  Remote and in-situ observations of cometary gases have revealed the presence
of a wealth of complex organic molecules, including carbon chains, alcohols,
imines and the amino acid glycine. Such chemical complexity in cometary
material implies that impacts by comets could have supplied reagents for
prebiotic chemistry to young planetary surfaces. However, the assumption that
some of the molecules observed in cometary comae at millimetre wavelengths
originate from ices stored inside the nucleus has not yet been proven. In fact,
the comae of moderately-active comets reach sufficient densities within a few
thousand kilometers of the nucleus for an active (solar radiation-driven)
photochemistry to ensue. Here we present results from our latest
chemical-hydrodynamic models incorporating an updated reaction network, and
show that the commonly-observed HC3N (cyanoacetylene) and NH2CHO (formamide)
molecules can be efficiently produced in cometary comae as a result of
two-body, neutral-neutral, gas-phase reactions involving well-known coma gases.
In the presence of a near-nucleus distributed source of CN (similar to that
observed by the Rosetta spacecraft at comet 67P), we find that sufficient
HC$_3$N and NH2CHO can be synthesized to match the abundances of these
molecules in previous observations of Oort Cloud comets. The precise origin of
these (and other) complex organic molecules in cometary comae can be verified
through interferometric mapping observations, for example, using the Atacama
Large Millimeter/submillimeter Array (ALMA).
