A radical route to interstellar propylene formation
  Complex organic molecules, such as propylene (CH3CHCH2), are detected in
molecular clouds (such as TMC1) with high fractional abundances (~2.10^{-9},
relative to hydrogen) that cannot be explained by gas-phase chemical reactions
under normal dark cloud conditions. To obtain such high abundances requires an
efficient grain-driven chemistry to be operating, coupled with effective
desorption of the complex organics back into the gas-phase. We propose that the
mechanism that we have previously described; rapid high density gas-phase
chemistry in the gas released following sudden, total, ice mantle sublimation -
can explain the high abundances, without recourse to ill-defined surface
chemical pathways. Our model provides a natural explanation for why it is that
some sources harbour high abundances of propylene, whilst others do not; based
on the age and level of dynamical activity within the source (which affects the
ice composition) and the chemical composition of the ambient gas.
