2-Butyne

{{chembox

| verifiedrevid = 403365671

| Reference=[https://www.sigmaaldrich.com/US/en/product/aldrich/254339] at Sigma-Aldrich[http://webbook.nist.gov/cgi/cbook.cgi?ID=C503173 NIST Chemistry WebBook page for 2-butyne]

| Name = 2-Butyne

| ImageFile1 = H3C-C#C-CH3

| ImageSize1 = 180px

| ImageName1 = Structural formula

| ImageFile2 = 2-butyne-3D-balls-B.png

| ImageSize2 = 150px

| ImageName2 = Ball-and-stick model

| PIN = But-2-yne

| OtherNames = Dimethylacetylene
Crotonylene

|Section1={{Chembox Identifiers

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID = 9990

| PubChem = 10419

| ChEMBL_Ref = {{ebicite|correct|EBI}}

| ChEMBL = 119108

| InChI = 1/C4H6/c1-3-4-2/h1-2H3

| InChIKey = XNMQEEKYCVKGBD-UHFFFAOYAO

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChI = 1S/C4H6/c1-3-4-2/h1-2H3

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| StdInChIKey = XNMQEEKYCVKGBD-UHFFFAOYSA-N

| CASNo_Ref = {{cascite|correct|CAS}}

| CASNo = 503-17-3

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = LKE6D3018E

| SMILES = CC#CC

}}

|Section2={{Chembox Properties

| Formula = C₄H₆

| MolarMass = 54.0904 g/mol

| Density = 0.691 g/mL

| MeltingPtC = -32

| BoilingPtC = 27

}}

}}

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 infrared spectrum{{cite journal | last = di Lauro | first = C.|display-authors=etal| year = 1997 | title =The rotation-torsion structure in the ν11/ν15 (Gs) methyl rocking fundamental band in dimethylacetylene

| journal = J. Mol. Spectrosc.| volume = 184 | issue = 1

| pages = 177–185 | doi =10.1006/jmsp.1997.7321}}

leads to a determination that the torsional barrier is only 6 cm⁻¹ (1.2{{e|-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{{cite journal | last1 = Longuet-Higgins | first1 = H.C. | year = 1963 | title = The symmetry groups of non-rigid molecules | journal = Molecular Physics | volume = 6 | issue = 5| pages = 445–460 | doi = 10.1080/00268976300100501 | bibcode = 1963MolPh...6..445L | doi-access = free }}{{cite journal | author1=P. R. Bunker | year=1964

| title=The Rotation-Torsion Wavefunctions of Molecules that have two Identical Rotors

| journal=Mol. Phys. | volume=8 | page=81 | doi = 10.1080/00268976400100091}} G₃₆ 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.{{cite journal | last = Jacovella | first = Ugo|display-authors=etal| year = 2015

| title=Internal rotation, spin–orbit coupling, and low-frequency vibrations in the ground state of CH3–CC–CH+3 and CD3–CC–CD+3

| journal=Mol. Phys. | volume=113 | page=2115 | doi = 10.1080/00268976.2015.1005708| hdl=20.500.11850/182812| hdl-access=free}}

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