KIXETICSOF FLVORIKATION
Dee., 1963
The main advantage of the method over a visual technique would appear to be the objectivity in ascertaining the dew point. The sensitivity of the method-that is, the minimum amount of condensate that can be observed-is probably about the same as for a visual method. With other tracers, higher specific activity, and different geometrical arrangements it
2799
might be possible to increase the sensitivity beyond that achieved in this study. Acknowledgments.-The assistance of D. L. Haes, W. A. Stensland, and R. G. Clark in building and maintaining the apparatus is gratehliy acknowledged. Valuable discussions were had with Dr. P. Chiotti on the interpretation of the results.
KINETICS OF FLUORINATION. I. ADDITION OF FLUORINE TO 2,3-DICHLaROHEr;AFLUOROBUTEhT]E-2l BY ALANS.RODGERS Contribution No. $7’4 from She Central Research Laboratories, Minnesota Uining and Manujacturiiig Company, St. Paul, Minnesota 55119 Received July IW, 1963
+
The rate of the reaction, CF3-CCl=CCl-CFs Fz + CF3-CFCl-CFCl-CF3, was investigated in the gas phase a t temperatures between f15 and -20” with reactant concentrations between 1 and 10 X lo-* mole/l. The reaction was found to be homogeneous. Within this range of variables the rate of production of thn adduct was given by: c I [ C ~ F ~/dt C ~=~ ]1.1 X 10” exp( -12,500/RT)[C,iF~C1~]1/~[F~]a/2 mole/l. see. The reaction mas found to be inhibited by oxygen. The experimental results are interpreted in terms of a chain reaction in which initiation occurs by the bimolecular reaction CF-CCL=CCl-CF3 Fz +CF8-CFClCC1-CF, F.
+
Introduction The quantitative aspects of the kinetics and mechanism of the reaction of fluorine with organic compounds has received very little attention despite a sustained intereat in the synthesis of fluorocarbons by direct reaction with fluorine as well as by indirect methods.lb It is generally accepted that fluorine reacts with organic materials uiu a free radical mechanism. Such a mechanism consists of initiation, propagation, and termination reactions. Thus far, most attention has been centered upon the propagation reactions for substitution. The relative rates of hydrogen abstraction by fluorine atoms has been determined for several alkanes.2-6 Recently the absolute rate of reaction betwelen fluorine and carbon tetrachloride6 has been determined. The results indicated that the dissociation and recombination of fluorine mas the initiating and terminating reactions. Addition reactions, however, have thus far been ignored. This is, therefore, the first study which has been reported on the kinetics of addition of fluorine to carbon-carbon double bonds. Experimental The apparatus consisted of two parts; one part was a conventional glass vacuum system equipped with an oil diffusion pump for handling condensable organic materials; the second part was a vacuum system fabricated from 0.25 in 0.d. Monel tubing ,joined with Monel Swageloks and containing Hoke M-440 bellows valves where necessary. This part was equipped with a rough vacuum pump protected by a soda-lime trap and was used in all operations involving fluorine gas. These two systems were joined by means of a Swagelok fitting using Teflon ferrules. -__
(1) (a) This research was supported b y the Advanced Research Projects Agency under Contract NOrd 18688 and was monitored by the Bureau of Naval Weapons; (b) J. R I . Tedder, “Advances in Fluorine Chemistry,” Vol. 11, Butterwortha Publishing Co., London, 1961, pp. 104-138. ( 2 ) G. C. Fettis, J. H. Knox, and A. F. Trotman-Dickenson, J . C h e m Soc., 1064 (1960). (3) P. C. Anson, P. S. Fredricks (in part), 2nd J. M. Tedder, zbid., 918 (1959). (4) P. S. Fredricks and J. M. Tedder, ihid., 144 (1960). ( 5 ) P. D. Mercer and H. 0. Pritchard, J . Phys. Chem., 68, 1468 (1959). (6) D. T. Clark and J. M. Tedder, 2nd International Symposium on Fluorine Chemistry, July, 1962.
