ELECTRIC MOMENTS OF
THE
3969
HALOTRIFLUORETHYLENES
The Electric Moments of the Halotrifluoroethylenes’ by E. J. Gauss and Theodore S. Gilman2 Department of Chemistry, University of Colorado, Boulder, Colorado 80802
(Received February 21, 1969)
The dipole moments of CF2C12, CFZ=CFCl, CF2=CFBr, and CFz=CFI have been measured, with values of 0.50, 0.58, 0.76, and 1.04 D, respectively, being obtained. The moments of the trifluorovinyl compounds are discussed in terms of possible mesomeric and inductive effects. It is concluded that the magnitude of and trend in these moments are best explained on the basis of inductive effects.
Introduction The change in dipole moment when an alkyl group is replaced by an alkenyl or aryl group or when the hydrogens of an alkyl group are replaced by fluorines is often attributed to mesomeric or inductive ~ f f e c t s . ~ -For ~ example, mesomerism can be used to explain the lowering of the dipole moment when the ethyl group in CzH5C1is replaced by a vinyl or phenyl group. On the other hand, replacement of the hydrogens in the methyl halides by fluorines causes the trend in polarity to reverse. In the CH3X series (X = C1, Br, or I), the electric moment of the chloride is greatest and that of the iodide the least; whereas in the CF3X series the trend in moments is in the opposite direction. This reversal has been attributed to inductive effects.6 The dipole moments of the halotrifluoroethylenes are therefore of interest since both mesomeric and large inductive effects are possible. These moments are reported in this paper, and the values are interpreted in terms of the above-mentioned effects. Experimental Section Materials. (i) Bromotrifluoroethylene was prepared by a method, developed in these laboratories,’ which involved the bromination of trifluoroethylene and subsequent dehydrobromination of the adduct, 1,2-dibromo-l,1,2-trifluoroethane, in a suspension of KOH in mineral oil. The gaseous product, after fractional distillation, boiled at - 12.0’ at 630 mm pressure. (ii) The preparation of iodotrifluoroethylene has been reported elsewhere.8 The sample used was shaken with mercury after fractional distillation (bp 27” (632 mm)) . (iii) Gaseous dichlorodifluoromethane and chlorotrifluoroethylene (E.I. du Pont de Nemours and Co.) were used directly from cylinders. (iv) The ammonia used to calibrate the dielectric constant cell was purified on a vacuum line by three successive trapto-trap distillations, with the middle fraction being saved in each instance. Apparatus and Pyocedure. The heterodyne beat apparatus, dielectric constant cell, and procedure used have been described previo~sly.~The cell constant was 79.14 pF, as found by calibration with ammonia whose dielectric constant is accurately known. lo Each
of the gaRes studied was admitted to the cell at various pressures ranging from 200 to 650 mm at each of four or five temperatures between 25 and 150”. Capacitance readings were taken; then the cell was immediately evacuated and the empty cell capacitance was read. This procedure tended to minimize the effects of drift in the electrical circuits and of gas absorption on the plates in the cell; it was found that there was a slow decrease in capacitance of the empty cell upon evacuation which was presumably the result of slow gas desorption.
Results For each of the gases and at each temperature, values of AC, the difference in capacitance between the full and empty cell, were plotted vs. the pressure of the gas in the cell. A value of AC for 1 atm was obtained by extrapolation of the best straight line drawn through the points, thus minimizing the effect of possible gas imperfections. Values of E - 1 at 1 atm were obtained by dividing the corresponding AC‘s (1 atm) by the cell constant (e = dielectric constant). The total molar polarization P was then calculated by
P (cc/mol)
=
82.052’ - 3 ( E - 1)
in which ideal gas behavior is assumed and in which E 2 has been replaced by 3 since E is so close to unity. The values of P thus obtained at different temperatures for the four gases studied are given in Table I.
