G. 0. PRITCHARD AND J.
1016
longer observed. The n-electron interaction may persist after the amino group is complexed, but its effect upon the spectrum is not apparent, Since the Uelectron interaction results in a very small spectral change, it probably has little effect upon the constants
E(.
FOOTE
reported for the amino group interaction. These constants represent the average effects of hydrogen bonding of chloroform to the amino group, irrespective of whether the a-electrons are involved in a secondary interaction.
The Reactions of C,F, and C,F, Radicals with Hydrogen and Deuterium'
by G. 0. Pritchard and J. K. Foote2 Department of Chemistry, University of California, Santa Barbara, Goleta, California (Receiued August 26, 2963)
By use of the perfluoroaldehydes as the sources of CZF5 and C3F7 radicals, the hydrogen abstraction reactions of these radicals with H2 and D2 are reinvestigated. The previous anomalously high values for the activation energies of these reactions are confirmed. New data for the reaction C Z F ~ D2 --.+ CzFsD D are presented.
+
Introduction In a previous article3 the photolysis mechanisms of perfluoroalkylaldehydes were discussed, especially with regard to the H abstraction reactions
Rr
+ RrCHO
--+
RfH
+ RfCO
(1)
relative to the respective radical recombination reactions
Rr
+ Rr
--+
Rf,
+ H2
Rt
+ Dz
-+
RrH
+H
(3)
+
RrD
+D
(4)
and Reactions 3 and 4 have been quite extensively studied using perfluoroalkyl ketones as the radical sources: CF3 with H2 and D2,4CzFL with H2,5 C3F7 with H2 and DZ,6and C3F7 with Dz.7 The reactions of CF3 radicals with Hz and D2 have also been investiThe Soozrrnal of Physical Ch.emistry
gated, using hexafluoroazomethane as the radical source Calvertg has pointed out that the activation energies obtained for the reactions of C3F7 with H2 and D2'jv7 (and subsequently CzFs Hz5)are anomalously high, when compared to the general pattern of alkyl and perfluoroalkyl H abstraction reactions. I t therefore seemed desirable to redetermine values of E3and E4 for Rt = C2F5 and C3F7, using a different photolytic
+
(2)
where Rf = CF3, CzFs, or C3F7. In this work, using C2F&H0 and C3F7CH0 as the radical sources, we have investigated the abstraction reactions of C2F5 and C3F7 radicals with H2and with Dz Rr
+
(1) This work was supported by a grant from the National Science Foundation and is based in part on a thesis submitted by J. K. F. in partial fulfillment of the requirements for the M.A. degree. (2) Department of Chemistry, University of California, Riverside, California. (3) G. 0. Pritchard, G. H. Miller, and J. K. Foote, Can. J . Chem., 40, 1830 (1962). (4) P. B. Ayscough and J. C . Polanyi, Trans. Faraday Soc., 52, 960 (1956). (5) 9. J. W. Price and K. 0. Kutschke, Can. J . Chem., 38, 2128 (1960). (6) G. H. Miller and E. W. R. Steacie, b. Am. Chem. Soc., 80, 6486 (1958). (7) G. Giacometti and E. W. R. Steacie, Can. J . Chem., 36, 1493 (1958). (8) G. 0. Pritchard, H. 0. Pritchard, H. I. Schiff, and A. F. Trotman-Dickenson, Trans. Faraday SOC.,52, 849 (1956). (9) J. G. Calvert, Ann. Rev. P h w . Chem., 11, 41 (1960).
