2-Butyne

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

${\displaystyle {\ce {H3C-C#C-CH3}}}$
Names
Preferred IUPAC name But-2-yne
Other names Dimethylacetylene Crotonylene
Identifiers
CAS Number	503-17-3 Y
3D model (JSmol)	Interactive image
ChEMBL	ChEMBL119108 Y
ChemSpider	9990 Y
ECHA InfoCard	100.007.239
PubChem CID	10419
UNII	LKE6D3018E Y
CompTox Dashboard (EPA)	DTXSID3060116
InChI InChI=1S/C4H6/c1-3-4-2/h1-2H3 Y Key: XNMQEEKYCVKGBD-UHFFFAOYSA-N Y InChI=1/C4H6/c1-3-4-2/h1-2H3 Key: XNMQEEKYCVKGBD-UHFFFAOYAO
SMILES C(#CC)C
Properties
Chemical formula	C4H6
Molar mass	54.0904 g/mol
Density	0.691 g/mL
Melting point	−32 °C (−26 °F; 241 K)
Boiling point	27 °C (81 °F; 300 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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.