1748
Joseph W.
Bozzelli and M. Kaufman
ineties and Mechanisms of the Reactions of Atomic Fluorine with CF3I and CC13Br Joseph MI. Bozzelli and M. Kaufman”’ Frick Chemical laboratory, Princeton University, Princeton, New Jersey 08540 (Received February 20, 1973) Publication costs assisted by The Office of Naval Research
Using molecular beam analysis, the kinetics and mechanisms of the reactions of atomic fluorine with CF3I and CCl3Br have been investigated in a fast-flow reactor. Direct observation of perhalomethyl radicals and lack of detection of displaced halogen atoms proves conclusively that both of these reactions proceed primarily by abstraction, rather than displacement, of a halogen atom. In the CCl3Br system, successive “atom-switching” reactions rapidly convert CC13 to its fluorinated analogs. The rate constants are k(F CF3I) = 7.2 f 3.6 X 1013 cm3/mol sec and k(F CCl3Br) = 5.6 f 2.8 x 1013 cm3/mol sec.
+
Introduction The attack of an atom on an unstrained sp3-hybridized carbon usually results in the abstraction of some group from the carbon. For only a few thermal reactions, has it been proposed that the principle mechanism is displacement of a group from the carbon.2 Such displacement reactions are strongly exothermic for the reactions of atomic fluorine with perhalomethanes, and have, in fact, been observed in the hot-atom regime.3 However, it is still somewhat unclear whether displacement occurs at thermal energies in this series of reactions, especially since their rates seem to correlate with the energetics of the abstraction mechanism. The reactions of this group for which abstraction is endothermic are all found to be very slow. However, while abstraction has been indicated for F + CCl4 (at 298”K)4 and F iCCl3F (at 491-586°K),5 the displacement pathway has been strongly favored for F i CClzFz and F f CF3C1 a t flame temperatures (1500”K).6 Abstraction is exothermic for F +- CF31; this reaction has been found to be very rapid, and evidence has been obtained that it proceeds by the abstraction mechanism.? In the present work, we investigate the reactions of atomic fluorine with CFJ and CC13Br, using an instrument which is capable of direct observation of transient intermediates.$ Detection of CX3 radicals and lack of detection of displaced halogen atoms proves conclusively that these reactions follow primarily the abstraction pathway, which is exothermic in both cases. The room temperature rate constants of these two reactions are measured. Experimental Section The reactions of atomic fluorine with CFJ and CC13Br are studied by molecular beam analysis, a new method, in which molecular beam techniques are employed to preselect the neutral species entering a mass spectrometer.8,s With the beam measurements helping to identify the neutral parent of each ion observed in the mass spectrometer, there is no need to carefully control the energy of the ionizing electrons, in order to achieve “appearance potential discrimination.” Thus 100-250 mA of ionizing electrons can be employed, giving ionization efficiencies several orders of magnitude larger than those generally attained in free-radical mass spectrometry. The use of phase sensitive detection of the modulated beam and total pressures of the order of 10-9 Torr in the ionization region allows very The Journal of Physicai Chemistry, Voi. 77, No. 14, 1973
+
low concentrations of transient intermediates to be detected by this method. In this work, we have employed only the magnetic deflection mode of molecular beam a n a l y ~ i s , which ~ . ~ consists of blocking the entrance to the mass spectrometer with an obstacle, and then using a small permanent (hexapolar) magnet to deflect particles into the ionization region. Paramagnetic atoms and free radicals are effectively “focused” by such a magnetic lens, while diamagnetic molecuIes are hardly affected by the field. The observation of net magnetic focusing at any m l e in the mass spectrum indicates that a t least a few per cent of these ions arise from paramagnetic neutrals, since the focusing effect must overcome the decrease in signal resulting from blockage of some of the scattered beam by the permanent magnet. Except for addition of liquid nitrogen cryopumping