+
Similar fittings were used for all other metal-to-glass connections. After the metal parts were passivated with fluorine and the entire vacuum system outgassed, a pressure of 1 X mm. was readily obtained. The addition of fluorine to 2,3-dichl~rohexafluorobutene-2was followed by observing the pressure change during the reaction. This was accomplished by means of a pressure transducer (variable reluctance type) from the Pace Engineering Corp., Model CP-53. The output of the transducer, 0-5 v. over the range of 0-760 mm., was fed into a voltage divider and zero suspression circuit, then to a Sargent strip chart recorder, Model SR. The instrument was carefully calibrated with a cathetometer from 1 to 25 mm. and was accurate to =!=0.2 mm. in this range. From 50 to 700 mm. i t was accurate to i l mm. However, the slope of the curve, pressure v s . millivolts output, in this range did not vary by more than 1%. Therefore, pressure differences (i.e., rate measurements) were accurate to fO.l mm. ( A P measurements were of the order of 10 mni.). Temperature control was maintained by a constant temperature liquid bath regulated by a Bayley on-off regulator, Model 231. The reaction system consisted of two flasks, a reaction flask:and a storage flask, connected by a Hoke C-416K cam-operated diaphragm valve. The entire system was immersed in the bath liquid. Thereby, each reactant was maintained a t the reaction temperature. The reaction was started by pressure expansion from the storage flask t o the reaction flask. The dead space (approximately 8 cc.) from the reaction flask to the pressure transducer, which was a t room temperature, contained only the diluent gas. The volume of the storage flask (glass) was 505 cc. The volumes of the reaction flasks were Teflon-packed reactor 148 cc., Teflon-coated reactor 153 cc., and glass reactor 180 cc. The fluorine was obtained from Matheson Co. It was passed through a sodium fluoride scrubber and stored in a 2-1. Monel flask. The fluorine was analyzed in the following manner. To a 335432. flask containing 75 g. of Hg, 0.082 g. of gas to be analyzed was added; this corresponded to approximately 120 mm. of pressure. After the reaction with mercury was completed the residual pressure in the flask was about 0.5 mm. Thus the fluorine gas is a t least 99.5% pure. 2,3-Dichlorohexafluorobutene-2was obtained from Hooke Chemical Co., dried over PgOs,and distilled into a gas storage bulb. Gas-liquid chromatographic analysis on an 18 ft. X 0.25 in. 0.d. column packed with 30% Kel-F pentamer (Minnesota Mining and Manufacturing Co.) on firebrickat 65’ showed that the cis-trans isomer ratio was 1:9. The structure of the isomers was confirmed by infrared and n.m.r. analysis. IIexafluoroethane was obtained from E. I. duPont de Nemours and
2800
.&LAN
\
23
8. RODGERS
I
I 8
IO e\ 0
100
ZOO
300
400
500
600
700
TIME,SEC. Fig. l.-AP us. time for addition of Fzto CF3CC1:CC1CF3a t 0" in packed Teflon reactor: run 22, PcaFoClz = 5.9 mm., PrZ = 6.2 mm., P c ~ = F ~74 mm.; run 23, P C ~ F ,=C20.9 I ~ mm., P p 2 = 6.1 mm., P c ~ = F 64.5 ~ mm.; run 24, PczFeclz= 9.9 mm., P p Z = 14.6 mm., P c ~ F=, 64 mm. Co. Gas-liquid chromatographic analysis on a 9 ft. x 0.5 in. 0.d. column packed with 30y0 FC-43 (Minnesota Mining and Manufacturing Co.) on firebrick a t -20" as well as infrared analysis failed t o detect any tetrafluoroethylene in the hexafluoroethane. The former method should be sensitive at the 1 part per 1000 level. Oxygen and nitrogen were obtained from Xational Cylinder Gas Co. and were used wif,hout further purification. n'itric oxide was obtained from RIatheson Co. and purified by passage through sirupy sodium hydroxide and distillation a t -80". Prior t o each run $11 condensable materials were degassed a t liquid nitrogen temperature to a pressure between 1 x 10-3 and 1 X 10-6 mm.
