  We study the molecular abundance and spatial distribution of the simplest
sugar alcohol, ethylene glycol (EG), the simplest sugar glycoladehyde (GA), and
other chemically related complex organic species towards the massive
star-forming region G31.41+0.31. We have analyzed multiple single dish and
interferometric data, and obtained excitation temperatures and column densities
using an LTE analysis. We have reported for the first time the presence of EG
towards G31.41+0.31, and we have also detected multiple transitions of other
complex organic molecules such as GA, methyl formate (MF), dimethyl ether (DME)
and ethanol (ET). The high angular resolution images show that the EG emission
is very compact, peaking towards the maximum of the continuum. These
observations suggest that low abundance complex organic molecules, like EG or
GA, are good probes of the gas located closer to the forming stars. Our
analysis confirms that EG is more abundant than GA in G31.41+0.31, as
previously observed in other interstellar regions. Comparing different
star-forming regions we find evidence of an increase of the EG/GA abundance
ratio with the luminosity of the source. The DME/MF and EG/ET ratios are nearly
constant with luminosity. We have also found that the abundance ratios of pairs
of isomers GA/MF and ET/DME decrease with the luminosity of the sources. The
most likely explanation for the behavior of the EG/GA ratio is that these
molecules are formed by different chemical formation routes not directly
linked; although warm-up timescales effects and different formation and
destruction efficiencies in the gas phase cannot be ruled out. The most likely
formation route of EG is by combination of two CH$_{2}$OH radicals on dust
grains. We also favor that GA is formed via the solid-phase dimerization of the
formyl radical HCO, and a chemical link between MF and DME.
