  It is currently assumed that infrared dark clouds (IRDCs) represent the
earliest evolutionary stages of high-mass stars ($>$ 8 M$_{\odot}$).
Submillimeter and millimeter-wave studies performed over the past 15 years show
that IRDCs possess a broad variety of properties, and hence a wide range of
problems and questions that can be tackled. In this paper, we report an
investigation of the molecular composition and chemical processes in two groups
of IRDCs. Using the Mopra, APEX, and IRAM radio telescopes over the last four
years, we have collected molecular line data for CO, H$_2$CO, HNCO, CH$_3$CCH,
CH$_3$OH, CH$_3$CHO, CH$_3$OCHO, and CH$_3$OCH$_3$. For all of these species we
estimated molecular abundances. We then undertook chemical modeling studies,
concentrating on the source IRDC028.34+0.06, and compared observed and modeled
abundances. This comparison showed that to reproduce observed abundances of
complex organic molecules (COMs), a 0-D gas-grain model with constant physical
conditions is not sufficient. We achieved greater success with the use of a
warm-up model, in which warm-up from 10 K to 30 K occurs following a cold
phase.