Results Reaction Products.-2,3-Dichlorohexafluorobutene-2 and fluorine were found to undergo a smooth reaction a t 0" in a Teflon-coated, glass flask packed with Teflon turnings. The products of several runs were combined and separated by g.1.c. using a column with 30% Kel-F pentamer oil on firebrick. Two major peaks were eluted comprising 95% of the area (aside from unchanged olefin). These peaks had ail area ratio of 6 : 1. The more abundant peak was identified as the adduct, 2,3-dichlorooctaAuorobutaneby infrared analysis. Xass spectral analysis of the less abundant peak showed that it contained only one chlorine atom and was most probably a Cq fluorocarbon. A likely structure for this compound is C1 F
I I cy3-C-c-
butene-2. Some typical pressure-time traces which mere obtained at 0' are shown in Fig. 1. The measured pressure decrease for the reaction when allowed to reach completion mas 90-100% of the theoretical, the lower values occurring when fluorine was the limiting reactant. This is presumably due to the consumption of fluorine by the formation of 2-chlorononafluorobutane. I n Fig. 1 an inhibition period is quite prominent. Roughly 10% of the reaction occurred during this period. Rate measurements were made a t various times after the inhibition period and the concentration of reactants at these times mere calculated on the basis that the obserwd AP mas due only to the addition TABLE I THEEFFECT O F REACTANT CONCENTRATION AND TEMPERATURE UPON THE RATEOF ADDITIOS O F FZTO CFICCl : CCICFBAFTER THE INHIBITION PERIOD Rate
x T,OK. 288
I 1 1
In a like manner it mas found that the unreacted olefin still retained a cis to trans ratio of 1:9. Concentration and Temperature Dependence.-To avoid complications due to self-heating resulting from the large heat of reaction (about 120 kcal./mole), it was necessary to restrict the concentration of the reactants to 1-10 X low4mole/l., to add hexafluoroethane as a diluent, and to pack the reaction vessel with Teflon turnings (to provide additional cooling surface). Under these conditions me were able to obtain a smooth addition of fluorine to 2,3-dichlorohexafluoro-
Concn. reactant X 104 mole/l. CaF6C1z Fz CzFs
2.58 2.81 6.13
2.52 2.75 3.38
48 48 103
Rate/
106
mole/l.
[CaFsClz]'/? [Fz]'/z,
sec.
l./moie see.
2.41 2.56 5.59 Av.
=
38.6 34.4 36.8 36.5 f 2
=
34.5 31.0 32.7 f 2
284
5.09 4.75
2.03 1.70
44 44
2.24 1.49 Av.
273
3.41 2.88 2.59 11.8 11.2 4.12 3.70 3.82 10.6 9.70
3.35 2.73 2.82 3.00 2.53 6.88 6.47 6.35 6.89 6.00
42 42 44 38 38 38 38 29 35 35
1.12 10.0 0.99 12.7 0.90 11.9 1.78 10.2 1.45 10.7 4.36 11.9 4.25 13.5 4.31 13.8 6.13 10.5 5.20 11.3 Av. = 1 1 . 6 f l
264.5
4.73 4.13
4.25 3.64
44 44
1.10 5.80 0.94 6.iO Av. = 6.25 f 0 . 5
263.5
4.93 4.39 7.55 5.73
4.32 3.78 10.W 8.89
44 44 35 35
1.02 4.54 0.89 5.86 4.80 5.00 3.88 6.10 Av. = 5.38 f 0 . 6
259.5
4.08 3.59
6.19 5.09
65 05
1.20 3.83 1.04 3 . !N Av. = S.91 f O . l
263.5
3.99
8.10
44
0.76
3.86
5.06
40
0.40
1.81
3.42 3.55
4.56 12.66
49 41
0.33 1.65 Av.
