Reactions of Ethynyl Radicals with Alkynes in the System Sodium Vapour/Ethynylbromide/Alkyne

Abstract

The radicals were generated from the respective halide RX by the reaction Na + RX → R + NaX in a low‐pressure flow reactor fed by a multi‐slit diffusion burner. Test results are reported for the system Na/CH3I giving k(Na + CH3I) = 6.0 · 10−3 and k(CH3 + CH3) = 2.8·1013 cm3 mol−1 s−1. The main part deals with the production of C2H in the system Na/C2HBr and its consecutive reactions with C2HBr and added C2H2 and C4H2. The rate constant k(Na + C2HBr) was found to be 2.9 · 1013 cm3 mol−1 s−1. The bimolecular rate constants of C2H with the compounds above have been determined to 1.0·1014, 2.3 · 1013 and 4.0·1013 cm3 mol−1 s−1, respectively, at a temperature of 603 K. – The main products in this reaction system are polyynes with a minor formation of ethyne and very little hydrogenated polyynes. The formation of an aromatic compound (ethynylbenzene) is negligibly small, a fact which is discussed in its relation to aromates in hydrocarbon flames. The primary reaction Na + C2HBr and its products are discussed as well as the unimolecular decomposition and stabilization of energized adducts formed by C2H.

Reactions and Kinetics of Unsaturated C2 Hydrocarbon Radicals

“…Interestingly, the value of k 5 obtained by Homann et al 36 at 600 K was 3.7 × 10 -11 cm 3 molecule -1 s -1 , in agreement with the values published earlier.…”

“…The transient intermediate, with a lifetime of about 10 -11 s to dissociation to vibrationally excited C 4 H 2 + H, is formed via a barrierless addition process followed by the dissociation. The formation of an excited C 4 H 3 * had been previously proposed by Homann et al, who considered possible deactivation to be insignificant under the experimental conditions of their particular experiment and that loss of an H-atom was most probable. In addition, no reaction products of C 4 H 3 had ever been detected.…”

“…In addition, no reaction products of C 4 H 3 had ever been detected. Interestingly, the value of k 5 obtained by Homann et al at 600 K was 3.7 × 10 -11 cm 3 molecule -1 s -1 , in agreement with the values published earlier. There is, at best, a minimal temperature dependence for reaction 5 at temperatures from about 300 to 1475 K .…”

“…Cullis et al report a room-temperature value for k 5 of the order of 10 -15 cm 3 molecule -1 s -1 , while Lange and Wagner 22 suggest a lower limit close to 5 × 10 -11 cm 3 molecule -1 s -1 at 320 K. As noted earlier, Laufer and Bass, using the vacuum ultraviolet photochemistry of C 2 H 2 , obtained k 5 = (3.1 ± 0.3) × 10 -11 cm 3 molecule -1 s -1 at room temperature by monitoring the appearance of product C 4 H 2 through its absorption in the vacuum ultraviolet. There are, however, a host of more recent data for this reaction ,− ,− that agree with a value of k 5 = 1.3 × 10 -10 cm 3 molecule -1 s -1 at room temperature.…”