J. phys. Chem. m 3 , 87, 243-244
TABLE VI: Standard Enthalpies of Formation Hf0298.16,
compd C,H,OH NaOH NaOH NaOH HZ0 C,H,ONa
state
200 H,O 50 H,O
-(1)H,O 3 H,O
-H,O (1H , S 0 , / 1 1 0 0 H,O)
kcal mol-'
ref
-68.850 -112.154 -108.894 -112.236 -68.317 -98.200
14 14 14 14 14 15
lowest concentration of ethanol used on the calorimetric = 0.237. This is smaller measurements, eq 18 gives K'm,CSb than the above value, but it is still 3.2 times as large as the one, K , = 0.0726, obtained calorimetrically (Table IV). Hence the equilibrium constants for reaction 1apply only to a narrow range of ethanol concentrations. The spectrophotometrically determined values reported in Table V are probably valid only for ethanol concentrations around 0.086 M, while the calorimetric values in Table I1 should not be used outside the range from 0.008 to 0.016 M in ethanol. Table VI gives values of the standard enthalpies of formation (Mf,298.'6) of individual components that are needed in discussing the values of the enthalpy of reaction 1 ( A H R , Table IV). If these values are used to estimate AHRo,the variation of AHTNdHwith concentiation cannot be neglected. Values of AHfoNaOEt will therefore be calculated separately for the lowest and highest concentra-
243
tions of sodium hydroxide (50 H2O and 3 H20). With AHR = 3.2 kcal mol-' (the lowest value in Table IV) the values of AHfo are -109.5 kcal mol-' (50 H20) and -106.2 kcal mol-' (3 H,O); with AH = 3.86 kcal mol-' (the highest value in Table IV) the values of AHfoare -108.8 kcal mol-' (50 H20) and -105.5 kcal mol-'. All these values are considerably different from the results obtained by Blanchard et al.,l5which is AHf,NaoE, = -98.2 kcal mol-', and which when combined with appropriate values of AHfofor the reactants and products gives AHRo= +14.6 kcal mol-' for the heat of reaction 1 in an infinitely dilute aqueous solution. Our values are in better agreement with Murto's conclusion,' from the temperature dependence of his values of the equilibrium constant for reaction 1, that the value of A H R "is approximately zero". Since for this purpose he expresses his constant only in concentration terms (concentration constants K c ) , the numerical value he gives ( A H R = -3.55 kcal mol-') must be considered only a very rough estimate. In view of the dependence of the equilibrium constant ( K ) on the concentration of ethanol, A H R can also be expected to depend on the composition of the solvent. The differences among the values of A H R and AHf,NaOEt obtained by different workers may be caused by such a dependence on concentration of ethanol. Acknowledgment. This work was supported by Grant No. CHE-7727751 from the National Science Foundation. Registry No. EtOH, 64-17-5; NaOH, 1310-73-2.
Heat of Formation of the CHCI, Radical. Bond Dlssociatlon Energies in Chloromethanes and Chloroethanes Mala W e l m a n and Sldney W. Senson' Department of Chemistry, Wdmarrbon Research Instkute, Unlverslty Park-MC 1661, Unlversiiy of Southern Callfornla. Los Angeles, Callfornla 90089- 166 1 (Received: June 25, 1982; I n Final Form: October 1, 1982)
The first experimentally based value of the heat of formation of the CHC12radical derived in a recent analysis of the thermochemistry and kinetics of the pyrolysis of pentachloroethane (25.7f 1.0 kcal/mol) was used to calculate C-H and C-Cl bond energies in some chloromethanesand C-C bond energies in some chloroethanes. The bond strength for intermediate compounds of the series of chloroethanes are higher than the values obtained from the extreme compounds of the series through interpolation. C1 substitution in CzHsincreases C-C bond strengths for the first homologues.