to the detector chamber, the molecular beam analyzer was physically as described previously.8 An additional change consists of maintaining the exit electrode of the Weisstype ionizerlo at ca. 300-V negative with respect to the grids and anode. This has proved to be a more efficient means of collecting ions than the application of a similar positive voltage to the entrance electrode. The F f CF31 and F f CCl3Br reactions were studied in a fast flow system a t total pressures of ca. 1 Torr. Atomic fluorine was generated by a microwave discharge in dilute (0.1%) mixtures of F2 in He or Ar, using an alumina discharge tubell and alumina, Teflon or Tefloncoated 19-mm diameter flow tubes. The perhalomethanes were added through a Teflon inlet, whose position could be varied with respect to the 0.1-0.2-mm diameter sampling orifice of the molecular beam analyzer. In such systems, no products of the reaction of atomic fluorine with surfaces could be observed mass spectrometrically. Both reactions were found to be very rapid. In order to achieve the flow speeds of 20-50 m/sec necessary for spacial resolution of the reaction in convenient ranges of concentrations, it was necessary to modify the flow reactor used in ref 4 by increasing the diameter of the tubing in the trapping system t o 5 cm and using only a single trap a t liquid N2 temperature. Since F2 is not removed by such a trap, the oil in the Welch 1397 pump was changed every 3 days. Gas flow rates are determined by timing pressure increases in calibrated volumes or volume changes at known
1749
Reactions of Atomic Fluorine with CF31 and CC13Br pressures. Halocarbon oil (Halocarbon Products Corp., Series 13-21) manometers are employed for these measurements. The very small flows of the perhalomethanes are controlled by a Granville-Phillips Series 203 variable leak valve. Pressure is measured with a McLeod gauge and generally varies less than 10% across the reaction zone. The average pressure is used to convert flow rate measurements to partial pressures. Initiation of the discharge reduces the m / e 38 (Fz+)signal to less than 3% of its original intensity. Simultaneously, m / e 19 (F+)increases greater than threefold and shows 25-30% magnetic focusing, indicating that FZis efficiently dissociated by the discharge. This result is obtained with as little as 10 W of 2450-MHz power. However, measurements are generally made with 40 W, a t which power it is somewhat easier to stabilize the discharge and minimize the radiation reflected from the Evenson-type cavity.12 The almost complete dissociation of Fz observed here contrasts with dissociations of 20-40% achieved in our earlier F CC14 studies,4 where the low rate constant required the use of high atomic fluorine concentrations and low flow velocities. Since the microwave power per input Fz molecule is similar in the two experiments, the difference must undoubtedly be due to the greater importance of atomic fluorine recombination in the CC14 case. In ROSner's work,ll intermediate (ca. 1%) concentrations of F:! were employed and 78% dissociation was attained. Absolute concentrations of atomic fluorine are determined by titration with Hz. However, the interpretation of the titration curves is somewhat different from that apCC14 ~ t u d i e s due , ~ to the higher flow propriate to our F rates and lower Fz concentration in the present work. The fall off of the magnetically focused m / e 19 (atomic fluorine) signal, as HZ is added to the gas stream, is shown for three titrations in Figure 1. Although the initial decrease of this signal is linear with added Hz, some tailing can be observed in the latter part of the titration curve. This results from the fact that the titration reaction F+HZ - H F f H (1) does not have sufficient time to go to completion while the gas is flowing the 20 cm from the HZ inlet to the sampling orifice. Corrections for this effect, which becomes relatively more important toward the end point of the titration, are made by using the known rate constant of reaction 1, as illustrated in Appendix I. The corrected titration curves are linear to the equivalence point. In the current work, regeneration of atomic fluorine by the reaction
0.4
0.2
\
'.
+
std cm3
.
