Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense   clouds: a case of dust grain chemistry
  We present an extensive review of gas phase reactions producing methylacetylene and propene showing that these relatively abundant unsaturated hydrocarbons cannot be synthesized through gas-phase reactions. We explain the formation of propene and methylacetylene through surface hydrogenation of C3 depleted onto interstellar ices, C3 being a very abundant species in the gas phase. 
Kevin M. Hickson, Valentine Wakelam, Jean-Christophe Loison
astro-ph.GA physics.chem-ph