The planar propargyl radical CH₂CCH was discovered by Agúndez and co-workers and reported in 2021. The molecule was detected toward the molecule-rich source TMC-1 using the Yebes 40m telescope. In the initial study, a cluster of hyperfine components of the 2_0,2–1_0,1 rotational transition of CH₂CCH were identified. In 2022, Agúndez and co-workers reported additional observations using the IRAM 30m telescope of ten hyperfine components of the 5_0,5–4_0,4 rotational transition to confirm the detection.

Agúndez et al. noted that propargyl radicalis very abundant toward TMC-1 and is likely to serve as a precursor to benzene and other compounds. Zhao and co-workers demonstrated that two propargyl radicals can react to form benzene and three other C₆H₆ isomers. Byrnes and co-workers performed modeling studies of possible formation pathways to CH₂CCH under interstellar conditions.

The propargyl radical has unpaired electron character on both of the terminal carbon atoms of its three-carbon backbone, as shown in the orbital plot below. While much of the character is on the planar carbon to which two hydrogen atoms are bonded, some of it is also on one of the acetylenic carbon atoms. That means that hydrogen can add to either of these two carbon atoms, yielding CH₃CCH (methyl acetylene = propyne) or H₂CCCH₂ (propadiene = allene). The coupling diagrams show that there is resonance between two ways that the π bonding can occur.