GEORGEGLOCKLER
828
The present work demonstrates the possibility of very efficient ionic polymerizations with unsaturated hydrocarbons. Other work4J1 extends these observations to include ethylene, benzene and toluene. There is no indication of high specificity and it is plausible that chain reactions of the type CmH, C& = C m + zHn+z (4) +
+
+
Vol. 63
could readily proceed, where m/n = 1. For the small molecules examined at very low pressure, neutral fragments always formed. For higher pressures and for higher molecular weights, addition without elimination may occur. 22 The possibility of reactions of type (4) is supported by the available thermochemical datal7for m = n = 1, 2, 3, 4, 5, 6. (22) M. Burton and J. L. hlagee, THISJOURNAL, 66, 842 (1952).
(21) Unpublished observations by R. Barker.
CARBON-HALOGEN BOND ENERGIES AND BOND DISTANCES' BY GEORGEGLOCKLER Contribution from the Department of Chemistry, Duke University, Durham, North Carolina Received January 16, 1960
Bond energy estimates and distances of the cyanogen halides, halomethanes, chloroethanes, carbonyl and acetyl halides show a linear relationship. These regularities and earlier information on carbon-carbon, carbon-oxygen and carbonhydrogen bond energies and distances are transferred to study chloroacetic acids, chloroaldehydes and chloroalcohols. The method permits estimates to be made of the heats of atomization, formation and combustion of compounds containing carbon-halogen and the above mentioned bonds. Internuclear distances and hence moments of inertia can be estimated by the inverse calculation.
Introduction
It is believed that the type of calculation presented here will ultimately produce relations of bond energy vahes and internuclear distances from which estimates of various thermochemical quantities such as heats of atomization, formation and combustion can be obtained for compounds which have not yet been studied thermochemically and others which have not even been synthesized. It would be very timesaving and inexpensive if a proper basis for making such estimates were available for all kinds of molecules. It will obviously be impossible to measure the heats of combustion of all compounds of interest now and in the future. Studies of the type reported here would relieve this situation. For example in another field, empirical equations for calculating isomeric variations in the values of physical properties of hydrocarbons and related compounds are very helpful to the investigator interested in these quantities in cases where they have not yet been determined experimentally.2 It is even conceivable that bond lengths obtained by physical methods such as microwave spectroscopy and electron diffraction methods may be measurable with such high accuracy in the future that their use and a well established system of bond energies will be a more satisfactory method of determining heats of formation and combustion than the present day thermochemical experiments. The relation between bond energies and distances will then have to be represented by more complex functions than straight lines. The connection between chemical and physical measurements here discussed can serve both groups of scientists as a check on their own respective experiments. Both ( 1 ) This research is supported by the Office of Ordnance Research, U. 9. Army under Contract No. DA-31-124-ORD-1535. The paper was presented at the Southeastern Regional iMeeting, Gainesville, Florida, 11-13 December 1958. (2) J. B. Greenshields and F. D. Rossini, THIS JOURNAL, 62, 271 (1958).
must show that their observations fit the requirements of the other group. For many practical uses it is of great intterest to be able to make even rough estimates of thermochemical quantities. They can be made even without knowledge of internuclear distances, if there exist some definite ideas as to the bond picture of the molecule under consideration. As force constants must be transferred from molecule to molecule, so must bond energies be transferred in the present calculations. Notation.--B(AB, ABC) = AB-bond energy (kcal.) in ABC; D(Brz) = 53.4; D(Clz) = 58; D(F2) = 37.6; D(Hz) = 104.2; D(Iz) = 51.0; D(N2) = 225.2; D(O)2 = 118.2; L(C) = 171.7 kcal.; n A = number of A atoms; Qa(ABC) = atomic heat of formation (kcal.) of ABC; Qf(ABC) = usual heat of formation from the elements A, B and C in their standard states; R(AB, ABC) =AB-bond length (A,) in ABC; temperature = 25" (ref. 3,4). (Note: In Figs. 1-3,fulldotsindicate that R(ilB) is known experimentally. Open circles show that R(AB) has been estimated.) As is the case with carbon-carbon, carbonhydrogen and carbon-oxygen bonds15it was found here also that large bond energies occur with small internuclear distances and vice versa. Most of these distances were obtained by microwave spectroscopy6 and by electron diffraction by various authors as given in the text. Most Qfvalues come from the work of Rossini3 and for fluorocarbons from M a r g r a ~ e . ~ R(CH) and B(CH) are related by (3) Selected Values of Chemical Properties of Hydrocarbons, Cir. 500, Nat. Bur. Standards (U. 8. Government Printing Office, Washington 1952). (4) R. P. Iczkowski, C. A. Neugebauer and J. L. Margrave, Office of Ordnance Research Report No. 1428 (Feb. 1957); C. A. Neugebauer and J. L. Maigrave (May 1957) and R. P. Iczkowski and J . L. Margrave (Oct. 1958). 61, 31 (1957); 62, 1049 (1958). (5) G. Glockler, THISJOURNAL, (6) W. Gordy, W. V. Smith and R. F. Trambarulo, "Microwave Speotroscopy," John Wiley and Sons, Inc., New York, N. Y., 1953.
