  Recent interstellar detections include a significant number of molecules
containing vinyl (C2H3) and ethyl (C2H5) groups in their structure. For several
of these molecules, there is not a clear experimental or theoretical evidence
that support their formation from simpler precursors. We carried out a
systematic search of viable reactions starting from closed shell hydrocarbons
containing two carbon atoms (ethane, C2H6; ethylene, C2H4; and acetylene, C2H2)
with the goal of determining viable chemical routes for the formation of vinyl
and ethyl molecules on top of interstellar dust grains. Our results show that
both H and OH radicals are key in converting acetylene and ethylene into more
complex radicals that are susceptible to continue reacting and forming
interstellar complex organic molecules. The relevant reactions, for example OH
additions, present rate constants above 10$^{1}$ s$^{-1}$ that are likely
competitive with OH diffusion on grains. Similarly, H atom addition to
acetylene and ethylene is a very fast process with rate constants above
10$^{4}$ s$^{-1}$ in all cases, and greatly enhanced by quantum tunneling.
Hydrogen abstraction reactions are less relevant, but may play a role in
specific cases involving the OH radical. Reactions with other radicals NH2, CH3
are likely to have a much lesser impact in the chemistry of ethyl and vinyl
bearing molecules.
