Interestingly, the unsaturated C₃ aldehyde propynal, HCCCHO, was the first one to be detected in the interstellar medium in 1988:
W. M. Irvine, R. D. Brown, D. M. Cragg, P. Friberg, P. D. Godfrey, N. Kaifu, H. E. Matthews, M. Ohishi, H. Suzuki, and H. Takeo,
A New Interstellar Polyatomic Molecule – Detection of Propynal in the Cold Cloud TMC-1
Astrophys. J. 335, L89–L93 (1988).
The three J = 2 – 1 a-type transitions around 18.5 GHz were detected with the 43 m NRAO dish. In addition, the 4_0,4 – 3_0,3 transition near 43.2 GHz was detected with the Nobeyama 45 m dish.

The detection of all three aldehydes, propynal, propenal (CH₂CHCHO) and propanal (CH₃CH₂CHO) has been reported towards Sgr B2(N):
J. M. Hollis, P. R. Jewell, F. J. Lovas, A. Remijan, and H. Møllendal,
Green Bank Telescope Detection of New Interstellar Aldehydes: Propenal and Propanal
Astrophys. J. 610, L21–L24 (2004).
Absorption as well as emission lines were detected with the 100 m GBT dish between 18 and 26 GHz. This indicates that the molecules reside in the colder and less dense envelope of Sgr B2(N). While the detection of propanal appears to be secure, only two propenal lines have been detected thus far. Therefore, the detection of propenal should be viewed with some caution.However, since propanal is suggested to be formed on grain surfaces from propynal by addition of H₂ twice via propenal, the presence of the latter species in the ISM is very plausible.

All three molecule were also reportedly detected in up to three Galactic center molecular clouds which have low dust temperatures (10–20 K), even lower rotational temperatures (around 10 K), which are moderately dense (a few tenthousand molecules per cubic centimeter), but which are kinetically moderately warm (about 100 K or more). Three or four transitions were detected with the 100 m GBT telescope between 13 and 23 GHz by
M. A. Requena-Torres, J. Martín-Pintado, S. Martín, and M. R. Morris,
The Galactic Center: The Largest Oxygen-bearing Organic Molecule Repository
Astrophys. J. 672, 352–360 (2008).
Two lines each were detected in one source, zero to two lines each in the other two sources. Some caution concerning this report may be useful.

J.-C. Loison, M. Agúndez, N. Marcelino, V. Wakelam, K. M. Hickson, J. Cernicharo, M. Gerin, E. Roueff, and M. Guélin
reported on
The interstellar chemistry of H₂C₃O isomers
Mon. Not. R. Astron. Soc. 456, 4101–4110 (2016).
Propynal was detected in the 9_0,9 – 8_0,8 transition near 83.8 GHz toward six dark clouds in addition to TMC-1 employing the IRAM 30 m telescope. The six sources are B1-b, L483, Lupus-1A, L1495B, L1521F, and Serpens South 1a.

J. M. Lykke, A. Coutens, J. K. Jørgensen, M. H. D. van der Wiel, R. T. Garrod, H. S. P. Müller, P. Bjerkeli, T. L. Bourke, H. Calcutt, M. N. Drozdovskaya, C. Favre, E. C. Fayolle, S. K. Jacobsen, K. I. Öberg, M. V. Persson, E. F. van Dishoeck, and S. F. Wampfler
reported on
The ALMA-PILS Survey: First Detections of Ethylene Oxide, Acetone and Propanal toward the Low-mass Protostar IRAS 16293–2422
Astrophys. J. 597, Art. No. A53 (2017).
This report is the first one on the detection of warm (~125 K) propanal in the ISM as well as on the detection of propanal around a low-mass protostar. The Protostellar Interferometric Line Survey (PILS) was carried out with ALMA in its Band 7 (329 – 363 GHz). A total of 44 lines were assigned to propanal.

S. Manigand, A. Coutens, J.-C. Loison, V. Wakelam, H. Calcutt, H. S. P. Müller, J. K. Jørgensen, V. Taquet, S. F. Wampfler, T. L. Bourke, B. M. Kulterer, E. F. van Dishoeck, M. N. Drozdovskaya, and N. F. W. Ligterink
reported on
The ALMA-PILS Survey: First Detection of the Unsaturated Three-carbon Molecules Propenal (C₂H₃CHO) and Propylene (C₃H₆) towards the Low-mass Protostar IRAS 16293–2422 B
Astrophys. J. 645, Art. No. A53 (2021).
This report is actually the first one on an umambiguous (i.e. multi line) detection of propenal in the ISM. It is also the first one on the detection of warm (~125 K) propenal in the ISM and on the detection of propanal around a low-mass protostar. The Protostellar Interferometric Line Survey (PILS) was carried out with ALMA in its Band 7 (329 – 363 GHz) with additional observations in Band 6. A total of 15 different unblended (with respect to a different molecule) lines were identified.

Contributor(s): H. S. P. Müller; 07, 2004; 07, 2012; 02, 2016; 01, 2017; 02, 2021.