, b
CARBON-HALOGEN BONDENERGIES AND BONDDISTANCES
June, 1959 B(CH) = -36.4
+ 148.4/R(CH)
829
(1)
B(CC) and R(CC) are determined graphi~ally.~ Cyanogen Halides.-The quantities mentioned in Table I for CNF were estimated by graphical methods from the relations R(CX, CNX), R(CX, CX4) and R(CN, CNX) with atomic number. The result is R(GF, CNF) G 1.19 and R(CN, CNF) G 1.167 A.' From graphs of B(CF us. R(CF), B(CF, CNF) A 133.0 kcal. a t 1.19 A. From the relation B(CN) us. R(CN), B(CN, CNF) G 181.6 kcal. a t 1.167 B., whence Qa(CNF) 3 14.6 kcal. Using the R(CN) values and the relation B(CN) us. R(CN), the B(CN) quantities for the other halCH,COX (c) ides were found and finally B(CX) = &a(CNX)B(CN, CNX) was determined. The relation of B(CX) us. R(CX) for CNX is shown in Fig. la. These quantities and a similar l o o i set for the tetrahalomethanes (Fig. lb) are used to determine the four general relations B(CX) vs. 50 R(CX). It is seen that B(CF) > B(CC1) > BI5 1.9 (CBr) > B(C1) for CNX. R (cx)A'+ Tetraha1omethanes.-These molecules have only Fig. l.--B(CX) estimates us. R(RX) for: a, cyanogen halbeen studied by electron diffraction methods since ides; b, tetrahalomethanes; c, acetyl halides. their symmetry precludes light absorption. B(C1) was determined from an extrapolation of B(CX) us. R(CX) (Fig. lb). Qa(CI4) was found from the relation Qa(CXI)us. R(CX, (Q.) (CX,)). In general iiiternuclear distances obtained from electron diffraction are quoted to be accurate to A 0.01 to 0.02 A. The CF-distance in CF4 has been measured lately by two investigators who obtained 1.317 B.* and 1.323 0.005.9 The lower value seems to fit much better in a plot of R(CF) us. n(F) of the series CF, to CHBF. The CC1-distance a t 1.765 A.lo fits very well into the series CC14(1.765), CHC13 (1.767), CHzClz (1.772) and CH3C1 (1.782)
*
B.0
The CBr-distance in CBr4 has been determined by three investigator? to be 1.93 f 0.02,11 1.9212 and 1.942 0.03 A.I3 However, .by a simple extrapolation R(CBr,CBr4) = 1.927 A. The electron diffraction resulk on GI4 indicake R(C1, c14) = 2.12 f 0.02 A.11 The value 2.12 A. was chosen and RLCI, CHI3) = 2.128 and R(C1, C H A ) = 2.133 A. were interpolated (Table 11).
