  Much attention has been placed on the dust distribution in protostellar
envelopes, but there are still many unanswered questions regarding the
structure of the gas. We aim to start identifying the factors that determine
the chemical structure of protostellar regions, by studying and comparing
low-mass embedded systems in key molecular tracers. The cold and warm chemical
structures of two embedded Class 0 systems, IRAS16293 and VLA1623 are
characterized through interferometric observations. DCO+, N2H+ and N2D+ are
used to trace the spatial distribution and physics of the cold regions of the
envelope, while c-C3H2 and C2H from models of the chemistry are expected to
trace the warm (UV-irradiated) regions. Both sources show a number of striking
similarities and differences. DCO+ consistently traces the cold material at the
disk-envelope interface, where gas and dust temperatures are lowered due to
disk shadowing. N2H+ and N2D+, also tracing cold gas, show low abundances
towards VLA1623, but for IRAS16293, the distribution of N2D+ is consistent with
the same chemical models that reproduce DCO+. c-C3H2 and C2H show different
spatial distributions for the two systems. For IRAS16293, c-C3H2 traces the
outflow cavity wall, while C2H is found in the envelope material but not the
outflow cavity wall. In contrast, toward VLA1623 both molecules trace the
outflow cavity wall. Finally, hot core molecules are abundantly observed toward
IRAS16293 but not toward VLA1623. We identify temperature as one of the key
factors in determining the chemical structure of protostars as seen in gaseous
molecules. More luminous protostars, such as IRAS16293, will have chemical
complexity out to larger distances than colder protostars, such as VLA1623.
Additionally, disks in the embedded phase have a crucial role in controlling
both the gas and dust temperature of the envelope, and consequently the
chemical structure.
