J. Phys. Chem. 1981, 85, 89-94
89
Gas-Phase Reactions of Thermal Fluorine-18 with Propyne and 3,3,3-Trifluoropropyne Colman Concannon and F. S. Rowland' Department of Chemistty, Universiiy of California, Ifvine, California 92717 (Received: July IO, 1980)
The reactions of thermal 18Fatoms have been studied with propyne and with 3,3,3-trifluoropropyne.Energetic 18Fatoms have been formed by the 19F(n,2n)18Fnuclear reaction in gaseous SF6 and moderated to thermal energies by multiple nonreactivecollisions with SFB. The radicals formed by addition of 18Fto alkyne substrates are converted to stable alkenes by reaction with a hydrogen-donor molecule such as HzSe or HI and assayed by radiogas chromatography. The terminal/central addition ratio for 18Fwith CH3C=CH is 2.6 f 0.2, and with CF3C=CHit is 3.7 f 0.2, each over wide variations in pressure, moderator ratio, and scavenger/substrate ratio. The terminal/central reaction ratio is probably controlled chiefly by the thermochemical stability of the incipient radical, since the electron densities of the a-and ,&carbon atoms in CF3C=CH are quite similar. The cis/trans ratio of the isomeric CH3CH=CH18F molecules formed by terminal attack of 18Fon CH3C=CH is 2.1 f 0.1, which is the thermochemical equilibrium ratio at about 220-250 "C. The trans/& ratio of CF3CH=CH18F isomers after thermal 18F attack on CF3C=CH is 7.1 f 0.5, which is the thermochemical equilibrium ratio for these molecules at about 2W250 "C. The CX3C=CHlsF radical precursorsin each system presumably are able to isomerize until sufficient internal energy is removed, and the existing cisltrans equilibrium among the radicals in then "frozen in" during the remainder of the radical thermalization process. Approximately 33 f 3% of the thermal 18Fatoms react with CH,C=CH by abstraction of H, while the remainder add to the a-bond system. Approximately 14 f 3% of the thermal 18Fatoms react by abstraction with CF3C=CH, with the rest reacting by addition. No reverse loss of 18Ffrom either C3H418F*or C3HF318F*was observed, with an upper limit on the half-stabilizationpressure for each of about 50 torr of SFs. The rate constant for thermal addition to CF3C=CH is (6.0 f 0.6) X cm3molecule-l s-l at 10 "C, which is about (2.9 f 0.5) times slower than addition to C2H2. Addition to CH3C=CH occurs with approximately the same rate constant as addition to CzH2.
Introduction The gas-phase addition of thermal halogen atoms to a-bonded molecules has furnished examples of relatively indiscriminate attack, as with F-atom addition to propene,1,2and strong selectivity, as found with C1 addition to propynea3 In the latter instance, addition to the terminal position of propyne is favored over the central position by about a factor of 8, while the CH3C=CHC1 radical formed by terminal addition overwhelmingly leads to cisCH3CH=CHC1 in reaction with a hydrogen-donor molecule such as HI. In our present experiments, we have made an extensive study of the addition of thermal fluorine atoms to propyne and to 3,3,3-trifluoropropyne in order to provide further information about the selectivity of such halogen-atom addition reactions. An exploratory experiment had earlier shown that under some conditions three isomers of C3H5Fwere found in appreciable yields from F-atom addition to propyne with a hydrogen-donor molecule present.2 No measurements have previously been reported for F-atom addition to CF3C=CH, which provides a test of the influence of polar substituents upon the central/terminal addition process with alkynes. Our experiments have utilized the radioactive tracer technique in which fast neutron irradiation of SF6forms 18Fby the 19F(n,2n)18Fnuclear reaction, and the 18Fatoms are then thermalized by multiple collisions with SFB.17294-7
When the mole fraction of SF6exceeds 0.9, most 18Fatoms react in these systems by thermal processes with the minor components. The expected reactions of thermal 18Fwith propyne and 3,3,34rifluoropropyne include both terminal and central addition, and abstraction, as in (1) to (3). "F CX3CrCH CX3C=CH18F* (1)
--
+ 18F+ C X 3 C ~ C H 18F+ CX3C=CH
CX3C18F=CH*
H18F + C3X3
(3) The subsequent reactions of the highly excited radicals formed in (1)and (2) can include (a) collisional stabilization, as in (4),loss of 18F,as in (5), and loss of H, as in (6).
-- ++
+
(4)
18F C3X3H
(5)
C3H18FX3*+ M
C3H18FX3 M
C3H18FX3*
H C318FX3 (6) In the presence of a hydrogen-donor molecule such as HI or H2Se,the stabilized radicals from (2) plus (4)will form the 2-fluor0 isomer by reaction 7, while those from (1) plus CX3C18F=CH + HY CX3C18F=CHz + Y (7) C3H18FX3*
-
CX,C=CH'*F
+
HY
-7
+
=c
H
(1) R. L. Williams, R. S. Iyer, and F. S. Rowland, J.Am. Chem. SOC., 94, 7192 (1972). (2) F. S. Rowland, F. Rust, and J. P. Frank, ACS SvmD. - - Ser., No.66. 26 (1978). (3) F. S. C. Lee and F. S. Rowland, J. Phys. Chem., 84, 1876 (1980). (4) T. Smail, G. E.Miller, and F. S. Rowland, J. Phys. Chem.,74,3464 (1970). (5) T. Smail, R. S. Iyer, and F. S. Rowland, J. Am. Chem. SOC.,94, 1041 (1972). (6) R. L. Williams and F. S. Rowland, J. Am. Chem. SOC.,94, 1047 (1972). (7) R. L. Williams and F. S. Rowland, J.Phys. Chem., 76,3509 (1972).
(2)
Y
(8)
I