&\
\ \
B
4
2
n2/ sec
a
x 103
Figure 1, Three typical titrations of atomic fluorine with molecular hydrogen. T h e solid curve is through the experimental points; the dased line is corrected for incompleteness of the titration reaction by the method given in Appendix I. TABLE I: Comparison of Atomic Fluorine Titration with Total F2 Input Atomic fluorine
+
H S F 2 -3 H F + F (2) is negligible, due to the low Fz concentration and the short time available for this reaction, which has been reported to be somewhat slower than reaction l . l 3 Table I shows that the atomic fluorine flow measured by HZ titration corresponds to dissociation of between 80 and 100% of the added Fz. Considering the other indications that Fz is e€ficiently dissociated under these conditions, we feel that the titration values can be trusted to within %lo%. Ion intensities a t m / e 196 (CF3I+) and 163 (CCIzBr+) were taken as proportional to the concentrations of CF31 and CClaBr, respectively. The latter compound has a very low parent ion intensity, which is generally characteristic of totally halogenated alkanes. At each inlet position, the ratio of the ion signals with the discharge on to that with
1
! \
Pressure, Torr 0.79 0.90 1.14 0.90 1.13 0.92 0.92
0.78 0.85 1.16 0.84 0.82
flow (by
F2 input
titration),
(flowmeter), std cc/sec
cc/sec x 103
std
F flow
x 103
2 X Fa input
8.84
5.05
4.90 3.70 8.47 1 .a4 1 .a4 3.24 4.30 4.18 6.38 3.37 5.28
2.47
0.88 0.99
2.27 4.63 1.08
0.81
2.01 2.60
0.91 0.85 0.84 0.81 0.83
2.10
1.oo
3.26 1.97
0.98 0.86 0.94
10.9
2.80
i t off was assumed equal to the fraction of the perhalomethane unreacted a t that position. All the gases used in these experiments are purchased from Matheson Gas Products, except for CFsI, which i s obtained from Peninsular Chemresearch, Inc. Helium and hydrogen are ultrapure grade and are used without further purification. Argon was dried by passing it through a P205 trap. Fluorine was passed through a NaF trap to remove HF. The molecular beam analyzer indicated that it contained less than 1%of air impurity. CF31 was used without further purification. CCl3Br was subjected to several cycles of freezing, pumping, and thawing, and stored in a 3-1. bulb. In some experiments it was diluted with He to improve the mixing a t the inlet.
Results and Discussion F + CFJ Reaction Mechanism. In the F + CFJ reaction, magnetic focusing is observed for the ions listed in Table 11. The paramagnetic neutral parent of CF3+ is unThe Jaurnaiof Physical Chemistry. Vol. 77, No. 14, 1973
1750
Joseph W. Bozzelli and M. Kaufman TABLE I I : Magnetic Focusing in the F 4- CF31 Reaction % magnetic focusing*
Ion CF3+ CF2+
Relative focused signal*
Paramagnetic netural parent
0.025 0.127 1 .oo
CF3 CF3
1.3 6.6 28.5
F+
F
a [Increase in intensity when magnet is lowered (direct beam blocked by obstacle)/direct beam] X 100. blncrease in ion signal when magnet is lowered/increase at m/e 19 (at same multiplier voltage and amplifier gain).
TABLE 111: Rate Data for F 4- CF31
Run
Pressure, Torr
1 2
1.40 0.98 1.03 0.90 0.90 1.14 1.14 1.14 0.91 1.13
3
i
6
i
disfsnce
4
8
5
< C P l
Figure 2. Linear least-squares fit to data for the F 4- CFsl reaction: Upper curve, run 3; middle curve, run 4; lower curve, r u n 1; linear flow velocities 2690, 2940, and 4300 cm/sec, respectively (see Table I l l ) . doubtedly the CF3 radical. This species probably also produces the observed focusing at CFz+, due to dissociative ionization by the 70-110-V electrons used in the ionizer. CFz could not cause this focusing, since it has a singlet ground state14 and there is no reasonable mechanism for its formation in this system. The focusing at CFz+ is considerably larger than that a t CF3 +, indicating that the CF3 radical, much like all perhalomethane molecules, shows a pronounced tendency to fragment one carbon-halogen bond when it is ionized. Thus, when the direct beam is monitored, initiation of the reaction results in a marked increase in the CFz+ peak and a decrease in the CF3+ peak. This latter effect can also be observed in Leipunskii’s data.7 No magnetic focusing is detected for the I+ ion in this system, and the defocusing observed when the magnet is lowered is typical of that obtained for ions from diamagnetic species. The presence of CF3 radicals and absence of detectable concentrations of atomic iodine proves that this reaction proceeds primarily by iodine abstraction
F + CFJ
-
CF,
+ IF
AH
= -11
f 5 kcal/moll5
6 7 8 9 10
[He1
[F]o
X 10 ,
X 10’1,
mol/cm3 mol/cm3 8.2 5.7 6.0 5.3 5.3
6.7 6.7 6.7 5.3
6.6
3.4 7.3 7.4 4.5 4.5 3.2 3.2 3.2 7.8 1.5
[CF~IIO k X X IO”, cm3/mol mol/cm3
seca
1.38
8.1 7.6 4.0 5.4
6.00 1.20 1.60 0.48
5.1
0.50
5.9
0.92
10.0
1.05
8.9 6.4 10.6
1.15 0.81
= 7.2 X I O i 3 cm3/mol sec.