REACTIONS OF CzF5 AND C3F7RADICALS
1017
source of the radicals, and especially in view of the fact that in the C3F7 radical work CeF14 was not measured directly, but estimated by radical balance procedure~.~~~
Experimental The apparatus, purification of fluorine-containing materials, and the method of analysis have been described previously3 (volume of fully illuminated reaction cell = 152.6 cc.). Dz was purchased from Air Reduction Co. and usecl without further purification. Analysis on the mass spectrometer indicated 97.370 D2, 1.8% Hz, and 0.9% H D based on identical sensitivities for the m/e peaks at 2, 3, and 4. Hz was obtained from Victor Equipment Co. and purified by passing it through a Pcl thimble a t 300-350" No detectable impurities were found by analysis on the mass spectrometer. I n the experiments containing added D,, mixtures of RfH and R f D were analyzed by using the relative peak heights at m/e 51 (CFzH+) and 52 (CFzD+), assuming equal sensitivities on the mass spectrometer. I n some cases the C4F10 and C6F14 samples contained small traces (< 1%) of the aldehyde. This was corrected for using the 29 (CHO +) ion peak. Results CZFKwith H, and D2. The results of these experiments are presented in Tables I and 11. Table I : Photolysis of CzFsCHO with Hz [CzFs-
Time,
Temp.,
sec.
OK.
CHO]oQ
[Hzloa
910 240 660 300 210 120 120 60.0 50.5
419 441 459 465 491 520 539 568 586
99.6 78.2 88.3 72.8 78.1 73.7 93.4 82.1 81.6
581 624 603 583 375 290 386 380 298
R c ~ F ~ HRC,FIO~ ~ 141 188 213 276 369 646 1340 1860 2170
95.0 79.1 70.6 89.0 103 141 183 193 187
ka/kz'/''
0.512 1.41 1.47 2.39 3.91 9.64 15.8 24.0 37.4
+
and 30% (mean = 20%) based on (CZFKH 2C4F10)/ (C2F6CHO)o. The ratio kl/k2'/' has been determined and this method of treatment has been discussed previous1y.8 In the experiments reported in this paper the consumption of H2 or D2 was limited to less than 0.5% in any run. A least-squares treatment of an Arrhenius plot of the above expression yields k3/kZ1"
=
(1.59
f
exp(-112,400
0.16)106 X f
200/RT) mole-'/' cc.'/'
set.-'/'
For the reaction of CZFKwith Dz we have
h / k ~ ' /= ~ R c ~ F ~ D / ~ [Dzl c~P~~~/' Aldehyde decompositions varied between 14 and 44% (mean = 23oj,). A virtually neligible correction has been applied to allow for the small decrease in DZconcentration. A least-squares treatment of the Arrhenius plot of the expression gives k4/k;"
(2.83 f 0.31)105 X exp(- 12,600 f 200/RT) mole-"' cc.''' =
set.-'/'
We may also obtain a rate constant ratio for the reaction of CzFswith CzF&HO. The data give
k ~ / k z "=~ & 2 ~ , ~ / R ~ 4 [CZFKCHO ~ , , L / ' ]a" =
(5.5
f
200/RT) mole-'/' CC.'/'set.-'/'
1.5)103 X exp(-4900
f
which is in good agreement with our previously determined value in experiments on the aldehyde alone,s where
kl/kz'/' = (3.09 i 0.12)103 exp(-4500
f
x
200/RT) mole-'/' cc.'/'
set.-'/'
Finally, the rate constant ratio kl/k4 is given in Table 11,where kl/k4
[ D Z ] / R C [CZFKCHO ~F~ Iav
~ C ~ F , H
A least-squares treatment of the Arrhenius plot yields kl/k4
For the reaction of C2FK with H2we may write k a / k ~ ' /= ~ ~ C ~ F , H / & ~ F , ~'/'[~Z] kl [CZFKCHO 1av/kz1/' [Hz] where I? denotes mean rate of formation, and [CzFa,CHO],, denotes average aldehyde concentration during a run. Aldehyde decomposition varied between 10
=
(1.55
* 0.10>10-2X exp(7900
f
100/RT)
From our previous values3 we may derive E4 - l/zEz= 12,400 f 300 cal. mole-' and A4/Az'/Z= 2.0 X 105 mole-'/' cc.'/' set."'. There is excellent agreement between the differently derived sets of values. C3F7 with H2 and Dz. The results of these experiments are presented in Tables I11 and IV. The corresponding rate constant ratios which can be derived from these data are Volume 68,Number 6 May, 1064
G. 0. PRITCHARD AND J. K. FOOTE
1018
Table I1 : Photolysis of CzFbCHO with DZ
a
Temp., OK.
