ChemInform Abstract: GAS‐PHASE REACTIONS OF THERMAL FLUORINE‐18 WITH PROPYNE AND 3,3,3‐TRIFLUOROPROPYNE

Abstract

The radicals formed by addition of /sup 18/F to alkyne substrates are converted to stable alkenes by reaction with a hydrogen-donor molecule such as H/sub 2/Se or HI and assayed by radiogas chromatography. The terminal/central addition ratio for /sup 18/F with CH/sub 3/C==CH is 2.6 +- 0.2, and with CF/sub 3/C==CH it is 3.7 +- 0.2. The terminal/central reaction ratio is probably controlled chiefly by the thermochemical stability of the incipient radical, since the electron densities of the ..cap alpha..- and ..beta..-carbon atoms in CF/sub 3/C==CH are quite similar. The cis/trans ratio of the isomeric CH/sub 3/CH==CH/sup 18/F molecules formed by terminal attack of /sup 18/F on CH/sub 3/C== CH is 2.1 +- 0.1, which is the thermochemical equilibrium ratio at about 220 to 250/sup 0/C. The trans/cis ratio of CF/sub 3/CH==CH/sup 18/F isomers after thermal /sup 18/F attack on CF/sub 3/C==CH is 7.1 +- 0.5, which is the thermochemical equilibrium ratio for these molecules at about 200 to 250/sup 0/C. The CX/sub 3/C==CH/sup 18/F radical precursors in each system presumably are able to isomerize until sufficient internal energy is removed, and the existing cis/trans equilibrium among the radicals is then frozen in during the remainder of the radical thermalizationmore » process. Approximately 33 +- 3% of the thermal /sup 18/F atoms react with CH/sub 3/C==CH by abstraction of H, while the remainder add to the ..pi..-bond system. Approximately 14 +- 3% of the thermal /sup 18/F atoms react by abstraction with CF/sub 3/C==CH, with the rest reacting by addition. No reverse loss of /sup 18/F from either C/sub 3/H/sub 4//sup 18/F* or C/sub 3/HF/sub 3//sup 18/F* was observed, with an upper limit on the half-stabilization pressure for each of about 50 torr of SF/sub 6/. The rate constant for thermal addition to CF/sub 3/C==CH is (6.0 +- 0.6) x 10/sup -11/cm/sup 3/ molecule /sup -1/s/sup -1/ at 10/sup 0/C, which is about (2.9 +- 0.5) times slower than addition to C/sub 2/H/sub 2/. Addition to CH/sub 3/C==CH occurs with approximately the same rate constant as addition to C/sub 2/H/sub 2/.« less