Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2pi) radical with C2H2, C2H4, C3H4 (methylacetylene and allene), C3H6 (propene) and C4H8 (trans-butene).

The reactions of the CH radical with several unsaturated hydrocarbons C2H2 (acetylene), C2H4 (ethylene), C3H4 (methyl-acetylene and allene), C3H6 (propene) and C4H8 (trans-butene) were studied at room temperature, in a low-pressure fast-flow reactor. CH(X2pi, v = 0) radicals were obtained from the reaction of CHBr3 with potassium atoms. The overall rate constants at 300 K are CH + C2H2: (3.6 +/- 0.6) x 10(-10), CH + C2H4: (3.1 +/- 0.6) x 10(-10), CH + C3H4 (methyl-acetylene): (3.4 +/- 0.6) x 10(-10), CH + C3H4 (allene): (3.6 +/- 0.6) x 10(-10), CH + C3H6 (propene): (4.2 +/- 0.8) x 10(-10) and CH + C4H8 (trans-butene): (4.0 +/- 0.80) x 10(-10) cm3 molecule(-1) s(-1) (errors are cited at the level of +/- 1 sigma). Absolute atomic hydrogen production was determined by vacuum ultra-violet (VUV) resonance fluorescence, H production from the CH + CH4 reaction being used as a reference. Observed H branching ratios for these CH reactions were: C2H2: 0.90 +/- 0.08, C2H4: 0.94 +/- 0.08, C3H4 (methyl-acetylene): 0.98 +/- 0.08, C3H4 (allene): 0.97 +/- 0.08, C3H6 (propene): 0.78 +/- 0.10, C4H8 (trans-butene): 0.69 +/- 0.12 (errors are cited at the level of +/- 1 sigma). A compilation of the available kinetic data on these reactions has been made in order to propose rate coefficients for each possible channel of the different reactions for astrochemical models.