from successive reactions of IF with atomic fluorine and residual Fz in the system. F + CF3I. Rate Constant. The data for ten kinetic runs CF31 reaction are given in Table 111. Atomic for the F fluorine was always in excess, but its concentration could not be made large enough to be considered constant, without having the reaction proceed inconveniently fast. However, the ratio of CF3I consumed to atomic fluorine consumed was consistently measured to be 1.0, allowing the rate equation to be expressed in terms of a single concentration variable, the amount of either reagent that has reacted (x)
+
dx / d t = h&[CFJl, x)C[F]o - X ) (5) Integrating this expression by the method of partial fractions gives
PI
In ___
!CF,II
= ([Flo
- [CF,I],,)k,t
+ In [CF,IIo ‘F1fl
I _ _
(6)
Figure 2 shows that plots of the kinetic data have this form. Each set of data is fit by a linear least-squares analrather than by displacement ysis, and the rate constants so determined are given in Table 111. The average rate constant is 7.2 X 1013 cm3/ F CFJ --+ CF, I AH = -69 f 8 kcal /moll5 mol sec, with a standard deviation of the mean of 0.7 X (4) 1013 cm3/mol sec. No obvious dependence of the meaA similar inference was made by Leipunskii, et a t . , 7 ’ b a ~ e d sured rate constant on the initial concentration of F or CF31, or on the ratio of these concentrations, can be disupon the decrease in I+ and increase in IF+ upon initiation of the reaction. However, our conclusion is based cerned. Although the random scatter of the results is quite upon direct observation of CF3, rather than assumptions regarding fragmentation patterns and the importance of small, there are difficulties involved in measuring very large rate constants in a flow system such as ours, which subsequent steps in the mechanism. introduce numerous possibilities for systematic errors. With fast flows, the only new ion produced by the reaction is I F + . However, a t slower flows IFz+, IFa+, IF4+, Probably the most important of these is a somewhat IFs+, and CzF,+ appear in the mass spectrum. These ions poorly defined flow pattern for a reaction that approaches completion in a distance of three-five tube diameters, no doubt arise from recombination of CF3 radicals and
+
(3)
+
The Journalof Physical Chemistry, Vol. 77, No. 14, 1973
Reactions of Atomic Fluorine with
and CC13Br
CF3I
TABLE IV: Magnetic Focusing in the F
1751
+ CC13Br Reaction
1.0-
0 9-
ion
% magnetic focusing=
Relative focused signal0
Paramagnetic neutral parent
1.2 4.1 5.5 5.5 19.2
0.03 0.04
cc13
28.5
1.o
cc13+ CCIZ+ CCIF+ CFzf CI+ F+
0.8-
6.7-
CC13, (CC12F) CCIzF, (CCIFp) CFzCI, CF3 GI, CC13, CCIzF, CClFa F
0.05 0.04 0.24
0.6-
IF1 -
cF1o
See corresponding footnotes to Table Ii.
0 4-
with possibilities of incomplete mixing and radial concentration gradients. However, the good fit of the experimental data to the theoretical form tends to indicate that these effects are not of overwhelming importance. Making allowance for errors in the titration (*lo%), errors introduced by the pressure drop (