Time, sec.
IC2FaCHOlo"
LD2]ua
408 419 442 458 48 1 510 566 612
1500 1080 960 300 310 420 90 70
113 92.1 94.7 87.5 86.2 75.8 73.8 68.0
385 504 560 492 42 1 358 389 288
Mole
CC.-'
X 108.
R c ~ F ~ H ~RC~FD' 135 131 124 222 204 285 586 986
Mole ec.-l sec.-l X 1Ol2.
RC4Fiob
ki/kz'/2'
ki/k2'/zC
kl/k4
100 76.3 75.6 103 121 101 202 179
0,0431 0.0855 0.160 0.335 0.507 1.15 3.21 9.58
15.3 17.8 17.6 27.1 23.6 43.8 60.2 119
356 208 109 81.1 46.9 38.1 18.8 12.4
1.66 3.76 7.78 16.7 23.2 41.3 177 368
Mole-'/2 cc.-'/z see.-'/%.
~
h/kz'/'
Table I11 : Photolysis of CaF7CHO with Hz
=
[CsF7-
Temp.,
Time,
OK.
see.
CHO]oa
[H210a
430 43 1 462 494 549 559 591 592
300 300 120 120 40 50 30 25
99.2 94.2 84.0 83.8 72.2 63.3 80.4 65.0
545 553 490 390 344 309 251 276
Mole cc.-l X lo8. cc.'/' see. -'/z. a
=
* Mole
[Dz]
&a~,~/&,~,41/2
(9.6
f
0.5)105 X exp(-114,000
R c ~ F , H ~ R C S F M ~ ka/k2'/zc 225 217 283 448 1050 1100 1850 1600
133 132 147 153 270 228 262 186
cc.-l see.-' X 1Ol2.
0.662 0.654 0.855 2.82 8.99 13.7 26.8 28.4 O
Mole-'/?
f
1OO/RT)
mole-'/2 C C . " ~ sec.-1/2 The aldehyde decomposition varied between 5 and 12% (mean = 8.5%).
ki/kz'/2
=
R : c ~ , [C3F&HO],v H
=
(9.5
f
2.0)103 X exp(-5500 mole-1'2
=t 200/RT) CC."~
set.-'/%
This is only in fair agreement with our previous results, obtained by the photolysis of C3F7CH0alone.3
Table IV : Photolysis of C,F&HO with DZ Temp.. OK.
Time, sec.
438 441 466 473 506 528 532 567 570
610 600 250 240 190 400 100 50 70
[C~FICHOIO'
93.4 94.0 103 93.0 73.9 77.2 74.4 63.6 79.6
[Dzlu'
RCaF,Hb
415 459 500 510 356 378 371 333 310
177 180 295 235 316 383 40 1 783 890
RCaF,Db
4.36 5.37 l5,7 17.4 34.3 66.9 74.5 209 190
&Flab
CC.''~
sec.
