Bimolecular kinetic studies with high-temperature gas-phase F-19 NMR: Cycloaddition reactions of fluoroolefins

A gas-phase NMR kinetic, technique has been used for the first time to obta in accurate measurements of rate constants of some bimolecular, second-orde r cycloaddition reactions. As a test of the potential use of this technique for the study of, second-order reactions, the rate constants and the activ ation parameters for the cyclodimerization reactions of chlorotrifluoroethy lene (CTFE) and tetrafluoroethylene (TFE) were determined in the temperatur e range 240-340 degreesC, using a commercial high-temperature NMR probe. Ob taining excellent agreement of the results with published data, the techniq ue was then applied to the reaction of 1,1-difluoroallene with 1,3-butadien e, the results' of which indicate that the use of gas-phase NMR for reactio n kinetics is particularly valuable when a reagent is available only in sma ll amounts and in cases where there are several competing processes occurri ng simultaneously. The major processes observed in this reaction are regios elective [2+2] and [2+4] cycloadditions, whose rates and activation paramet ers were determined [k(2)= 9.3 x 10(6) exp(-20.1 kcal mol(-1)/RT) L/mol(-1) s(-1) and k(3) = 1.2 x 10(6) exp(-18.4 kcal mol(-1)/RT) L/mol(-1) s(-1), r espectively] in the temperature range 130-210 degreesC.