The first experimentally based value for the heat of formation of the CHCl, radical (AHfo3,-,,J was derived in a recent analysis of the thermochemistry and kinetics' of the rate data reported for the pyrolysis of the pentachlsroethane in excess of toluene:2 25.7 f 1.0 kcal/mol. With this we can evaluate C-H and C-Cl bond dissociation energies (DHom8)in some chloromethanes and C-C bond dissociation energies in some chloroethanes. Next-nearneighbor interactions in chloroalkyl radicals are presently neither well documented nor understood. Therefore, the accuracy and/or even validity of DHoB8values obtained through interpolation from known compounds is consid-
ered questionable. Until now, because experimental data for the heats of formation of the CHCl, and CH2Clradicals were missing from the literature, only estimated C-H, C-Cl, and C-C bond strengths have been used for many chloroalkanes. Thus, Goldfinger et al.3 have assumed a smooth variation of the DHo29s(C-Cl) in the chloromethane series of compounds and, from the known values of the extremes of the series (CH3C1and CC14),they have derived, through direct interpolation, DHoB8(CH2C1-C1) and DH0298(CHC12-C1).The. accuracy was estimated as f4 kcal/mol. With AH; for CH2Cland CHC12estimated in this way, C-C bond energies for chloroethanes were ~~~
(1)S.W.Benson and Maia Weisaman, Znt.J. Chem. Kinet.,in press. (2) T. J. Houser and T.Cuzcano, Znt. J . Chem. Kinet., 7,331 (1975).
~
~
~
(3) P. Goldfinger and G. Martens, Trans. Faraday SOC.,57, 2220
(1961).
0022-3654/03/2007-0243$0 1.5Q/O 0 1903 American Chemical Society
244
The Journal of Physical Chemistry, Vol. 87, No. 2, 1983
tab~lated.~ The accuracy of these values, neglecting the errors in the heats of formation of the chloroalkanes, becomes f 4 to f 8 kcal/mol. Extending the method proposed by Bernstein5 to account for next-neighbor interactions in chloromethanes to chloroalkyl radicals, Benson et have estimated AHfo for CH2C1and CHC12 which suggest that DH0B8(C-H) and DHoB8(C-C1)for CH2C12and CHC1, are higher than the interpolated values. The accuracy of these estimates is f 2 kcal/mol. In their study of C1 abstraction by cyclohexyl radicals from chloromethane, Katz et al.' found a Polanyi-Evans correlation factor a N l.13 Because this value is too high for this type of reaction they suggested that the DH0298(CH2C1-C1) and DHom8(CHC12-C1)might be higher than the values obtained through interpolation from the known DHo,,(CH,-C1) and DH0,(CCl3-C1). However, in order to obtain an a 31 0.5 (close to the values found for other neutral radicals abstracting C1 from chloromethanes, (0.2: 0.42: 0.651°), the DHom(CH2C1-C1)and DH0,(CHCl2-C1) would have had to be increased to unreasonably high values (-78 and -120 kcal/mol, respectively). On the other hand, the large value of E might be due also in part to the stabilizing effect of the increasing number of chlorine substituents. As the magnitude of this effect is not known and is not presently predictable, an estimate of what part (if any) of it is attributable to an increase in the DHozg8(C-C1) cannot be made with any significant accuracy. Therefore, these data cannot provide any meaningful experimental evidence that DHo2g8(CH2C1-Cl) and DH0,8(CHC12-C1) are larger than the values obtained through interpolation from DHo,8(CH&1) and DHP,8(CC13-C1). The preferred value derived by us: AH? (CHC12)= 25.7 f 1kcal/mol, is within the accuracy limits of the estimated values: 22.9 f 4 kcal/mo13 and 24.2 f 2 kcal/mol.6 In Table I we list bond dissociation enthalpies, DHoB8, for compounds in the series Y-CH,Cl, where Y = H, C1, CH,C13-,. The values are based on known values of AH; for all species" except for AHf0(CH2C1)which has been (4)J. A. Franklin and G.H. Huybrechta, Int. J. Chem. Kinet., 1, 3 (1969). (5)H. J. Bernstein, J. Phys. Chem., 69,1550 (1965). (6) S. Furuyama, D. M. Golden, and S. W. Benson, J.Am. Chem. Soc., 91,7564 (1969). (7)M.G.Katz, A. Horowitz, and L. A. Rajbenbach, Int. J. Chem. Kinet., 7, 183 (1975). (8)P. Cadmar, G.M. Tilsley, and A. F. Trotman-Dickenson,J. Chem. Soc., Faraday Trans. I, 69, 914 (1973). (9)E.Warhurst, Q.Reo. Chem. SOC.,5, 44 (1951). (10)J. A. Kerr, B. J. Smith, A. F. Trotman-Dickenson, and J. C. Young, J . Chem. Soe. A , 510 (1968).