+
(1) Based in part on the M.A. Thesis of E. J. Gauss, University of Colorado. (2) Deceased on February 11, 1969. Reprint requests may be addressed to Chairman, Department of Chemistry, University of Colorado, Boulder, Colorado 80302. (3) C. P. Smyth, “Dielectric Behavior and Structure,” McGraw-Hill Book Co., Inc., New York, N. Y., 1955,Chapters VIII-X. (4) R. J. W. Le Fevre, “Dipole Moments,” 3rd ed, John Wiley & Sons, Inc., New York, N. Y., 1953,Chapter IV. (5) J. W. Smith, “Electric Dipole Moments,” Butterworth and Co. Ltd., London, 1955,Chapter 7. (6) A. Di Giacomo and C. P. Smyth, J . Amer. Chem. SOC.,77, 774 (1955). (7) R.J. Seffl, Ph.D. Thesis, University of Colorado, 1954. (8) J. D. Park, R. J. Seffl, and J. R. Lacher, J . Amer. Chem. SOC., 78, 59 (1956). (9) E.J. Gauss and T. S. Gilman, Rev. Sci.Instr., 31, 164 (1960). (10) A. van Otterbeek and K. de Clippelier, Physica, 14,349 (1948).
Volume 78, Number 11 November 1969
3970
E. J. GAUSSAND THEODORE 5. GILMAN
Table I : Total Polarization a t Various Temperatures T ,OK
T,OK
P , oc/mol
CFzClz 298.2 323.2 359.0 373.2 423.2
22.88 22.12 21.46 21.54 21 - 3 5
297.6 324.2 373.2 423.3
CF2=CFBr 298.1 323.0 373.3 424.2
P , eo /mol
CFz=CFCl 21.19 20.62 19.73 19.16
(D)
CFz=CFI
27.82 27.01 25.03 24.48
298.2 326.2 363.2 393.6
51.37 45.54 42.18 39.45
With the exception of CFz=CFI, plots of P us. 1/T extended over a sufficiently wide temperature range to justify determination of the slopes B and intercepts PD, the distortion polarization, by the least-squares method. The values thus obtained for PD and for the dipole moment p, calculated from B by means of p (D) = 0.012812/B1are given in Table 11. ~~
~-
Table I1 : Distortion Polarizations and Dipole Moments PD (ee/mol)
CFzClz CFz=CFCl CF2=CFBr CFn=CFI
arrangement a t the higher temperature. I t was therefore necessary to resort to the refractivity method" for the determination of the dipole moment of CFz=CFI. In this method PD is estimated from refraction or other data and subtracted from the total observed polarization at a certain temperature to obtain the orientation polarization. The dipole moment is then calculated with the relation
-+
17.44 14.27 15.90 24"
)c
-+
(D)
0 . 5 0 f: 0 . 0 6 0 . 5 8 f:0 . 0 4 0 . 7 6 f:0.05 1 . 0 4 f:O.lOb
The uncertainties in the values of p were computed by determining by how much the slopes B would be increased if the least-squares values of P at Tminand T,,, were increased and decreased, respectively, by the calculated standard deviation in P. Least-squares treatment of the data for CFz=CFI yielded a dipole moment of about 1.5 D and a value of 2.85 cc/mol for PD. Since the distortion polarization of CFz=CFI can hardly be less than that of CFZ= CFBr, it was clear that the slope of the polarization us. 1/T data could not be used to evaluate the dipole moment in this instance. Reproducible polarization data were obtained only between 326 and 394"K, which is too small a temperature interval for the evaluation of a reliable slope and intercept. Dielectric constant measurements for CFz=CFI were in fact made at 298 and 423°K as well, but a large scatter of the data points was observed at each of these temperatures. This was attributed to partial condensation of the gas at the lower temperature (even though the pressure in the cell never exceeded 430 mm) and to decomposition or reT h e Journal of Physical Chemistry
=
O.O1281[(P - ~ D ) T ] ' / ~
The following evidence was used to arrive at a value of 24 cc/mol for PDof CFz=CFI. (a) The values of P D for CFZ=CFCl and CF2=CFBr given in Table I1 differby 1.6 cc/mol; the same differencein PDhas been found for CF3C1 and CF3Bre6 Furthermore, PD for CFSI exceeds that of CFaBr by 7.7 cc/mol.6 Thus, if this same excess holds for the corresponding trifluorovinyl compounds, PD for CFz=CFI should be 23.6 cc/mol. (b) The molar refraction (Dline) of CFz=CFCL has been found12to be 15.75 cc/mol. The D line atomic refraction of iodine exceeds that of chlorine by 7.93 cc/ mol.13 Thus the molar refraction of CF, CFI should be about 23.7 cc/mol, which indeed is the value one obtains by adding