2-Butyne (dimethylacetylene, crotonylene or but-2-yne) is an alkyne with chemical formula CH<sub>3</sub>Câ¡CCH<sub>3</sub>. 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 infrared spectrum leads to a determination that the torsional barrier is only 6àcm<sup>âÂÂ1</sup> (1.2àJ or 72àJ mol<sup>âÂÂ1</sup>). However, it has not been determined whether the equilibrium structure is eclipsed (D<sub>3h</sub>) or staggered (D<sub>3d</sub>). Symmetry analysis using the Molecular Symmetry Group G<sub>36</sub> shows that one would need to analyse its high resolution rotation-vibration Raman spectrum to determine its equilibrium structure. Pulsed-field-ionisation zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of 2-butyne and its fully deuterated isotopomer have been recorded and analysed.
2-Butyne (dimethylethyne) forms with 5-decyne (dibutylethyne), 4-octyne (dipropylethyne) and 3-hexyne (diethylethyne) a group of symmetric alkynes.
2-Butyne can be synthesized by the rearrangement reaction of ethylacetylene in a solution of ethanolic potassium hydroxide.
2-Butyne, along with propyne, is used to synthesize alkylated hydroquinones in the total synthesis of Vitamin E.