2.05 1.95 1.9f0.1
CFa
F F
Vol. 67
1.65
=
reaction. Because of the formation of 2-chlorononafluorobutane (I) only rates taken during the first 25% of the reaction were used. The relevant data are given in Table I. It appeared that the order of the over-all addition reaction was '/2 with respect to the olefin and 3 / 2 n-ith respect to the fluorine. There appeared t o be 110 effect due to total pressure. These orders were confirmed by the constancy of rate/ [CJ?~C121'/'[F2Ia/'
KINETICS OF FLUORIXATION
Dec., 1963
in column 6 of Table I. The rate of reaction may be expressed by eq. 1 for any given temperature.
2801
'I-----0
The values of the over-all specific rate constant ( K ) in units of l./mole see. for various temperatures are given in column 6 of Table I. A least squares treatment of log K as a function of 1/T resulted in log K (I./moIe sec.) = 11.05
Therefore
K
=
(1.1
0.04 -
! f
2.72
* 0.1 T
0.1 X loll) exp
___
0
0 eJ
-2
B
3 (-12,500
-I
f
0
500)
20
60
80
100 120 t40 I
TIME,SEC.
RT
l./mole sec.
40
(3)
The errors quoted are standard errors. Effects of Surface and Total Pressure.-When an unpacked reaction vessel (glass or Teflon-coated glass) was used, the reaction of fluorine with 2,3-diclilorohexafluorobutene-2 still exhibited an inhibition period, but this was now followed by a small pressure increase, reaching a maximum in 10-15 see. prior to the expected pressure decay. Some typical results are shown in Fig. 2. When the concentration of both reactants was raised to 25 mm. (14.7 X low4mole/l.), a thermal ignition followed a 31-sec. inhibition period. This behavior suggested that the initial pressure increase observed in the unpacked reactor mas due to self-heating of the reaction gases. The applicable equations for a nonisothermal reaction using convective heat transfer' are
(4'
Fig. 2.-AP us. time, excluding inhibition period for reaction of Fz with CFd3Cl:CClCFz a t 0' in unpacked glass reactor: 0, run 4 Pc4F,ci2 = 5.5 mm., P F = ~ 5.7 mm., PczFa = 259.5 mm.; 0, run 12 P c ~ F=~5.5 c mm., ~~ PF= ~ 6.0 mm., P C ~ F=$87.5 mm.; A, run 13 Pc,s,oi, = 6.1 mm., P F = ~ 6.0 inm., Po2p6 = 88 mm.; -, calculated for circular data points.
I
I
v
zoo
I50
IO0
50
0
in mhich X = mole/l. of product A B
= =
X U CJ"
= =
initial concn. of olefin in mole/l. initial coricn. of fluorine in mole/l. mole/l. of total gases temperature of gas in OK. temperature of the wall in OK. herit of reaction in cal./mole coefficienl of heat transfer in cal./deg. sec. cm.2 surface axea in cm.2 heat capacity at constant volume in cal./deg. mole volume of reactor in 1.
Fig. 3.--Inhibiton times us. reciprocal of CF&Cl: CCICFl pressure a t 0' for packed and unpacked reactors: 0, packed Teflon reactor; 0 , unpacked glass reactor.
Two additional equations are obtained from the stoicliiomctry of' tlic reaction aiid thc ideal gas law.
Equations 4 through 7 were solved at 3-see. intervals by Eulers' method using Ai =: O.OG see. on an IBM 705 computer with a S.A.L.E. I1 library. The values used for the constants in eq. 4 and 5 were: K as given in eq. 3; C, = 22 cal./deg. mole [C,(C2F6) = 2121, Q = 120 kcal./mole, and aS/VC, = 12 X mole/l. sec. The value of aS/VC, was adjusted so that the calculated maximuin pressure was equal to tlic cxperiinciital maximuin. The calculated prcssurctime relation for ruii 4 is shouii in Fig. 2. IVhen OM' considers that the real rate does not achieve its maximum value a t time t = 0 because of inhibition, the agreement between the calculated and observed curves is quite satislactory. Thus, tlie kinetics aiid specific
(7) N. N. Scrnenov, "Some Problems in Chernicill Kinetics and Roactivity," Vol. 2 , Perearnon Press, Xeu York, N Y , 1939, pp. 1-10.
