Expanding the C

Aims. The tentative detection of 3-hydroxypropanal (HO(CH2)2C(O)H) toward the Galactic center molecular cloud G+0.693-0.027 prompts a systematic survey in this source aimed at detecting all C3H6O2 isomers with available spectroscopy. Methods. We used an ultra-deep broadband spectral survey of G+0.693-0.027, carried out with the Yebes 40 m and IRAM 30 m telescopes, to conduct the astronomical search. Results. We report the first interstellar detection of lactaldehyde (CH3CH(OH)C(O)H) and methoxyacetaldehyde (CH3OCH2C(O)H), together with the second detections (i.e., confirmation) of methyl acetate (CH3C(O)OCH3) and hydroxyacetone (CH3C(O)CH2OH), and new detections in this source of both anti - and g auche - conformers of ethyl formate (CH3CH2OC(O)H; the latter being tentative). For these species, we derived a fractional abundance relative to H2 of ~(0.81, 0.24, 16, 1.6, 1.3, 1.4) × 10−10, respectively. In contrast, neither propionic acid, CH3CH2C(O)OH, nor glycidol, c-CH2OCHCH2OH (i.e., the most and the least stable species within the C3H6O2 family, respectively) were detected, and we provide upper limits on their fractional abundances of ≤1.5 × 10−10 and ≤3.7 × 10−11. Interestingly, all C3H6O2 isomers can be synthesized through radical-radical reactions on the surface of dust grains, ultimately tracing back to CO as the parent molecule. We suggest that formation of the detected isomers is mainly driven by successive hydrogenation of CO, producing CH3OH and CH3CH2OH as the primary parent species. Conversely, propionic acid is thought to originate from the oxygenation of CO via the HOCO intermediate, which help us rationalize its non-detection. Overall, our findings notably expand the known chemical inventory of the interstellar medium and provide direct observational evidence that increasingly complex chemistry involving O-bearing species occurs in space.