atomic refraction^,'^ using 1.1 cc/mol for the atomic refraction of fluorine.12 (c) On the basis of refraction data for a number of perhalogenated olefinic compounds, Fainberg and Miller14 give 10.15 cc/ mol for the refraction of the CFz=CF- group. Adding 13.9 cc/mol for atomic refraction of iodine13 gives 24.2 cc/mol for CF2=CFI. Hence to the extent that the distortion polarization is approximately equal to the D line molar refraction, a value of 24 cc/mol should be a reasonable estimate of the former quantity for CFz=CFI. Actually, since the total polarization is about twice as great as the distortion polarization for this compound, considerable uncertainty in the latter can be tolerated. For example an uncertainty of 2 cc/mol in PD leads to one of only about 0.06 D in the dipole moment. l5
+
Discussion The dipole moments of both CFzClz and CFZ=CFCl have been determined previously. That of the first compound was measured to check the reliability of our method and apparatus, while the other was determined in order to have data for the three members of (11) See, for example, ref 4,pp 13-15,25,and 26. (12) M. T. Rogers, J. G. Malik, and J. L. Spiers, J.Amer. Chem. SOC., 78, 46 (1956). These authors measured the refractive index for the mercury green line. Adjustment to the D line amounted to only 0.02 cc/mol. (13) K.Fajans, "Technique of Organic Chemistry," A. Weissberger, Ed,, 2nd ed, Vol. I ("Physical Methods") part 11, Interscience Publishers, Inc., New York, N. Y.,1949,p 1163. (14) A. H.Fainberg and W. T. Miller, Jr., J . Org. Chem., 30, 864 (1965). (15) A value of 22.8 cc/mol for the molar refraction can be calculated from the refractive index (D line) and density of CFz=CFI at 0' measured by Park, Seffl, and Lacher.*
ELECTRIC MOMENTS OF THE HALOTRIFLUORETHYLENES the CFz=CFX series measured under the same conditions. For CFzClz there is good agreement with the value 0.51 D reported by Smyth and McAlpine;16 a somewhat higher value (0.55 D) was found by Epprecht” and Fuoss.’* A greater spread of moment values has been reported in the literature for CFZ=CFCl: 0.38,19 0.40,*and 0.16 D.20 From one point of view, a dipole moment of about 0.6 D for CFz=CFC1 might seem unexpectedly large were it not for mesomeric effects involving structures such as
+F
F
\\ :/ c-c / \
F
C1
I
F+
F
F
F
\: // c-c / \
/ c-c / \ F c1+
I1
I11
F
C1
\=
One can cite several sets of data which suggest that the moment should be smaller than the observed value in the absence of mesomerism. In this regard Rogers and Pruett have pointed out19 that the moment might be expected to be close to the difference between the moments of fluorobenzene and chlorobenzene, about 1.6 and 1.7 D,21respectively, in the gaseous state. Other evidence supporting this argument is the small difference between the moments of (a) vinyl fluoride and vinyl chloride, both about 1.43 D,21and (b) 1,ldifluoroethylene, 1.37 DjZ1and 1,l-dichloroethylene, 1.2-1.3 D (from solution measurements.)21 On the basis of this type of argument, then, contributions from mesomeric structures I, 11, or I11 are needed to explain the observed moment of CFZ=CFCl. Bond length considerations have been used to decide whether structures with the double bond to fluorine (I or 11) are more significant than those with the double bond to chlorine (111). Since the decrease in C-F bond distance which occurs in methane derivatives as the degree of fluorine substitution increases has been interpreted as indicating increasing double bond character,22Rogers and Pruett c o n ~ l u d e dthat ~ ~ the contribution of structures with the double bond to fluorine is more important. (They also inferred that a structure similar to I, wit,h the chlorine end negative, would be preferable to I1 or I11 because the dipole moment of chlorobenzene is somewhat greater than that of fluorobenzene.) Other bond distance evidence which supports this viewpoint is the unusually short C-F bond length in CFz=CFz (1.3O-1.31Az3), as compared to 1.34 or 1.35 A in vinyl fluoride,Z4 whereas no such similar effect has been observed in the corresponding chloro compounds-26 Furthermore, with increasing atomic size, there seems to be a decreasing tendency toward the formation Of bonds the Overlap Of p Orbitals; thus, On this basis struttures 1 and 11 are preferable to 111, particularly Since
