  We present detailed gas-phase chemical models for the envelope of the
low-mass star-forming region IRAS 16293-2422. By considering both time- and
space-dependent chemistry, these models are used to study both the physical
structure proposed by Schoier et al. (2002), as well as the chemical evolution
of this region. A new feature of our study is the use of a detailed,
self-consistent radiative transfer model to translate the model abundances into
line strengths and compare them directly with observations of a total of 76
transitions for 18 chemical species, and their isotopes. The model can
reproduce many of the line strengths observed within 50%. The best fit is for
times in the range of 3e3 - 3e4 yrs, and requires only minor modifications to
our model for the high-mass star-forming region AFGL 2591. The ionization rate
for the source may be higher than previously expected -- either due to an
enhanced cosmic-ray ionization rate, or, more probably, to the presence of
X-ray induced ionization from the center. A significant fraction of the CO is
found to desorb in the temperature range of 15-40 K; below this temperature,
\~90% or more of the CO is frozen out. The inability of the model to explain
the HCS+, C2H, and OCS abundances suggests the importance of further laboratory
studies of basic reaction rates. Finally, predictions of the abundances and
spatial distributions of other species which could be observed by future
facilities (e.g., Herschel-HIFI, SOFIA, millimeter arrays) are provided.
