The reaction of the dicarbon molecule (C-2) in its (1)Sigma(+)(g) electronic ground state with hydrogen cyanide HCN(X-1 Sigma(+)) is investigated in a crossed molecular beam setup to untangle the formation of the cyanoethynyl radicalCCCN(X-2 Sigma(+)) in hydrocarbon-rich atmospheres of planets and their moons such as Titan. Combined with electronic structure and rate theory calculations, we show that this elementary reaction is rapid, has no entrance barriers, and yields CCCN via successive rearrangements of the initial HC3N collision complex to the cyanoacetylene intermediate (HCCCN) followed by unimolecular decomposition of the latter without exit barrier. New photochemical models imply that this radical could serve as a key building block to form more complex molecules as observed in situ by the Cassini spacecraft, ultimately leading to organic aerosol particles, which make up the orange-brownish haze layers in Titan's atmosphere.