Pressure and Temperature Effects on Product Channels of the Propargyl (HC⋮CCH₂) Combination Reaction and the Formation of the “First Ring”

Abstract

The propargyl (HC⋮CCH2) radical is among the critical intermediates in hydrocarbon reaction systems pertinent to both the high temperatures of combustion systems and the low temperatures of planetary atmospheres. This work reports experimental results on the nature and the relative yields of the final products of the propargyl combination reaction, C3H3 + C3H3 → C6H6 (1). Propargyl radicals, for most experiments, were generated by the 248 nm excimer laser photolysis of propargyl bromide. The 193 nm photolysis of propargyl chloride and of allene were also used in a number of experiments, particularly at higher temperatures. Product studies were performed at a pressure range of 27 mbar (20 Torr) to 933 mbar (700 Torr) and at a temperature range of 295−623 K. Final reaction products were separated, identified, and quantified using an on-line gas chromatograph/mass spectrometer system. Five isomeric C6H6 final products were detected including 1,5-hexadiyne, fulvene, dimethylenecyclobutene, and benzene. The relative yields of the major reaction products showed significant pressure and temperature dependencies. Under high-pressure conditions 1,5-hexadiyne is a major product with a relative yield of 51 % at P = 933 mbar and T = 295 K. However, its yield decreases to 27 % at P = 933 mbar and T = 623 K and to 1 % at P = 27 mbar, T = 623 K. Dimethylenecyclobutene, has a relative yield of 6% at 295 K and 933 mbar. It becomes the most abundant product with a relative yield of nearly 90% at 623 K and 133 mbar. Fulvene appears to be a minor product at all conditions of this study and its relative yield (∼1.5% or less) is significantly lower than the reported computational predictions. Interestingly, an appreciable amount of benzene is also formed, particularly at lower pressures with relative yields considerably higher (43% at 27 mbar and 623 K) than those predicted computationally (∼3% at P = 27 mbar T = 650 K). The results of this work suggest that the formation of dimethylenecyclobutene and benzene from the propargyl combination reaction is significantly more efficient than previously predicted. Implications of these results on modeling of combustion processes as well as planetary atmospheric processes are discussed.

Formation of fulvene in the reaction of C2H with 1,3-butadiene

“…When only 1,3-butadiene is present, a small signal appears at m/ z = 78 that has a very slow rise time that accounts for no more than 10% of the total when both reactants are present. This signal is likely due to the recombination of C 3 H 3 radicals (m/z = 39) that are formed by 1,3-butadiene photolysis, a process that has been observed before [53], or a reaction of C 3 H 3 with 1,3-butadiene. Nevertheless, it is clear that formation of C 6 H 6 (m/z = 78) is the predominant reaction channel in the C 2 H + 1,3-butadiene reaction.…”

Photochemistry and diffusion in Jupiter's stratosphere: Constraints from ISO observations and comparisons with other giant planets

“…Propargyl‐radical recombination (R351: C 3 H 3 + C 3 H 3 + M → l ‐C 6 H 6 + M) is relatively fast [e.g., Fahr and Nayak , 2000; Atkinson and Hudgens , 1999; Morter et al , 1994; Alkemade and Homann , 1989], but the reaction preferentially forms linear C 6 H 6 isomers at laboratory pressures and temperatures of 67 mbar and 295 K [ Fahr and Nayak , 2000]. However, a recent experimental study by Howe and Fahr [2003] demonstrates that the relative yields of different C 6 H 6 isomers exhibit significant pressure and temperature dependencies. Benzene was observed to form preferentially at low pressures.…”

Formation and distribution of benzene on Titan

“…They detected two linear C 6 H 6 (1,5 -hexadiyne and 1,2 -hexadiene -5yne) with yields of 60 and 25%, respectively and a third unidentified isomer. Howe and Fahr [2003] performed product studies at a pressure range of 27 mbar to 933 mbar and at a temperature range of 295 -623 K. The same two linear and three new cyclic C 6 H 6 isomers (dimethylenecyclobutene, fulvene and benzene) were identified. Benzene was a major product particularly at low pressure ($25%).…”