  We present the results of a spectral-line survey of the W51e1/e2 star-forming
region at 68-88 GHz. 79 molecules and their isotopologues were detected, from
simple diatomic or triatomic molecules, such as SO, SiO, and CCH, to complex
organic compounds, such as CH$_3$OCH$_3$ or CH$_3$COCH$_3$. A number of lines
that are absent from the Lovas list of molecular lines observed in space were
detected, and most of these were identified. A significant number of the
detected molecules are typical for hot cores. These include the neutral
molecules HCOOCH$_3$, CH$_3$CH$_2$OH, CH$_3$COCH$_3$ etc, which are currently
believed to exist in the gas phase only in hot cores and shock-heated gas. In
addition, vibrationally excited C$_4$H and HC$_3$N lines with upper-level
energies of several hundred Kelvins were found. Such lines can arise only in
hot gas with temperatures on the order of 100 K or higher. Apart from neutral
molecules, various molecular ions were also detected. Some of these
(HC$^{18}$O$^+$, H$^{13}$CO$^+$, and HCS$^+$) usually exist in molecular clouds
with high visual extinctions. Potential formation pathways of complex organic
molecules (COMs) and of hydrocarbons, along with nitriles, are considered.
These formation routes are first discussed in the context of laboratory
experiments elucidating the synthesis of organic molecules in interstellar ices
in cold molecular clouds, followed by sublimation into the gas phase in the hot
core stage. Thereafter, we discuss the predominant formation of hydrocarbons
and their nitriles in the gas phase through bimolecular neutral-neutral
reactions.