( 8 ) J. 8. Wicklund, H. F. Flieger, and J. F. hiasi, J . Res. A'atl. BUT.Std., 61, 91 (1953).
(2
= =
O(
=
S = C, = V =
ALANS. RODGERS
2802
rate constant for the addition of fluorine to the olefin are the same in both the packed and unpacked reaction vessels. Comparison of run 4 with runs 12 and 13 (Fig. 2) further confirms that the rate of reaction is independent of the total pressure. Inhibition Period.-A striking feature of the reaction between fluorine and 2,3-dichlorohexafluorobutene-2 is the inhibition period, which at times was even longer than the reaction itself. Results for the inhibition period are summarized in Table 11. The reproducibility of this period suggested that it was due to an impurity in one of the reactants rather than to random contamination. This was confirmed by an experiment in which the olefin was reacted with excess fluorine; after the reaction was completed additional olefin and hexafluoroethane were added and no inhibition was observed. Consequently, it was concluded that the inhibitor was present in the fluorine. Figure 3 shows that the duration of inhibition is independent of fluorine partial pressure and inversely proportional to the olefin partial pressure, as one might expect if the inhibitor were in the fluorine. Figure 3 also indicates that the inhibiting reactions are somewhat dependent upon surface, surface-to-volume ratio, or both. Since oxygen is a likely impurity in fluorine and a well-known inhibitor of free radical reactions, some experiments were made with small amounts of added oxygen. These results are given in Table I11 with run 12 added for comparison purposes. The results in Table I1 are closely fit by the equation
Po PFZ The inhibition times ( I t ) calculated by the above are 182, 274, and 369 sec. for the observed values of 180, 274, and 365 sec., respectively. IcPC4F,C1, =
1 x io3 f 8.2
x
io4*-
TABLEI1 EFFECTOF EXPERIMENTAL CONDITIONS UPON THE INHIBITION PERIODOF THE REACTIONOF FLUORINE WITH 2,3-DICHLOROHEXAFLlJOROBUTESE-2AT 0 Inhibition Run Initial pressures of reactants, mm. period, aec. Fz CZF6 no. C4FaClr
Type of reactor
74 243 Packed 5.9 6.2 64.5 62 Packed 20.9 6.1 64 114 Packed 9.9 14.6 10.0 14.3 49.5 115 Packed 58.5 55 Packed 20.1 13.8 24.7" 31 Unpacked 26 2 24.7 91 Unpacked 4.9 212 10.2 5.6 213.5 142 Unpacked 3 10.0 259 160 Unpacked 5.7 4 5.5 5 5.1 11.4 210 180 Unpacked 12 5.5 6.0 87 180 Unpacked Unpacked 71 190b 18 6.1 6.3 * XZwas used instead of CzFe; inhibition period was followed This run was made in a Teflon-lined flask. by ignition. 22 23 24 25 26 1 2
TABLE 111 EFFECT OF ADDED OXYGENUPON INHIBITION TIMEFOR REACTION OF FLUORINE WITH 2,3-DICHLOROHEXAFLTJOROBUTEXE-2 AT 0' IN AN UNPACKED GLASSREACTOR Run no.
12 10 11
Inhibition time, sec.
Added Oa, mm.
180 274
0 0 034
365
0.068
Initial pressures, CIF6Ch
nim.
5.5 5.15 5.38
6.0 6 72 5.73
F2,
Vol. 67
These results support the hypothesis that oxygen is indeed responsible for the inhibition period. The amount of oxygen in the fluorine can be estimated from the data in Table 111. Thus, 34 p of oxygen increased the inhibition time by approximately 90 sec. Run 12 had an inhibition time of 180 sec. and, therefore, about 68 p of O2 for 6.0 mm. fluorine, or about 1% oxygen. This appears high in view of the fluorine analysis. However, the oxygen was measured by pressure expansion and the absolute value of its pressure (but not the ratio of the two pressures) could be in error. When 6.1 mm. of C4FsC12,3.0 mm. of NO, and 85 mm. of C2Fs were added by expansion to 6.1 nim. of Fz, a smooth reaction mas observed to take place with no 160 measurable inhibition time (