Weissman and Benson
TABLE I: Bond Dissociation Enthalpies (DH',,,) Expressed in kcal/mola bond H-CH, H-CH,Cl H-CHCI, H-CC1,
105.1 (102.9) 100.6 95.2
bond CH,C1-CH, CH,Cl-CH,C1 CH,Cl-CHCl, CH,C1-CCl,
Cl-CH, Cl-CH,Cl CI-CHC1, Cl-cc1,
83.6 (82.8) 79.2 70.3
CHC1,-CH, CHC1,-CH,Cl CHC1,-CHCl, CHC1,-CCl,
91.9 (92.1) 87.9 79.0
CH,-CH, CH,-CH,Cl CH,-CHCl, CH,-CCl,
90.4 (92.9) 91.9 87.7
CC1,-CH, CC1,-CH,CI CC1,-CHCl, CC1,-cc1,
87.7 (85.3) 79.0 70.7
DH0298
DH029R
(92.9) (93.2) (92.1) (85.3)
a Values in parentheses are based on AHf"(CHgCl) = 31.1 kcal/mol obtained from the interpolation of DH ,,"(H-CH,) and DHo,98(H-CHCl,).
assigned the value of 31.1 kcal/mol on the basis of interpolation of DH0298values from H-CH, and H-CHC12. This is within the accuracy limits of the value reported by De Corpo et al.:12 30 f 2 kcal/mol. We see, comparing the DHO, of Y-CH, and of Y-CCI,, that the effect of substituting H by C1 is to lower the C-C bond strength (Y = CH,Cl,-,). The amount of lowering increases as the number of chlorine substituents in Y increases. The lowering in the series Y-CH3 throughout Y-CC13 is not smooth; the DHO,is of the intermediate members of the series are larger than the values interpolated from the extreme compounds of the series. In some cases they even surpass DHom(Y-CH3). This appears to parallel the AH;,,, of the CH,Cl.+, compounds which decrease by about 2.5 kcal/mol for n = 1except at CC14for which it increases by 2.0 kcal/mol. (11)S. W. Benson, 'Thermochemical Kinetics", 2nd ed, Wiley, New York, 1976;J. Chao, A. S. Rodgers, R. C. Wilhoit, and B. J. Zwolinski, 'Ideal Gas Chemical Thermodynamic Properties: The Six Chloroethanes with a Symmetric Top", NBS-OSRD, Thermodynamics Research Center, Tx, 1972;A. S. Rodgers, R. C. Wilhoit, and B. J. Zwolinski, "Ideal Gas Chemical Thermodynamic Properties: The Light Chloro and Fluoromethanes", NBS-OSRD, Thermodynamics Research Center, Tx, 1972; "JANAF Thermochemical Tables", Dow Chemical Co., Midland, MI; J. D. Cox and G. Pilcher, 'Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, 1970; D. R. Stull, E. F. Westrum, and G. K. Sinke, 'The Chemical Thermodynamics of Organic Compounds", Wiley, New York, 1969; J. A. Franklin and G.H. Huybrechta, Int. J. Chem. Kinet., 1, 3 (1969). (12)J. J. De Corpo, D. A. Bafw, and J. L. Franklin, J. Chem. Thermodyn., 3,125 (1971). (13)E = a(DHom8(C-Cl))where E is the activation energy.
