  Advances in infrared and submillimeter technology have allowed for detailed
observations of the molecular content of the planet-forming regions of
protoplanetary disks. In particular, disks around solar-type stars now have
growing molecular inventories that can be directly compared with both
prestellar chemistry and that inferred for the early solar nebula. The data
directly address the old question whether the chemistry of planet-forming
matter is similar or different and unique relative to the chemistry of dense
clouds and protostellar envelopes. The answer to this question may have
profound consequences for the structure and composition of planetary systems.
The practical challenge is that observations of emission lines from disks do
not easily translate into chemical concentrations. Here, we present a
two-dimensional radiative transfer model of RNO 90, a classical protoplanetary
disk around a solar-mass star, and retrieve the concentrations of dominant
molecular carriers of carbon, oxygen and nitrogen in the terrestrial region
around 1 AU. We compare our results to the chemical inventory of dense clouds
and protostellar envelopes, and argue that inner disk chemistry is, as
expected, fundamentally different from prestellar chemistry. We find that the
clearest discriminant may be the concentration of CO$_2$, which is extremely
low in disks, but one of the most abundant constituents of dense clouds and
protostellar envelopes.
