  In recent years, organic molecules of increasing complexity have been found
toward the prolific Galactic center source Sagittarius B2. We wish to explore
the degree of complexity that the interstellar chemistry can reach in
star-forming regions. We carried out a complete line survey of the hot cores
Sgr B2(N) and (M) with the IRAM 30 m telescope in the 3 mm range. We analyzed
this spectral survey in the LTE approximation. We modeled the emission of all
known molecules simultaneously, which allows us to search for less abundant,
more complex molecules. We compared the derived column densities with the
predictions of a coupled gas-phase and grain-surface chemical code. We report
the first detection in space of ethyl formate (C2H5OCHO) and n-propyl cyanide
(C3H7CN) toward Sgr B2(N). The abundances of ethyl formate and n-propyl cyanide
relative to H2 are estimated to be 3.6e-9 and 1.0e-9, respectively. Our
chemical modeling suggests that the sequential, piecewise construction of ethyl
and n-propyl cyanide from their constituent functional groups on the grain
surfaces is their most likely formation route. Ethyl formate is primarily
formed on the grains by adding CH3 to functional-group radicals derived from
methyl formate, although ethanol may also be a precursor. The detection in Sgr
B2(N) of the next stage of complexity in two classes of complex molecule,
esters and alkyl cyanides, suggests that greater complexity in other classes of
molecule may be present in the interstellar medium. {Abridged}
