  Methyl cyanide (CH3CN) and propyne (CH3CCH) are two molecules commonly used
as gas thermometers for interstellar gas. They are detected in several
astrophysical environments and in particular towards protostars. Using data of
the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30m single-dish
telescope, we constrained the origin of these two molecules in the envelope of
the source. The line shape comparison and the results of a radiative transfer
analysis both indicate that the emission of CH3CN arises from a warmer and
inner region of the envelope than the CH3CCH emission. We compare the
observational results with the predictions of a gas-grain chemical model. Our
model predicts a peak abundance of CH3CCH in the gas-phase in the outer part of
the envelope, at around 2000 au from the central star, which is relatively
close to the emission size derived from the observations. The predicted CH3CN
abundance only rises at the radius where the grain mantle ices evaporate, with
an abundance similar to the one derived from the observations.
