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@ Copyright, 1972, by the Ameriean Chemical Society
VOLUME 76, NUMBER 24 NOVER,IBER 23, 1972
Fluorine-18 Atoms with Ethylene] J
Ronald L. Williams and F. S. Rowland"
Department of Chemistry, University of California, Pivine, California 92664 (Received June 26, 1972j Publication costs assisted by Division of Research, U . S . Atomic Energy Commission
Energetic l8F atoms formed by the 19F(n,Zn)'8F nuclear reaction can undergo both hot arid thermal addition reactions with ethylene to form excited CH2WCHz*radicals. The hot reactions of ISFc a n be almost entirely suppressed by moderation with SF6or CF4, and the excited CH21*FCHz*radicals exhibit normal st'ahilizationB) behavior us. pressure for monoenergetically excited radicals. The decomposition (to CH18F=CH2 stabilized radicals are observed as CH3CHa18Fafter reaction with scavenger H I . The half-stabilization pressures for SFe and CF, are 80 and 135 Torr, respectively, and are essentially unchanged for moderator fractions >0.75. The difference in half-stabilization pressures indicates that SFe is more efficient, per molecule, in removing the excitation energy of CH21BFCH2*radicals. I n less-moderated systems, hot reactions are &so observed: (a) the formation of CH18F=CH2 by a more energetic route; (b) the formation of CH18F from secondary decomposition of CH18F=CH2; and (c) the formation of CBPF by decomposition of CK21aFC1112*. The rupture of the C-C bond in CH218FC112*probably occurs predominantly after capture of I8F :it high-impact parameters with consequent high-angular momentum in the excited radical. Similarly, substitution of l8F/H a t high-impact, parameters is postulated to lea,d to the C - C bond rupture in CH1BF==CB2*.
+
Introduction Study of the reactions of atomic fluorine with various olefins has not progressed very rapidly, largely because of the experimental difficulties involved with the handling of Fz,EIF, and other highly reactive fluorinated reactants and protlucts. Valuable information concerning the c1emer.t ary processes involved in the reactions of atomic fluorine with ethylene has been obtained as a by-product t o the use of C2H4as a scavenger for 18F atoms formed by the I9F (y,n)I8F reaction in CF4.2,a In this work, the IcF atoms which failed t o react with CF4 were effectivel:y removed by addition to ethylene, as in (1). "F
+ G'w2=GHz
-d-
CH2"FCHZ"
(1)
The subsequent fate of these excited CHz18FCHz*radicals was then determined by a pressure-dependent competition betn een collisional stabilization (eq 2 ) and decomposition by hydrogen atom loss (eq 3).
+
+ +H
CHz18FCHz* nir +CH218FCHz &I CR,'SFCE-I,* --+CR'8F-CH2
(2)
(3)
The stabilized CHz18FCHz radical was then detected after reaction with scavenger molecular I, as CH218FCH21,as in (4). CRz'*FCHz
+
I2
--s CW21TCHZI
4- P
(4)
The pressure dependence of the cHl8F=@H2/CH2I8FCH2Iratio in excess CF, further showed that the excited CHzlsFCH** radicals were almost uniformly monoenergetic, indicating that the radicals were excited chiefly by the exothcrmicity of the leF atom addition with small or negligible additional contribution from extra translational energy of the I8F atom. A similar F atom addition-plus-decomposition mechanism has been invoked in explanation of Gz]E13F as a product from the photolysis of OXF in the presence of C2W4.4 Very (1) This research was supported by A.E.C. Contract No. AT(04-3)-34, Agreement No. 126. (2) N. Colebourne, J. F. J. Todd, and E . Wolfgang, "Chemical Effects of Nuclear Transformations," Val. 1, International Atomic Energy Agency, Vienna, 1965, p 149.
(3) J. F. J. Todd, N. Colebourne, and R. Wolfgang, J . P h y s . Chem., 71, 2875 (1967). (4) A. L. Flores and B. deB. Darwent, ibid.,73, 2203 (1969).
R. L. WILLIAMS AND F. S.
3510 recent studies with crossed beams of F atoms (from thermal dissociation of Fz)and CzH4 or CZD4 have convincingly shown that the substitution of F for H (or D) proceeds through a long-lived intermediate, CZH4F (or C2D4F).5 The relative kinetic energy of F atoms and ethylene molecules averaged only 1.98 and 1.94 kcal/ mol, respectively, with CzH4 and CzD4 in these experiments. We have recently reported the results of some experiments on fluorine atom additions to olefins and acetylene, using '*F atoms generated a t tracer levels by and obtained some the nuclear reaction I9F(n,2n)18F,e-8 information on reactions with ethylene in the process. We have now ca,rried cut a more extensive study of the reactions of "F atoms with ethylene with careful attention to the effects of several parameters. First, we hsve utilized H I as the scavenger molecule instead of 1 2 , because of the much greater ease of radio gas chroma,tographic determination of its scavenged radical product, CMaCJ3218F, than of the iodo compound formed in (4). 111 general, our measurements of the CHZ"FCHZ
+- HIE --+ CH2'8FCH3 + I
(5)
yields for reaction with ethylene in HI-scavenged excess CF4 are in excellent agreement with the earlier Iz-scavenged experiments. Secondly, subsequent investigations of the chemistry of energetic '*F atom reactions with CF4 have shown the presence of appreciable amounts of CF'*F indicative of (a> extensive decomposition of excited CF318F* m o l e c ~ l e an ~ , ~as ~ ~a ~corollary (b) that CF, is not quite as inert a reactant as originally believed. Accordingly, most of our experiments have been carried out with 6F6 as the nearly jnert) source of '*F atoms. Approximately 1% of the 18Fatoms formed in SFG react while translationally "hot" to form SF5I8F,and the remainder are available for reaction a t much lower energies with minor components mixed with the SF6moderator.6-B Finally, we have been concerned with CHz=CH2 as the primary substrate, and have made measurements in excess GH,, thereby permitting observation of both thermal and hot reactions with the substrate molecule.
E x p ~ r ~ ~Seetion ~~tal A detailed discussion of most of the experimental aspects of IEF pecoil chemistry as conducted in this laboratory has recently been Our present experiments have been carried out with gas samples in quite rknilar manner, and only a brief summary is given here. As i.1 eadier systems, the sample irradiation temperature could only be 10" because of the present design characteristics of the target area of the fast neutron generator. The 18F atoms were formed by the l9F(n,2n)'T nuclear reactior: on the fluorinated gaseous substrates SFs or CF4 during the irradiation of 15-ml glass ampoules with fast neutrons produced by a Kaman A711 The JaurnaE of Phyaical Chemistry, Vol. 76,N o . 34, 1972
neutron generator. The total absolute I8Fproduction from SFe or CF4 was evaluated with an accuracy of f10% using the external Teflon-sleeve monitor system. Loss of 18Ffrom the gas phase by recoil into the walls of the glass bulbs is unimportant (
