  Comets are a rich reservoir of complex organic molecules. Ground and
space-based observatories have recently greatly enhanced the cometary molecular
inventory. Although these molecules' origin is believed to be the cometary
nucleus, they can be partially synthesised in the coma. We studied organic
molecules' nucleus versus coma origins for various initial conditions, using a
multifluid chemical-hydrodynamical model and an updated chemical network. For
the study, we considered four comets [C/1996 B2 (Hyakutake), C/2012 F6
(Lemmon), C/2013 R1 (Lovejoy), and C/2014 Q2 (Lovejoy)] due to their relatively
high activity and observation of large number organics species. We emphasised
the C-H-O and N-bearing species, including the simplest amino acid, glycine. We
discuss the formation pathways of the organics and the conditions for their
formation in the coma and find that the abundance varies from one comet to
another due to differences in the initial conditions, relative abundances of
the reactants and temperature. We compare the organic abundances when they are
present as parent volatiles to their formation solely due to gas-phase
chemistry. Their abundance purely due to the coma chemistry is moderately to
significantly lower compared to that when they are parent volatiles. However,
we find that the production rates of some of the coma-synthesised organic
molecules can reach peak values of 1e22 - 1e26 molecules/s , which is in the
realm of detection by in situ/space-based observations, and can therefore be
important considering future missions to comets. We also compare our modelled
abundances with those observed in 67P/C-G by Rosetta, which detected several
organics at a large heliocentric distance and low production rate.
