Ethyl cyanide, C₂H₅CN, aka propanenitrile, is a fairly prototypical hot core molecule. It was detected first toward OMC-1 and Sgr B2:
D. R. Johnson, F. J. Lovas, C. A. Gottlieb, E. W. Gottlieb, M. M. Litvak, P. Thaddeus, and M. Guélin,
Detection of Interstellar Ethyl Cyanide
Astrophys. J. 218, L370–L376 (1977).

More recently, however, it has also been detected toward a low-mass star-forming region:
S. Cazaux, A. G. G. M. Tielens, C. Ceccarelli, A. Castets, V. Wakelam, E. Caux, B. Parise, D. Teyssier,
The Hot Core around the Low-mass Protostar IRAS 16293-2422: Scoundrels Rule!
Astrophys. J. 593, L51–L55 (2003).

Ethyl cyanide in its two lowest vibrational states v₁₃ ≡ v_b = 1 (in plane CCN bend; ca. 206.5 cm^–1) and v₂₁ ≡ v_t = 1 (torsion; ca. 212.7 cm^–1) has been detected in a 3, 2, and 1 mm line survey carried out with SEST:
E. Gibb, A. Nummelin, W. M. Irvine, D. C. B. Whittet, and P. Bergman,
Chemistry of the Organic-Rich Hot Core G327.3–0.6
Astrophys. J. 545, 309–326 (2000).
This account apparently did not receive the appropriate attention. Later, observations toward Sgr B2(N-LMH) were reported:
D. M. Mehringer, J. C. Pearson, J. Keene, and T. G. Philips,
Detection of Vibrationally Excited Ethyl Cyanide in the Interstellar Medium
Astrophys. J. 608, 306–313 (2004).

These states as well as higher vibrational states up to v₁₂ = 1 at 770 K were detected toward Orion KL by
A. M. Daly, C. Bermúdez, A. López, B. Tercero, J. C. Pearson, N. Marcelino, J. L. Alonso, and J. Cernicharo,
Laboratory Characterization and Astrophysical Detection of Vibrationally Excited States of Ethyl Cyanide
Astrophys. J. 768, Art. No. 81 (2013).
They were also detected in Sagittarius B2(N) by
A. Belloche, H. S. P. Müller, K. M. Menten, P. Schilke, and C. Comito:
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).

The three ¹³C isotopic species have been reported first in observations toward Orion IRc2 by
K. Demyk, H. Mäder, B. Tercero, J. Cernicharo, J. Demaison, L. Margulès, M. Wegner, S. Keipert, and M. Sheng,
Isotopic Ethyl Cyanide ¹³CH₃CH₂CN, CH₃¹³CH₂CN, and CH₃CH₂¹³CN:\\ Laboratory Rotational Spectrum and Detection in Orion
Astron. Astrophys. 466, 255–259 (2007).
They were also detected in Sagittarius B2(N) by
H. S. P. Müller, A. Belloche, K. M. Menten, C. Comito, and P. Schilke,
Rotational Spectroscopy of Isotopic Vinyl Cyanide, H₂C=CH–C≡N, in the Laboratory and in Space,
J. Mol. Spectrosc. 250, 319–325 (2008).

The detection of the ¹⁵N isotopolog (in Orion KL) was reported first by
L. Margulès, R. Motiyenko, K. Demyk, B. Tercero, J. Cernicharo, M. Sheng, M. Weidmann, J. Gripp, H. Mäder, and J. Demaison,
Rotational Spectrum of Deuterated and ¹⁵N Ethyl Cyanides: CH₃CHDCN and CH₂DCH₂CN and of CH₃CH₂C¹⁵N
Astron. Astrophys. 493, 565 (2009).
The isotopolog was also detected in Sagittarius B2(N2), the northern, secondary hot-core of Sgr B2(N), by
A. Belloche, H. S. P. Müller, R. T. Garrod, and K. M. Menten,
Exploring Molecular Complexity with ALMA (EMoCA): Deuterated Complex Organic Molecules in Sagittarius B2(N2)
Astron. Astrophys. 587, Art. No. A91 (2016).

A. Belloche, H. S. P. Müller, R. T. Garrod, and K. M. Menten
were
Exploring Molecular Complexity with ALMA (EMoCA): Deuterated Complex Organic Molecules in Sagittarius B2(N2)
Astron. Astrophys. 587, Art. No. A91 (2016).
They report the tentative detection of deuterated ethyl cyanide in the course of their 3 mm line survey with ALMA. Their ratios of the normal species to the one deuterated at the out-of-plane methyl group position and at the methylene group are about 2000, the upper limit to the one deuterate at the in-plane methyl group position is 4000, consistent with the other positions which occur twice each. The number of clean lines was so small that the authors refrained from claiming a firm detection.

L. Margulès, A. Belloche, H. S. P. Müller, R. A. Motiyenko, J.-C. Guillemin, R. T. Garrod, and K. M. Menten,
reported on the
Spectroscopic Study and Astronomical Detection of Doubly ¹³C-Substituted Ethyl Cyanide
Astron. Astrophys. 590, Art. No. A93 (2016).
The astronomical detection was made in Sagittarius B2(N2) in the course of their 3 mm line survey with ALMA.

Contributor(s): H. S. P. Müller; 07, 2004; 03/04 2007; 10, 2010; 06, 2016