2-Butyne (dimethylacetylene, crotonylene or but-2-yne) is an alkyne with chemical formula CH₃C≡CCH₃. Produced artificially, it is a colorless, volatile, pungent liquid at standard temperature and pressure.

2-Butyne is of interest to physical chemists because of its very low torsional barrier and the problem of determining that barrier using high-resolution infrared spectroscopy. Analysis of its spectrum leads to a determination that the torsional barrier is only 6 cm⁻¹ (1.2×10⁻²² J or 72 J mol⁻¹). However, it has not been determined whether the equilibrium structure is eclipsed (D_3h) or staggered (D_3d). Symmetry analysis using the Molecular Symmetry Group G₃₆ shows that one would need to analyse its high resolution rotation-vibration Raman spectrum to determine its equilibrium structure.

2-Butyne (dimethylethyne) forms with 5-decyne (dibutylethyne), 4-octyne (dipropylethyne) and 3-hexyne (diethylethyne) a group of symmetric alkynes.