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Pressure and Temperature Effects on Product Channels of the Propargyl (HC=CCH2) Combination Reaction and the Formation of the First-Ring



P -. Howe, Askar Fahr


Propargyl (HCidentical with}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 (400 Torr) and at a temperature range of 295 K to 623 K. Final reaction products were separated, identified and quantified using an on-line gas chromatography/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. At high pressure conditions 1,5-hexadiyne is a major product with a relative yield of 51 % at P=933 mbar and T=295K. However, its yield decreases to 27% at P=933 mbar and T=623 K and to 1 % at P=27 mbar, T=623K. Dimethylenecyclobutene, has a relative yield of 6 % at 295K 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 (difference of} 1.5 % or less) is significantly lower than the reported computational predictions. Interestingly, an appreciable amount of bezene is also formed, particularly at lower pressures with relative yields considerably higher (43 % at 27 mbr and 623K) than those predicted computationally (difference of} 3 % at P=27 mbar T=650K). 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.
Journal of Physical Chemistry A
No. 45


1, 5-hexadiyne, benzene, C<sub>3</sub>H<sub>3</sub>, combination reaction, dimethylenecyclobutene, fulvene, products, propargyl, radicals


Howe, P. and Fahr, A. (2003), Pressure and Temperature Effects on Product Channels of the Propargyl (HC=CCH<sub>2</sub>) Combination Reaction and the Formation of the First-Ring, Journal of Physical Chemistry A (Accessed June 15, 2024)


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Created October 31, 2003, Updated October 12, 2021