-4445 from the National Science Foundation. Preliminary presentation: F. E. Rogers and J. Hine, Abstracts, 152nd National Meeting of the American Chemical Society, New York, N . Y., Sept 1966, No. S168. (2) Cf.L. 0. Brockway, J . Phys. Chem., 41, 185, 747 (1937). (3) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed, Cornell University Press, Ithaca, N. Y . , 1960, pp 314-315. (4) J. Hine, J . Am. Chem. Soc., 85, 3239 (1963).
Hine, Rogers
analogous compounds containing hydrogen instead of fluorine. For example, the carbon-nitrogen bond oin tris(trifluoromethy1)amine (length, 1.43 f 0.03 A)5 appears to be shorter than the typical bond betw$en sp3carbon and tricovalent nitrogen (1.472 i 0.005 A),6 and the carbon-oxxgen bond in bis(trifluoromethy1) ether (1.36 f 0.04 A)7 appears to be shorter than the typical )and between sp3 carbon and oxygen (1.426 -I 0.005 A).6 In contrast, the bonds from the central atom to the trifluoromethyl groups in bis(trifluoromethyl) sulfide (C-S bond length 1.828 f 0.015)7 and tris(tri$luoromethyl)phosphine (C-P bond length 1.94 f 0.02 A)6 are longer, if anything, than typical bonds between sp3 carbon and divalent sulfur (1.817 f 0.015 A)6 and betweeq sp3 carbon and trivalent phosphorus (1.841 f 0.005 A),6 respectively. However, the differences in bond lengths brought about by replacing hydrogen by fluorine in these cases are on the order of the experimental uncertainty. Furthermore, the exact relationship between bond lengths and thermodynamic stabilities is unknown. Therefore, we have obtained thermochemical data on the extent of interaction of fluorine atoms with other atoms and groups attached to the same saturated carbon atom by measuring the enthalpies of addition of various compounds to 1,ldichloro-2,2-difli~oroethylene.
Results A Bunsen ice calorimeter was used to determine the enthalpies of addition of methanol, 1-butanethiol, and pyrrolidine to 1,l -dichloro-2,2-difluoroethyleneat 0”. Attempts to add hydrogen fluoride to this olefin at room temperature and below were unsuccessful, even though both alcohol and dipolar aprotic solvents were used, and the reaction was attempted with and without fluoride (5) R. L. Livingston and G . Vaughan, ibid., 78, 4866 (1956). (6) L. E. Sutton, Ed., “Tables of Interatomic Distances and Configuration in Molecules and Ions, Supplement 1956-1959,” Special Publication No. 18, The Chemical Society, London, 1965, pp S19s-S23s. (7) L. E. Sutton, Ed., “Tables of Interatomic Distances and Configuration in Molecules and Ions,” Special Publication No. 11, The Chemical Society, London, 1958, p M120.
Stabilizing Interactions between Substituents on Saturated Carbon
6702
ions (sometimes from the organic-soluble tetrapropylammonium fluoride) as catalysts. To obtain the enthalpies of reaction of the pure liquid reactants to give the pure liquid products, and to obtain the enthalpies of reaction in the gas phase, enthalpies of solution and of vaporization are needed. The following thermochemical scheme is used. CF2=CCI2(soln)
+ HX(so1n)
t A H 5
t*H%
CFZ=CCl,(l)
HXO)
JbHod
4AH'b
CFz=CCIz(g)
+
HX(g)
AHor(soln) ___f
XCFzCHClz(soln) pH.*
XCF2CHCI?(1)
AHor(g)
4
AH06
XCFXHCIz(g)
In the reactions of methanol and butanethiol
AHoexptl= AH",(sdn) AHo&)
=
AHoexptl
+ AHol
+ AHOz - AH", -
AH'i - AH"56
+ AH"
In the reaction of pyrrolidine
AHoexptl = AH",(soln) AHor(g> = AH"expt1
+ AH"I -
+ AHoz
AH", - AH"4 - AHOj
+ AH06
The values of the various AH'S determined at 0" are listed in Table I, as are the values of AHo,(g) calculated from them. The enthalpy of solution of 1-(2,2Table 1. Enthalpies at 0" MeOHb
HX BUSH
Pyrrolidined
