Ethanal, CH₃CHO, also known as acetaldehyde, was among the early molecules to be detected in space. Its first detection was reported in a conference proceeding:
C. A. Gottlieb,
Detection of Acetaldehyde in Sagittarius
Molecules in the Galactic Environment eds. M. A. Gordon and L. E. Snyder, Wiley-Interscience, New York, 181–186 (1973).
The a-type Q-branch transition 1₁₀ – 1₁₁ near 1065 MHz was detected toward Sagittarius A and Sgr B2.

Soon thereafter, the observation of the 2₁₁ – 2₁₂ transition near 3195 MHz toward Sgr B2 with the Parkes 64 m telescope was reported:
N. Fourikis, M. W. Sinclair, B. J. Robinson, P. D. Godfrey, and R. D. Brown,
Microwave Emission of the 2₁₁ → 2₁₂ Rotational Transition in Interstellar Acetaldehyde
Aust. J. Phys. 27, 425–430 (1974).

Many more transitions were detected toward hot cores such as Sgr B2 or OMC-1. However, the rotational temperatures were usually quite low (50 K or even considerably lower) and suggest an extended distribution. A fairly recent mapping of the 1₁₀ – 1₁₁ transition toward Sgr B2 with the GRMT demonstrates this rather nicely:
J. N. Chengalur and N. Kanekar,
Widespread Acetaldehyde near the Galactic Centre
Astron. Astrophys. 403, L43–L46 (2003).

Ethanal has also been detected in cold clouds:
H. E. Matthews, P. Friber, and W. M. Irvine,
The Detection of Acetaldehyde in Cold Dust Clouds
Astrophys. J. 290, 609–614 (1985).
Both A and E internal rotation components of the 1₀₁ – 0₀₀ transition near 19264 MHz were detected with the NRAO 140-foot telescope toward TMC-1 and L134N.

It was also found in one of three translucent clouds (CB 17):
B. E. Turner, R. Terzieva, and E. Herbst,
The Physics and Chemistry of Small Translucent Molecular Clouds. XII. More Complex Species Explainable by Gas-Phase Processes
Astrophys. J. 518, 699–732 (1999).

A. Belloche, H. S. P. Müller, K. M. Menten, P. Schilke, and C. Comito
investigated
Complex Organic Molecules in the Interstellar Medium: IRAM 30 m Line Survey of Sagittarius B2(N) and (M)
Astron. Astrophys. 559, Art. No. A47 (2013).
They observed several transitions in the first excited torsional state. The vibrational temperature of about 150 K is higher than the rotational temperature of about 100 K. The difference was viewed as an indication of (far-) infrared pumping.

J. K. Jørgensen, H. S. P. Müller, H. Calcutt, A. Coutens, M. N. Drozdovskaya, K. I. Öberg, M. V. Persson, V. Taquet, E. F. van Dishoeck, and S. F. Wampfler
reported on
The ALMA-PILS Survey: Isotopic Composition of Oxygen-containing Complex Organic Molecules toward IRAS 16293–2422B
Astron. Astrophys. 620, Art. No. A170 (2018).
The results were obtained in the course of a line survey of the solar-type protostellar binary IRAS 16293–2422 with ALMA covering the frequency range 329.1–362.9 GHz. The authors only investigated source B. They detected for the first time both ¹³C isotopomers as well as the CH₃CDO species. The D/H ratio per H atom is about 0.08. The ¹²C/¹³C ratio is compatible with the local ISM ratio. Ethanal is among the molecules with colder rotational temperatures of around 125 K in that hot corino.

L. H. Coudert, L. Margulès, C. Vastel, R. Motiyenko, E. Caux, and J.-C. Guillemin
extended the CH₂DCHO line list considerably. Subsequently, they were able to detect this isotopolog in the PILS data as well. They estimated the column density to be slightly higher than that of CH₃CDO, as expected.
Astrophysical detections and databases for the mono deuterated species of acetaldehyde CH₂DCOH and CH₃COD
Astron. Astrophys. 624, Art. No. A70 (2019).
The observations are based on PILS.

J. Ferrer Asensio, S. Spezzano, L. H. Coudert, V. Lattanzi, C. P. Endres, J. K. Jørgensen, and P. Caselli
described the
Millimetre and Sub-millimetre Spectroscopy of Doubly Deuterated Acetaldehyde (CHD₂CHO) and First Detection towards IRAS 16293−2422
Astron. Astrophys. 670, Art. No. A177 (2023).
The observations are based on PILS. The authors determine a CHD₂CHO to CH₂DCOH ratio of ~0.21. It is also worthwhile to compare the column densities with that of the main isotopic species. This translates to a deueration degree of ~0.017 per H atom for CH₂DCOH and ~0.060 for CHD₂CHO. Such enhancement in the degree of deueration in doubly or higher deuterated isotopologs compared with sinly deuterated variants has been found commonly.

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Contributor(s): H. S. P. Müller; 11, 2009; 03, 2012; 01, 2014; 10, 2017; 01, 2019; 04, 2019; 03, 2023.

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