568
G. 0. PRITCHARD AND R. L. THOMMARSON
The C-H Bond Dissociation Energies in CF,H, C,F,H, and C,F,H1
by G. 0. Pritchard and R. L. Thommarson Department of Chemistry, University of California, Santa Barbara, California
+
(Received September $0, 1968)
+
By determining E L for the reaction, Rf CH, -+ R f H CH3, where R f = CF3, CaFs, or C3F7, and using previously determined values for El, we find D(Rf-H) = 102 f 2 kcal. mole-', based on D(CH3-H) = 103 f 1 kcal. mole-'.
Introduction The growing body of data (see Discussion) on the difference in activation energies El - l/zEzfor the reactions
Rf
+ R H +R f H + R
(1)
Rf
(2)
+ Rf
-+ Rf,
where Rf is CF3, CzFs, or C3F7, indicates that El does not vary with R f for a particular substrate RH, although Hz and Dz are exceptions.2 Values of El 1/2Ezcan be compared directly, since although Ez may have a small positive value, it does not vary with Rf.3 As reliable data are available for reaction 1 for R H = methane, it is possible to measure D(Rf-H) by the classical kinetic method by determining E-l. Based on Tedder's review4 we have previously suggested3s5 that D(Rf-H) will be constant. Corbett, Tarr, and Whittle6 have recently reviewed the literature on D(CF,-H), which yields a value of 102 f 2 kcal. mole-'. However, in these authors' work on the bromination of fluoroform they obtain a much higher value of D(CF8-H) = 109.5 f 1.5 kcal. mole-'. In view of this discrepancy, besides determining E-' for R f = C2F5 or C3F7, we have redetermined the previous value7 for D (CF3-H) obtained by this method. Also, the three results obtained will now be internally consistent.
Experimental The apparatus has been described p r e v i o ~ s y . ~ The fully illuminated reaction volume was 152.6 ml. Acetone-& was obtained from Llerck Sharp and Dohme of Canada, Ltd. ; mass spectrometric. analysis indicated that it contained 37, acetone-&. It was decided to use acetone-d6 rather than acetone as the radical source in determining E-' so that the methane T h e Journal of Physical Chemistry
(CD3H) formed in reaction 1 could be determined directly on the mass spectrometer, rather than having to allow for H abstraction from the radical source. It has recently been pointed outs that in the latter case systematic errors of up to 1 kcal. mole-' may occur in the derived activation energies. However, correction must be made to the CD3H due to H abstraction from the acetone-&. It is well established that the values of E-' will be unaffected by isotopic substitution in the attacking radidaLg CF3H was obtained from Columbia Organic, and CzF5H and C3F7H from Merck Sharp and Dohme. They were checked for purity on the mass spectrometer. No significant impurities were present. After photolysis of the mixtures of acetone-& and RfH, CD3H, CD,, and CO were collected at -210" and analyzed on the mass spectrometer. The ratio of CD3H to CD, in the mixture was estimated from the m/e 19 (CD3H+)and 20 (CD,+) peak heights, assuming equal sensitivities for the two compounds. Periodically, blank runs with acetone-& alone were carried out (1) This work was supported by a grant from the National Science Foundation, performed while R. L. T. was a candidate for t h e M.A. degree. (2) G. 0. Pritohard and J. K. Foote, J . P h y s . Chem., in press. (3) G. 0. Pritchard, G. H. Miller, and J. R. Dacey, C a n . J . Chem., 39, 1968 (1961). (4) J. M . Tedder, Quart. Reu. (London), 14, 336 (1960). (5) G. 0. Pritchard, G. H Miller, and J. K. Foote, Can. J . Chem., 40, 1830 (1962).
(6) P. Corhett, A. M. Tarr, and E. Whittle, T r a n s . Faraday Soc., 59, 1609 (1963); Bull. sac. chim. Belges, 71, 778 (1962).
(7) G . 0. Pritchard, H. 0. Pritchard, H , I. Schiff, and A. F. TrotmanDickenson, T r a n s . Faraday SOL,52, 849 (1956). (8) H. 0. Pritchard and G. 0. Pritchard, Can. J . Chem., 41, 3042 (1963). (9) E. Whittle and E. W. R. Steacie, J . Chem. P h y s . , 21, 993 (1953); J. R. 1McNesby and A. S. Gordon, J . Am. Chem. Soc., 76, 1416 (1954).
