  Hydrogen addition and abstraction reactions play an important role as surface
reactions in the buildup of complex organic molecules in the dense interstellar
medium. Addition reactions allow unsaturated bonds to be fully hydrogenated,
while abstraction reactions recreate radicals that may undergo radical-radical
recombination reactions. Previous experimental work has indicated that double
and triple C--C bonds are easily hydrogenated, but aldehyde -C=O bonds are not.
Here, we investigate a total of 29 reactions of the hydrogen atom with
propynal, propargyl alcohol, propenal, allyl alcohol, and propanal by means of
quantum chemical methods to quantify the reaction rate constants involved.
  First of all, our results are in good agreement with and can explain the
observed experimental findings. The hydrogen addition to the aldehyde group,
either on the C or O side, is indeed slow for all molecules considered.
Abstraction of the H atom of the aldehyde group, on the other hand, is among
the faster reactions. Furthermore, hydrogen addition to C--C double bonds is
generally faster than to triple bonds. In both cases, addition on the terminal
carbon atom that is not connected to other functional groups is easiest.
Finally, we wish to stress that it is not possible to predict rate constants
based solely on the type of reaction: the specific functional groups attached
to a backbone play a crucial role and can lead to a spread of several orders of
magnitude in the rate constant.
