Cartoon of the temperature structure of protostellar envelopes showing where O-bearing and N-bearing species are most likely to be found in the gas. This is based on the findings of Crockett et al. (2015) for massive protostars, who showedthat higher excitation temperatures for N-bearing COMs are found towards several high-mass protostars, and van ’t Hoff et al. (2020a), who proposed a top-down carbon chemistry for nitriles. It is not yet clear whether this structure also applies to low-mass protostars.