C-H BONDDISSOCIATION ENERGIZE>
569
to ensure that the CD3H/CD4 correction had not changed.
in the acetone-de. The rate constant ratio is obtained from
_ k3 - d[CD,l/dt -
Results The reactions of interest involving CD3 radicals are
+ R f H +CD3H + Rt (-1) CD3 + CDaCOCD, -+- CD4 + CDzCOCD3 (3) CD3 + CD3 --+ C:!De (4) CD,
I n Table I .we have presented the data for the competitive reactions --I and 3 for the experiments with CFBH, CZFBH, and C3F7H, respectively. The CD3B/ CD4ratios have been corrected for the H atom impurity
4
[CFsH] d [CD3H]/dt [acetone-do]
k-,
+
CF3H A cetone-de Experiments. C3"H pressures used varied between 5.24 and 20.7 cm., while those of acetone-& were between 2.18 and 5.66 cm. A leastsquares treatment of the Arrhenius plot gives k3/k-l
=
52 exp[(-1230
+
k3lk-l = 75 exp[(-1800
=
144 176 195 201 207 237 255 257 275 309
3.66 1.57 2.99 1.88 3.09 4.07 7.26 2.25 1.42 4.06
:RrH 135 161 170 192 208 230 253 272 305 318
=
1.77 1.52 1.62 1.21 1.27 2.52 1.32 1.28 1.06 1.16
129 155 168 171 199 207 219 250
1.07 1,13 0,962 1.29 1.05 1.26 0.960 1.15
260
1.15 1.10
1.13 1.07 1.12
=
11.9 13.7 14.3 15.1 14.8 16.2 16.7 16.8 17.3 18.1
CPFBH 2.19 1.68 1.69 1.13 1.12 2.03 0.954 0.857 0,673 0.697
ltfH
280 287 312 323
CFiH 0.309 0.115 0.205 0.125 0.209 0.251 0,435 0.134 0,082 0.231
0,807 0,909 0.957 1.06 1.14 1.24 1.39 1.49 1.57 1.67
CiF7H 1.35 1.31 1.02 1.35 2.94 1.10 0,767 0.883 0.771 0.648 0.841 0.598 0.594
f
D(CH3-H) - (10.3
'/zEz =
20)/RT]
Our previous determination7 gave
D(CF3-H) RrH
f
150)/RT]
We may takee>l0EB - '/2E4 = 11.4 - 0.2 kcal. mole-' and El - l/zEz = 10.3 f 0.2 kcal. mole-I for R f = CF3 and R H = methane." Using these results we have
Table I : Data For the Competitive System: CD3 RfH CD3H -1 Rf ( - 1), and CD3 CDSCOCDi -,CDa CDzCOCD3 (3)
+ +
k3/Yc-1
0.789 0.865 0.941 0,952 1.12 1.14 1,25 1.30 1,50 1.70 1.66 1.79 1.89
+ (11.4
= D(CH3-H)
f
f
0.2) -
0.2) - 1.2
- (0.1 f 0.4) - '/zEz
+
'/2E4
+ '/zE4
Assuming D(CH3-H) = 103 f 1,6E z = 1 f 1,3and E 4 = 0 kcal. mole-', respectively, D(CF3-H) = 102.4 f 1.9 kcal. mole-'. CzF5H Acetone-& Experiments. CzF5H pressures varied between 5.47 and 12.8 cm. and acetone-do pressures between 4.99 and Z.67 cm. The data yield
+
k3/k-1
= 8.4 exp[(-1940
f
20)/RT]
For the reaction of CZF5 radicals bith methane, Price and KutschkeLZfind El - l / z E ~ = 10.62 kcal. mole-' Hence D(CzF5-H) = 101.4
f
1.7 kcal. mole-'
+
C3F7H Acetone-de Experiments. C3F7H pressures varied between 4.12 and 7.38 cm., and those for acetone-de between 3.86 and 5.73 cm. The data give k3/k-l
=
13 exp[(-2270
=t 20)/R1']
For the reaction of C3F7 radicals with methane, Giacometti and SteacieI3 obtain E1 - l / z E ~= 9.5 f 0.5 kcal. mole-'. Hence D(C,F?--H) = 102.1
f
2 . 2 kcal. mole-'
The results obtained for the CzIc5H and C3F7H (10) 3. R. hIcNesby, T. W. Davis. and A. S. Gordon. J . Am. Chem. Sci., 76,823 (1954); M.H. J. Wijnen, J . Chem. P h y s , 2 2 , 1074 (1954). (11) P. B. Ayscough, J. C. Polanyi. and E. W. R. Steacle, Can. J . Chem., 33, 743 (1955). (12) S. J. W.Price and K . 0. Kutschke. ibid., 38, 2128 (1960). (13) G. Giacometti and E. W. R. Stencie, ibid.,36, 1493 (1958)
Volume 6S, L\-umber 3
M a r c h , 1964
570
G. 0. PRITCHARD AND R. L. THOMMARSON
-
Table 11: Activation Energy Data (Et - 1/2E2,kcal. mole-') for the Competitive Systems: Rf RH RfH R ( l ) , and Rf Rr + Rf, (2)
+
+
+
RH
CFB
C2Fi
CsF7
CH4 CzHs cyclo-CsHlz Acetone RfCHO"
9 . 5 f 0 . 7 , " 8 . 6 f 0.5b 10.2 f. 0.7," 9 . 5 f 0 . 4 b 1 0 . 3 f 0 . 2 , l 10.48 7 . 5 f. 0.21 5 . 0 f 0.Zi 6 . 9 f. O . l k 8.28
11.9,