3971
an interpretation is sought which will take into account the moments of the bromo and iodo compounds as well. Bond length evidence supporting any of the structures I, 11, or I11 is, however, somewhat inconclusive in that a slightly longer than usual C=C bond distance should result from the contribution of such structures. Yet in the similar compounds, CFFCFZ and CFz=CHZ, this distance is, if anything, shorter than the normal C=C double bond distance of 1.34 On the other hand, as the data in Table I1 indicate, the dipole moments in the CFz=CFX series increase as X changes from C1 to Br to I; this strongly suggests that the nonfluorine halogen atom X is a t the positive end of the dipole. If such is the case, then contributions from structures like I, which have X at the negative end, must be relatively unimportant; otherwise, the CFz=CFX molecules would have moments which, in the absence of mesomeric effects, would be even greater than those observed. This seems highly unlikely in view of the fact that the polarities of the various carbon-halogen bonds are generally regarded as differing at most by only 0.2 or 0.3 D. Moreover, structures like I1 fail to account for the substantial increase in moments noted above. In contrast to the problems encountered with finding a satisfactory mesomeric explanation of the magnitude of and trend in the electric moments in the CF2=CFX series, a qualitative interpretation in terms of inductive effects is fairly straightforward. For example, were it not for induction, the dipole moments of t~ans-1,2chlorofluoroethylene and chlorotrifluoroethylene should be about the same, since the over-all moments in both molecules might be regarded as the resultant of a C-C1 and a C-F bond moment pointing in opposite directions. Indeed the moment of the first compound should be close to aero, because as indicated earlier, the moments of the C-C1 and C-F bonds differ by only 0.1 D or less.27 But, with CFz=CFCl this should no longer be A.23326
(16) C. P. Smyth and K. B. McAlpine, J . Chem. Phys., 1 , 190 (1933). (17) G. W. Epprecht, Z . Angew. Math. Phys., 1 , 138 (1950). (18) R. M. Fuoss, J . Amer. Chem. SOC.,60, 1633 (1938). (19) M. T. Rogers and R. D. Pruett, ibid., 7 7 , 3686 (1955); ibid., 79, 6575 (1957). (20) J. E. Boggs, C. M. Crain, and J. E. Whiteford, J. Phys. Chem., 61, 482 (1957). (21) A. L. McClellan, “Tables of Experimental Dipole Moments,” W. H. Freeman & Co., Ban Francisco, Calif., 1963. (22) L. Pauling, “The Nature of the Chemical Bond,” 2nd ed, Cornel1 University Press, Ithaca, N. Y., 1940, p 235. (23) I. L. Karle and J. Karle, J. Chem. Phys., 18, 963 (1950); T. T. Brown and R. L. Livingston, J.Amer. Chem. SOC.,74,6084 (1952). (24) H. W. Morgan and J. H. Goldstein, J. Chem. phys., 30, 1025 (1959); B. Bak, D. Christensen, L. Hansen-Nygaard, and J. RastrupAndersen, Spectrochim,Acta, 13, 120 (1958), (25) L. E. Sutton, Ed., “Tables of Interatomic Distances and configuration in Molecules and Ions,” Special Publication NO.11, The Chemical Society, London, 1958. (26) A. Roberts and W. F. Edgell, J . Chem. Phys., 17,742 (1949). (27) The dipole moment of trans-l,>chlorofluoroethyleneapparently has not been determined. Volume 78,Number 11
November 1969
3972
E. J. GAUSS AND THEODORE S. GILMAN
the case; the opposing C-C1 and C-F bond moments Table 111: Comparison of Dipole Moments (and also the opposing C-F moments for that matter) X II of CFaX,' D p of CFz=CFX, D should not effectively cancel each other out. This is because through mutual induction, carbon-halogen c1 0.46 0.58 bonds lower the moments of other carbon-halogen bonds Br 0.65 0.76 I 0.92 1.04 to the same carbon.