*
TABLE TI TETRAHALOMETHANES &a B(CX), Qf, R(CX), kcal. kcal. kcal. A. 116.0 217.2' 464.1 1.317d CF4 CCI, 25.5b 313.2 78.3 1.765e CBr4 -12.0b 266.5 60.6 1.927, CT., -61.0' 213.lc 53.4c 2.120 a Ref. 4. Ref. 3. CEstirnated. Ref. 8; see also ref. 9. " R e f . 10. 'Estimated; see ref. 11, 12, 13. 0 Ref. 11. (7) W. J. 0. Thomas, J . Chem. Phys., 20, 920 (1952). (8) C . W. W. Hoffman and R. L. Livingston, J . Chem. Phys., 21, 565 (1953). (9) 0 . Brockway, et al., reported by R. L. Livingston, Ann. Reu. Phys. Chem., I , 397 (1954). ( I O ) L . S. Bartell, L. 0. Brockway and R. H. Schwendernan, J . Chem. Phys., 23, 1851 (1955). (11) Chr. Finbak and 0. Hassel, Z . ghysik. Chem., B36, 301 (1937). (12) L. R. Maxwell, J . Opt. SOC.Am., 30, 374 (1940). (13) Chr. Finbak, 0. Hassel and 0. J. Olaussen, Tids. Kjemi Bergueeen, 9 , 13 (1943).
6
9
9
56.1 6
52 2.05 2.1 0 R( C X ) A o 4 Fig. Z.-B(CX) estimates as functions of R(CX), X = F, C1, Br, I; (CNX, CX4 to CHaX).
Methyl Halides.-The bond energies and distances can be fitted into the straight line determined by the corresponding points from the cyanogen halides and the tetrahalomethanes (Fig. 2). The resulting data are given in Table 111. The sections CX4 to CH3X are shown in Fig. 3. Two estimates have been made of Qf(CH3F): 59 2 and 58.0 k ~ a l . The ~ R(CX) values (X = F),6 (X = C1, Br, 1)14were determined by micro-
*
(14) S. L. Miller, L. C . Aatnodt, G. Dousmanis, C. H. Townes and J. Kraitchman, J . Chem. Phys., 20, 1112 (1952).
820
GEORGEGLOCICLER
Vol. 63
copy on account of the influence of zero point vibrations. Internuclear distances determined by electron diffraction measurements are subject to variations of f 0.01 to 0.02 A. Of course thermochemical experiments also are affected by errors of 0.1 t o 1.0% and even more with many fluorine containing compounds, for example. It is rather interesting that such diverse measurements should yield as good agreement in the determination of molecular distances. Methylene Halides (Table IV) .-A preliminary report gave R(CF, CH2F2) as 1.36 and R(CH, CH2F2)as 1.09 A.17 An interpolation R(C!F, CF,) +R(CF, CHBF) resulted in R(CF, CH2F2) = 1.36 A.and R(CH, CH2F2,calculated) was 1.10 A. I n the molecule CH2Clz a larger than usual difference in the thermochemical and microwave values of the CH-distance was found. The reason may lie in a discrepancy in the heat of formation of the compound. One value is 21 k ~ a l .while ~ another is 32 kcal., derived from a heat of combustion (106.5 kcal.) due to Berthelot.I8 TABLE IV METHYLEKE HALIDES CHzFz Fig. 3.--B(CX) estimates as functions of R(CX), X = F, C1, Br, I; (CXa to CH,X).
TABLE I11 METHYL HALIDES Qr, kcal.
Qa,
kcal.
B(CX), R(CX), kcal. A.
CHPCI~
CHIF
57.0" 403.8 107.0 1 . 3 8 5 ~ 98.9 1.108 l.llC,f CH&I 19.6' 376.6 76.7 l . 7 8 2 d 99.9 1,090 1. IOdJ CHSBr 8.56 363.2 66.4 1.939d 98.9 1.10. 1.10d*f CHJ -4.9' 348.7 52.6 2 . l R Y 98.7 1 . 1 O e 1.10d.f a Estimate; see ref. 4. Ref. 3. Ret. 0. Ref. 6, 14. "Estimate by eq. 1. 1 Ref. 15, 16. ,
CHzTkC
kcal.
105.5" 419.0 21.0'
CH2Br2 B(CH), R(CH), kcnl. A.
B(CX), R(CX), B(CH), R(CH), kcal. A. kcal. A.
Qa,
Qft
kcal.
l.Ob
-24.0'
330.3
98.5 1.10' 1.09e 77.0 1.7i2d 99.9 1.0gc I.07d 06.4 3.93-P 98.7 l . l O c
303.0
53.0 2.133'
354.9
111.0 1.30'
9 8 . 5 1.10'
... Ref. 4. 6Ref. 17. (I
Ref. 3.
Estimated by eq. 1.
Ref. 6.
,
TABLE I CYANOGEN HALIDES Qi,
&a,
kcal.
kcal.