Aldehyde decomposition varied between 8 and 15% (mean = 11%). The ratio kl/kz'/2was obtained from experiments in which C3F7CHO was photolyzed doneea The Journal of Phgsical Chemistry
ki/kz'/2'
0.104 0.117 0,255 0.297 0.812 1.55 1.53 4.09 4.38
19.9 20.3 24.0 22.7 37.6 47.9 42.2 82.6 82.9
102 100 152 132 141 132 172 236 197
exp ( - 4000 mole-'/2
kr/kn'/lc
f
ki/kr
191 174 94.1 76.4 46.3 30.9 27.6 20.2 18.9
300/ R T ) mole - "' cc .12' sec . -
There appears to be a compensating effect between the respective values of the hrrhenius parameters in the two determinations of kl/k2"2. The rate constant ratio in Table IV, kl/k4 is given by
REACTIONS OF C2F5 AND C3F7RADICALS
~
-
_
Table V :
_
1019
_
Comparison of Rf with H,, D,, and CHI Data
CFa CzFs C3F7
10.2 f 0 . 7 9.5 f 0.4
5.9 2.9-6.1
4
28 60
9
6
12.6 f 0.2 13.8 f 0 . 5
9
12.9 f 0 . 8 14.0 f 0.1
49 96
9.5 f.0.7 8.6 f 0.5 11.9 12.4 f 0 . 2 12.3 f 10.4
14 4.2-8.9 53 158 44
4
12.1 f 0.2
72
8d
8d
0.41 0.6ge
5 g
6 6 7h 9
10.3 f 0.2 10.4 10.6
10 40 4.2
C
f 5
0.18 1.37 1.326 9.5 It 0.5
0.97
7
1.33
a E, - ‘/,E2 in kcal. mole-1. A,/AZ’/~in CC.~’’ set.-'/* X P. B. Ayscough, J. C. Polanyi, and E. W. R. Steacie, n-butane reaction as reference point. A,/A2’/’ values are for either Can. J . Chem., 33, 743 (1955). E. values are given for CFs butane or ethane reaction as reference point. See G. 0. Pritchard and G. H. Miller, ,I. Chem. Phys., 35, 1135 (1961),and P. B.AysHZ Dz. cough and E. W. R. Steacie, Can. J.Chem., 34, 103 (1956). e Direct experimental determination in competitive system Rr R.E.Dodd and J. W. Smith, J . Chem. Soc., 1465 (1957). This work. * Competitive experiments, C3F7 Dz CH,.
+
’
h/k4
+ +
[D~]/Rc~F,D [C3FiCHO]av (8.5 & 0.4)KO-3 X exp(8600 f. 100/RT)
= &F,H =
Combining with our previous resultb we obtain E4 -‘/,Ez = 12,600 f 400 cal. mole-l and A4/Az‘/z == 2.2 X lod mole-’/’ cc.’/’ sec.-l/’. These results are again only in fair agreement with those obtained for these parameters by determining k4/JCZ‘/’ directly.
Discussion The per cent decompositions of the aldehydes in this work tend to be somewhat high for the rigorous use of the steady sta,te approximation. Due to the low activation energies required for the abstraction of the aldehydic hydrogen (reaction 1)) the aldehydes are not particularly good sources of CzF5 and C3Fi radicals for studies of reactions 3 and 4, which have considerablly higher activation energies. Because of this factor, before there is sufficient build-up of product from reaction 3 or 4 for analysis, more aldehyde is decomposed than is desirable. The aldehyde is not only consumed in the initial photolytic step, but by any reaction between an atom or a radical with an aldehyde molecule, e.g., reaction 1; this is a chain propagating step, and the rate of aldehyde decomposition increases greatly with rise in temperature. This is offset somewhat by the relative increase in rate of reactions 3 and 4 over reaction 1, with temperature. However, even a t the highest temperatures it did not seem feasible ‘to conduct experiments of less than -30 sec. in duration, leading to about 10% aldehyde decomposition. The ratios of the rate constants for reactions 1 and 4 are given in Tables I1 and IV. There is a factor of ovler lo2involved a t the lowest temperatures. In Table V we have compared the available data for E, (a = abstraction) and A , / A 2 ’ / 2for the rie-
+ +
actions of CF3, cZF5, and C3F7 radicals with Hz, D,, and CHI. The various values of E , can be compared directly, as, although E , may have a small positive value, it is invariant for the three radical specics. l o It is seen that the (‘high” values for E , for the reactions of CzFsand C3F7radicals with Hz and D, are confirmed. For the reactions of the radicals with CH,, the value of E , appears to be constant. Similarly, for other substrates R-H, the value of E , appears to be independent of Rf.3,10,11 In all cases, AE, for Rr radical attack on Dz and H2 is within the limits of the difference in zero-point energy between the two molecules. The general pattern of the results suggests that there is some compensation between the Arrhciiius parameters obtained in our C2Fb D, system, and that they are low. It appears that E , - 1/2E2should be about 1 kcal. mole-’ greater, making the ratios Aa/A2’/’ and A4/A3 correspondingly greater. The errors quoted are the deviations obtained in the least-squares treatment, and are not meant to signify that the values are absolute. Simple collision theory predicts that the ratio of the frequency factors A4/A3 = 2-‘lz. The values given in Table V are for the cases in which the Arrhenius parameters were derived in the same manner, or for competitive systems. The correlation for the C IC3 radical reactions is good, but the c31iiresults arc coiisistently high. ( AIiller and Steacies erroneously state that their experimental values agree with the ratio of the relative collision numbers, citing 2”‘, rather than 2 - ?)