** The C-F bond has the greatest H 1.62 1.32' ability to lower the moments of other bonds, and the extent to which the moment of a C-X bond is lowered a See ref 6. ' 0. L. Stiefvater and J. Sheridan, to be published. increases with the polarizability of X. Thus a C-F bond has a greater effect than a C-C1 bond in lowering the C-X bond. Hence the fact that the moments of the the moment of a C-X bond to the same carbon, and the trifluorovinyl halides are greater suggests that the inmoment of a C-C1 bond is lowered more than that of ductive influence of the CF2 group at one end of the C-F bond by a C-X bond. molecule is transmitted through the double bond and As a result of this type of interaction, then, the polarthat the double bond itself is polarized, thus giving rise ity of the C-C1 bond in CF2==CFC1will be lowered more to a charge separation over a greater distance than is than that of the opposing C-F, and a moment for the possible with the trifluoromethyl compounds. The whole molecule greater than that expected on the basis constant difference between the moments in the two of simple bond moment considerations will result. series can thus be regarded as the contribution of this Moreover, as a consequence of the increase in polarizcharge separation, plus possibly a small contribution ability, replacing the chlorine with bromine or iodine from mesomerism. should cause increasingly greater lowering of the C-Br For the first three members in each series (X = or C-I bond moment and hence an increase in the mohalogen only), the data in Table I11 exhibit a parallelism ment of the molecule as a whole, as has been found by which becomes more evident and meaningful when the experiment. A similar type of argument was used to dipole moments in each series are plotted against the explain the increase in moment in the CF3X series.6 A rough estimate of the magnitude of the moment of atomic refraction of the corresponding halogen. The fact that two parallel lines result supports the plausibilCF2=CFC1 resulting from inductive' effects can be ity of an inductive effect interpretation of the dipole made with the aid of the following bond moment lowermoments since atomic refraction can be considered to be ing data computed by Smyth and McAlpine.lB For proportional to polarizability. Such a simple correlachlorine and/or fluorine bonded to the same carbon tion between dipole moment and polarizability can only with tetrahedral angles, the lowerings were calculated be expected in a series of compounds in which one eleto be: C-F by C-F, 0.21 D; C-C1 by C-F, 0 3 6 D; ment is replaced by another with the same outer elecC-F by C-C1, 0.11 D. With these figures, one can tron configuration and with about the same bond moestimate that, at the CF2end of the molecule, the lowerment. The last line of Table 111,referring to the hydroing should be about 0.4 D and at the CFCl end, about gen compound, seems to indicate that a mutual polariza0.7 D, leaving a net induced moment of about 0.3 D, with the CF2end being the negative end of the d i p ~ l e . ~ ~tion , ~ of~ bond moments is transmitted and intensified by the C=C bond only to halogens, but not to hydrogen. It thus appears that roughly one-half of the total moment of CF2=CFC1 is attributable to isolated inductive Acknowledgments. The financial support of a grant effects at each end of the molecule, since as mentioned from the Research Corporation is gratefully acknowlearlier, on the basis of bond moments alone the total edged. We are also indebted to Professor Joseph D. moment be Only Or less* The rest Of the Park for available samples of the compounds total moment of about 0.6 D can be accounted for in studied. terms of effects which are best revealed by a comparison of the moments in the trifluoromethyl and trifluorovinyl (28) See ref 3, pp 235-239. series, which is given in Table 111. (29) The fact that the pertinent bond angles in CF2=CFCl are about 114'ao instead of 109.5' causes small decreases in the calculated lowerOn the basis of inductive action, one would expect ings which can be neglected in this approximate treatment. the in the trifluoromethyl series to be larger (30) P. A. Akisin, L. V. Vilikov, and Ju. I. Vesnin, Dokl. Akad. Nauk because there are three fluorines to lower the moment of SSSR, 126,310 (1969).
The Journal of Physical Chemistry