B ( C X ) , R ( C X ) , BfCN), R(CN), lical. A. kcal. A.
CNF. - 8 . 7 = 314.G" 133.0" 1.19Oa 181.6" CNCl -34.5' 278.8 89.8 1.629c 189.0 CYBr -43.36 267.7 70.2 1.79OC 197.5 CNI -54.6b 255.2 p57.7 1.9956 197.5 a All values for CNF are estimat,cs. Ref. 3.
1.167n 1.163c 1.15gC 1.159c Ref. 6.
wave spectroscopy. The B(CX) quantities were taken from Fig. 2. The energies B(CH, CH3X) were obtained from the difference Qa - B(CX). The results are shown iii the last column of Table 111. They can be compared with the ones determined from microwave spectroscopy work6J6J6 (Table 111,column 7). It is seen that these R(CH) quantities check to about 0.01 A. which is the variation of measurement by microwave spectros( 1 5 ) D. P. Stevenson and J . A. Ibers, Ann. Rev. Phus. Chem., 9 , 359 (1958). (16) C. W. N. Cumper, T r a m . Faraday SOC.,64, 12G1 (1958).
Ha1oforms.-The Qf-values (Table V) lead to the Qa's in the usual manner. From Fig. 3 the bond energies are derived from the corresponding distances (R(CX)). The CH-distances calculated are compared with the microwave values (column 7, Table V). In bromoforin, chloroform and fluoroform, the CH-distances are progressively shorter (1.068, 1.073, 1.098) due presumably to the influence of resonance structures containing double b o n d ~ . ~ ~However, ~g this effect also may be ascribed to the difference of s- and p-hybridization affecting the carbon atom Ch1oroethanes.-The heat of formation of chloroethane is 25.1 kcal.a resulting in Qa = 658.5 kcal. The assumption that B(CC, C2H6) = 85.9, B(CH, C2H6) = 98.4 and B(CC1, CH3CI) = 76.7 kcal., leads to a calculated Qa = 654.6 kcal. (0.6% low). 1,l-Dichloroethane is assumed to have B(CC, C2H6) = 85.9, B(CH, CH2C12) = 99.9, B(CH, C2H6) = 98.4 and B(CC1, CH2C12) = 77.2 kcal. Since Qf = 29.1 kcal.,8 Qa = 639.3 kcal. while the value from bond energies is 636.2 kcal. (0.6% low). (17) D. R. Lide, J . A m . Chem. SOC.,74, 3548 (1952). (18) P. E. Berthelot and J. Ogier, Ann. chim. phya., [51 2S, 197 (1881). (19) S. N. Chosh, R. Trambarulo and W. Cordy, J. Chem. Phye., 20, 605 (1952). (20) C. A. Coulson, Commemorative Volume for Viotor Henri "Contribution t o Molecular Structure Studies," Desoer, Li@, Belgium, 1948.
a
CARBON-HALOGEN BONDENERGIES AND BOND DISTANCES
Julie, 1959
R(CC1) = 1.775 0.02 .kZ6 Bond energies to satisfy these data are given in Table VII.
TABLE V HALOFORMS Qf,
QP, kcal.
B(CX), R(CX). B(CH), R(CH),
CHCb CHBrr
-3.0bse
CHFi
1.07d
300.8
G6.5 1.030d 101.3 1.08"
258.0
5 3 . 2 2.128" 98 4 1.10'
l.Oid
CHI8
e
-42.36
Ref. 4 . Estimate.
TABLE VI1 CHLOROFLUOROMETHANES
kcal.
A. kcal. A. 16!!.Ga 442.8 114.3 1.332d 99.9 1.09' 1.10d 24.0b 334.8 78.0 l . i G i d 100.8 1.08c kcal.
Ref. 3. CEstimated by eq. 1.
831
Ref. 6.
Qt.
kcal.
Q.,
kcal.
B(CCl), R(CCl),
kcal.
B(CF), R(CF),
A. kcal. CCl, 25.5a 313.2 7 8 . 3 1.565d . . . CCliF 67.7' 345.2 77.7 1.770' 112.1 CCliFz 112.5' 379.8 77.1 1.777f 112.9 CClR 163.2* 420.3 76.58 1 . 7 8 5 ~ 114.6 CF, 217.2' 464.1 . . . ... llG.0 a Ref. 3. b Estimate; see also ref. 4, 21, 25. dRef. 10. eEstimate. f Ref. 26. UAssumed; 27,28. hRef.8; seealsoref.0.