-28.8 & 0 . 3 -33.8 f 0.4 AHoeaprl -31.2 f 0.7 AH32 0.0 0.35 + 0.03 -0.57 f 0.09 -0.79 f 0.13 AH'S AH"a 6.42 6.42 6.42 AH"6 9.40 9.12 8.90 AH06 9.25 13.19 12.79 -30.0 -36.3' AHo&) - 31.2
In kilocalories/mole. "soln" refers to methanol solution. "soln" refers to N,N-dimethylformamide solution. d "soln" refers to ether solution. e Assuming that AHol and A H o 8are equal. c
dichloro- 1,l -difluoroethyl)pyrrolidine was not determined because we were not able to prepare the material in a sufficiently pure form, free from significant amounts of acidic impurities. Therefore the enthalpy of solution of this compound was assumed to be the same as that of pyrrolidine. The other enthalpies of solution are average values, determined at concentrations around those present in the determinations of enthalpies of addition. The enthalpies of vaporization at O", AHo4,AHo5,and AHo6, were estimated from the boiling points of the compounds, using the method of Klages,8 in all cases except those of methanolg and l-butanethiol'O where the values at 25" and the method of Klages for estimating the change in enthalpy of vaporization with changing temperature were used. We estimate the uncertainty of AHol(g) for the reactions of methanol and I-butanethiol t o be on the order ( 8 ) F. Klages, Chem. Ber., 82, 358 (1949). (9) J. H. S. Green, Quart. Reo. (London), 15, 125 (1961). (IO) H. Mackle and P. A . G. O'Hare, Tetrahedron, 19, 961 (1963)
Journal of the American Chemical Society
1 90.24
of 2 kcal/mol. For pyrrolidine, where the reaction product is less stable and where the product analysis was less satisfactory, the result may be in error by several kilocalories/mole.
Discussion The only other additions to 1,I-dichloro-2,2-difluoroethylene for which we have found enthalpy measurements recorded are those of hydrogen bromide" and chlorine, which were found to be exothermic by 23.0 and 41.1 kcal/mol, at 94 and 90", respectively, in the gas phase. In order to learn more about the extent to which the differences in enthalpies of addition of the various compounds to difluorodichloroethylene are d u i to interactions involving the fluorine atoms, these enthalpies should be compared with the enthalpies of addition to 1, I-dichloroethylene. Unfortunately, neither the enthalpies of addition to 1,l-dichloroethylene nor the data from which they may be calculated appear to be available, and they cannot be determined with the equipment at hand. We have chosen ethylene as the reference olefin. This is equivalent to defining hydrogen as the reference substituent, interactions with which are defined as zero. Enthalpies of addition to ethylene were calculated from the enthalpies of formation of ethylene, the reactants, methan01,~ butanethiol, lo and hydrogen bromide, l 4 and the products, ethyl methyl ether,'j butyl ethyl sulfide,'O and ethyl bromide,I6 giving the values - 16.1, -21.8, and - 18.6 kcal/mol, respectively (all in the gas phase at 25"). The enthalpy of addition of chlorine to ethylene at 82" has been found to be -43.7 kcal/mol by direct measurement. The enthalpy of addition of pyrrolidine to ethylene to give 1-ethylpyrrolidine was estimated to be -17.4 kcal/mol by use of the group contributions of Benson and Buss. 18, l 9 Comparison of an enthalpy of addition to dichlorodifluoroethylene with an enthalpy of addition to ethylene is equivalent to subtracting one thermochemical equation from another. For the five adding reagents under consideration such subtraction, after correction to 25", gave the following equations. 2 1 (11) J. R . Lacher, K. R. Lea, C. H. Walden, G. G. Olson, and J. D. Park, J . Am. Chem. Soc., 72, 3231 (1950). (12) J. R . Lacher, J. J. McKinley, C. Walden, K. Lea, and J. D. Park, ibid., 71, 1334 (1949). (13) F. D. Rossini, I
