ALANS. RODGERS
3400
cystine. The data in this laboratory indicate that the reaction rate of H atoms with methionine was lower than with cystine (Table II).) Sulfur atoms in cystine and methionine must react with the radiolytic H atoms extremely rapidly. This would be expected, since some sulfur compounds, including cystine, are known as good biological radioprotectors. The aromatic group present in some amino acids also scavenges H atoms very rapidly, and the presence of various substituents on the ring affect these reaction rates. It was found upon introduction of an OH group on phenylalanine to form tyrosine that the rate is increased. This is consistent
with other measurements, indicating that the addition of the OH group to the aromatic moiety increases the reaction rate with both H atoms and OH radicalsn8 I n summary, the rate constants of several selected amino acids with radiolytically produced H atoms were measured. The rate constants and the relative effects of the structure and functional groups present on the side chain of the amino acid group were qualitatively discussed. However, valid mathematical analysis of the data concerning the observed effects and the parameters which affect abstraction reactions will require :t more extensive and systematic study.
Kinetics of Fluorination. 111. The Unimolecular Decomposition of Chemically Activated sec-2,3-DichloroperfluorobutylRadicals102 by Alan S. Rodgers Thermodynamics Research Center, Department (Received February 14,1968)
of
Chemistry, Texas A & M University, College Station, Texas 77848
The mechanism for the formation of 2-chloroperfluorobutane observed in the fluorination of 2,3-dichloroperfluorobutene-2 (near-stoichiometricmixtures) has been investigated and has been found to result from the ks hot-radical decomposition of the sec-butyl radical. CF,CCl=CClCF, -p F 3 CF3CFClCC1CF3*-+ CFaCCl= CFCF, dl and CF,CCl=CClCF, -t CF&FC1CClCF3*ke(M_) CF3CFC1CClCF3. Under the assumption that deactivation occurs at each collision, ks = 1.45 x 109 sec-l. Analysis of the isomer ratio of CFaCF= CClCF, permitted ICs to be further broken down into ks" = 5.4 x lo8 sec-' and ks' = 9.1 x lo8 sec-l. These results are shown to be in accord with the quantum-statistical theory of unimolecular reactions of Marcus and Rice, if all internal degrees of freedom are taken as active.
+
+F
F
Introduction I n previous publication^,^*^ kinetic data on the addition of fluorine to perfluoroolefins was presented and interpreted by a mechanism schematically represented by F
>C=C
c=c< + F --t -c--C< I F
I
-C--C< I
The Journal of Physical Chemistry
I I
-C-CI
1
+F
I
(4)
The fluorination of perfluorobutene-2 was found to be adequately described by this mechanism, with respect to both kinetics and product^.^ On the other hand, the detection of an appreciable quantity of 2chloroperfluorobutane in the products resulting from the reaction of stoichiometric mixtures of trans-2,3dichloroperfluorobutene-2 and fluorine indicated a more complex mechanism for this reaction., In an
(2)
F F
+ Fz
I
~-C--C< -+ termination
(3)
(1) This research was supported by the Advanced Research Projects Agency under Contracts NOrd 18688 and NOW64-0399-0, monitored by the Bureau of Naval Weapons. (2) Work done at 3M Co., St. Paul, Minn. (3) A. S. Rodgers, J . Phys. Chem., 67, 2799 (1963). (4) A. 8. Rodgers, ibid., 69, 264 (1965).
KINETICSOF FLUORINATION
3401
effort to minimize the effects of such side reactions, only rate data taken in the initial portions of the reaction were considered. However, for such a treatment to meet with any degree of success, it is necessary that the ignored reactions result in neither a net increase nor a net decrease of chain carriers. Should either of these stipulations not be met, the interpretation of the kinetic data would be open to some doubt. For this reason, it waE; of interest to investigate the origin of the 2-chloroperfluorobutane in this reaction. Further interest for this investigation was generated from the conclusion that chlorine abstraction by fluorine atoms from the adduct, eq 5, must be very slow compared with eq 2, as lit is surely 10 kcal/mol endothermic and
+F +
CF3CFCICFCICF3
CF3CFC1cFCF3
+ FC1
(5)
possibly 20 kcal/mol. The value of DH" (C-C1) in CF3CFC1CFC1CF3 certainly lies between the extremes of and DH"(CzC15-Cl) = DHo(CF3-C1) = 84 kcal/m01~,~ 73 kcal/moL7 This, combined with DH"(C1-F) = 61 kcal/mol, leads to A H ( 5 ) 2 10 kcal/mol. Clark and Tedders have concluded that the reaction
CX314
+ I;, -*
ccl3
+ FC1
is very fast, with IC = 2 x 10'0 l./mol sec a t 300°K and an estimated activation energy of 3 kcal/mol. However, this conclusion is now untenable, as DH"(CC13Cl) = 74.5 kcal/molg and, therefore, E* 2 AH300 = 13.5 kcal/mol. An alternative mechanism, and the one which proved correct, is that the chemically activated sec-butyl radical, formed in step 2, decomposes to yield 2-chloroperfluorobutene-2. This is subsequently fluorinated to yield 2-chloroperfluorobutane. Such examples of chemical activation or hot-radical reactions have been noted in the literature with increasing frequency in the last few years and have been recently reviewed by Rabinovitch and Flowers.9b
Experimental Section The apparatus has been described in detail previously.a The reaction vessels used in this work were 495- and 1065-ml round-bottom glass flasks. Each vessel was conditioned by exposure to 100-200 torr of fluorine prior to use initially and reconditioned after any exposure to the atmosphere. The diluent gas in all experiments was hexafluoroethane. Prior to each experiment, all condensable materials weire degassed at liquid N2 temperatures. The gases were allowed to react for 40-45 min, the time required for 95-99% completion; then the reactantproduct mixture was condensed into a 100-ml (nominal) flask fitted with a greaseless West Glass Teflon plug. The sample was flash evaporated, was mixed convectively, and then was stored (1 hr to a few days) for analysis. The samples were analyzed by glpc, using a 12-ft X 0.25-jn, stainless steel column packed with a 33
wt % ethyl ester of Kel-F Acid 8114 (334 Co.) on Chromasorb P (column A) with an F & h4 Model 500 chromatograph equipped with a disk integrator. Immediately before analysis, t h e b t 5% of the sample was discarded and then 25-ml aliquots at autogenous pressure were analyzed. In several cases duplicate analyses were made. The good agreement between these indicated that a homogeneous sample had been obtained. The reaction products were determined by comparison of retention times (relative to reactant) with authentic samples on two different glpc columns: column A, described above, and column B, a 10-ft x 0.25-in. stainless steel column packed with 33 wt % Kel-F grease on Chromasorb P. The relative sensitivity factors (relating peak area to partial pressure) for CF3CFClCFClCF3 and CFaCF= CClCF3 were determined to be 0.97. Consequently, it was felt that little error was introduced by assuming the relative sensitivities of all materials to be 1.00. Reactants. The fluorine was obtained from the Matheson Cooand was passed through an NaF scrubber and was stored in a 2-1. h/Ionelflask. A 100-torr X 300cm3 aliquot was treated with 10 ml of Hg. The pressure of the residual gases was less than 0.5 torr and the composition, obtained by mass spectrometric analysis, was found to be: 02,7.2%; Nz,88.6%; COz, 1.0%; and SiFa, 1.7%. Hexafluoroethane, obtained from E. I. du Pont de Nemours and Co. was used as the diluent in all experiments. Analysis by glpc on a 24-ft X 0.25-in. column containing 33% Kel-F tetramer oil (31% Co.) on Celite indicated an impurity peak a t the