A.
... 1.348' 1.340"' 1.328g 1.317' Ref. 4. ref. 6,
The heat of formation of trifluorochloromethane has been determined twice, Qf = 171 f 1 2 1 and 167 kcal.25 The average value of Qa is then 426 kcal. Microwave spectroscopy in two determinations indicates that R ( C F ) = 1.323 and R(CC1) = 1.765 or R(CF) = 1.328 and R(CC1) = 1.74 A.6 or R(CF) = 1.328 f0.002 and R(CC1) = 1.751 f 0.004 k by electron diffraction.28R(CF) is definite and leads to B(CF) = 114.6 kcal. Then B(CC1) = 426 - (3 X 114.0) = 82.5 kcal., avery Btriking increase indeed since the other B(CC1) values are 78.3, 77.7 and 77.1 kcal. It seems unlikely that the substitution of another fluorine for a chlorine atom should produce such a marked increase in the B(CC1) value in CClF3. It is TABLE VI possible that the Qf values are not accurate. At any rate such a discrepancy should lead to a HEXAH ALOETHANES reiiivestigation of the pertinent data. Here it is Qr, Qa, B(CX), R(CX), B(C-C), R(C-C). kcal. kcal. kcal. A. kcal. A. assumed that B(CC1,CCIF3) is 76.5 kcal. from the 759.2 112.7 1.342' 82.0 1.56' 303a CzFs trend noted above. Then Qa(CC1FB) = 420.3 551.4 77.6 1.772f 8 5 . 9 1.543dJ and Qt = 163.2 kcal., lower by 3 kcal. than the C&la 34b C2Br6 -16.6, 487.0 GG.9 1.415' 8 5 . 9 1.543eJ experimental values mentioned above. 85.9 1.543",f 400.0 5 2 . 3 2.15' Carbonyl Halides.-Two &f values of COF2 have CJej -100' 0Ref. 21. 6 Ref. 3. GRef. 22. dRef. 23. 'Ref. 24. t w n determined lately, 1434 and 150.35 kcal. 2 9 f Assumed. (Qf = 146.7 and Qa = 415.6 kcal.). The geometry of the molecule wns studied by electron d i f f r a ~ t i o n . ~ ~ The distaiices for C2CI6 as given by electron The latter information leads to the following diffraction are R(CC) = 1.57 f 0.06 and R(CC1) molecular constants: Qa = 417.7 kcal. with B= 1.74 f 0.01.23 It was assumed however that (CF, 1.32 k.) = 115.6 and B(C=O, 1.17 A.) = B(CC,oC2C16)= B(CC, C2&) = 85.9 kcal. Gt 186.5 ked. 1.543 A. I n CzBr6 (crystal), R(CBr) = 1.93 A. From the microwave spectrum for phosgene it is if R(CC) = 1.52 A . 2 4 is assumed. However the found that R(C=O) = 1.166 f 0.002 and R(CC1) ethane value for the CC-bond was taken as in C&. = 1.746 f 0.004 A.3l Electron diffraction studies The halogens except fluorine are unlikely to change give 1.18 f 0.03 and 1.74 f 0.02 k.,r e ~ p e c t i v e l y . ~ ~ the CC-bond. The microwave lengths correspond to B(C=O) Chlorofluoromethanes.-The thermochemistry of = 190.0 and B(CC1) = 79.8 kcal., whence Qa = trichlorofluoromethane has been studied by three 349.6 kcal. From thermochemistry &f = 53.33 (Qa investigators: Qf = 70.0 f 4,21 6725 and 6G.2.4 = 342.6) and 60.633 (Qa = 349.6) kcal. Hence the The average value is 67.7 kcal. so that Qa = 346.2 higher value would appear to be the better choice kcal. Its bond energies and distances have been although Qf = 53.3 kcal. has been found by several taken as the averages of CFI and CC12F2 since its iave~tigators.~ geometry is not known. ( 2 6 ) R. L. Livingston and D. H. Lyon, J . Chem. P h y a . , 24, 128d For the dichlorodifluoro compound two values are (185G). known for Qf, 112 f 221 and 113 kcal.2b Their (27) D.I