+
(10) G. 0. I’ritchard, G. H. Mller, and J. R. Dacey, Can. J . Chem., 39, 1968 (1961). (11) G . 0. I’ritchard, Y. P. Hsia. and G . H. Aliller, J . Am. Chem.
Sot., 8 5 , 15G8 (1963); G. 0. Pritchard and It. L. Thornmarson, J . Phys. Chsm., 6 8 , 5G8 (19G4).
Volume 68,A’umber 6
M a y , 19F4
G. 0. PRITCHARD AND J. E(. FOOTE
1020
I n the perfluoro ketone system^^-^ the fate of the H and D atoms generated in reactions 3 and 4 was not established. In the CF3 radical system Ayscough and Polanyi4contend that rather than RE
+ H + (AI) +RrH + (M)
(5)
Rr
+ D + (11)
+ (34)
(6)
and +RfD
H
+ H + M +H~ + nf
(7)
D
+ D + M + D ~ + n4
(8)
and
and that all four reactions require a third body. I n the czIi’6and CSF7 radical systems6r6the opposite point of view is taken, that reactions 5 and 6 do not require a third body, and as a limiting value, the rates of formation of R f H and R f D were divided by a factor of two, in order to obtain the true rates of formation of these products in reactions 3 and 4. It has been assumed that whichever standpoint is taken, the derived values of E , are unaffected, but the rate constant ratios ka/kZ1/2, and thus the pre-exponential ratios Aa/A21’2, may vary between a factor of one and two. However, this is an oversimplification of the situation.12 In the present work it was anticipated that the H and D atoms would be removed rapidly by H
+ RrCHO +Hz + RfCO
(9)
D
+ RfCHO -+ H D + RfCO
(10)
and
In the C3F7CH0 Dz mixtures we analyzed for HD enrichment in the Dz a t the end of the runs.
The Journal of Physical Chemistry
CHO ---f CO
+H
followed by the exchange reaction
being the fate of the H and D atoms, they are removed from the system by
+
Values varied from 60 to 100% H D formation based on C3F7D produced. It thus appears that reactions 9 and 10 are the main fates of H and D atoms in the aldehyde systems. It should be pointed out that the decomposition of the formyl radical
H
+ Dz +H D + D
will also lead to H D enrichment. Finally, we may comment on two further types of errors occurring in our experiments, which render them less reliable than the perfluoro ketone systems. Firstly, in the experiments a t higher per cent decompositions, especially so in the CzF6CH0 systems, possible loss of products may occur due to subsequent reactions. In the Hz systems, H CzFsH+. H2 CzF6 can occur, and some of the radicals released may eventually dimerize to C4F10. In the Dz systems, D and H (from H) atoms may attack the C2F6H and CHO +. CO C2F6D which is produced. I n view of the rapid occurrence of reactions 9 and 10, these effects are probably minor, but not negligible. They may help to account for the apparently low E, obtained in the CZF6 Dzsystem.13 Secondly, if reactions 5 and 6 compete significantly with reactions 9 and 10 as modes of removal of H and D atoms, errors will be introduced due to the differing temperature dependences of the respective pairs of reactions. A comparison of the various Aa/AZ1” ratios (excluding CzFS Dz) in Table V indicates that reactions 7 and 8 may not be insignificant in the perfluoro ketone systems.
+
+
+
+
+
(12) K. 0. Kutschke, Can. J . Chem., in press. (13) We are indebted to a referee for discussion on this point.
