View source for Vinyl alcohol

{{Distinguish|ethanol|ethynol}}
{{Chembox
| verifiedrevid = 470630576
| Name = Vinyl alcohol
| PIN = Ethenol
| OtherNames = Hydroxyethene<br>Hydroxyethylene<br>Ethylenol
| ImageFileL1 = Ethenol-2D.png
| ImageClassL1 = skin-invert 
| ImageSizeL1 = 120px
| ImageNameL1 = Structural formula of ethenol
| ImageFileR1 = Ethenol-3D-balls.png
| ImageClassR1 = bg-transparent  
| ImageSizeR1 = 130px
| ImageNameR1 = Ball-and-stick model of ethenol
|Section1 = {{Chembox Identifiers
| InChI = 1S/C2H4O/c1-2-3/h2-3H,1H2
| InChIKey1 = IMROMDMJAWUWLK-UHFFFAOYSA-N
| InChI1 = 1S/C2H4O/c1-2-3/h2-3H,1H2
| CASNo_Ref = {{cascite|correct|CAS}}
| CASNo = 557-75-5
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = GO1N1ZPR3B
| PubChem = 11199
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 10726
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = IMROMDMJAWUWLK-UHFFFAOYSA-N
| SMILES = OC=C
| ChEMBL_Ref = {{ebicite|correct|EBI}}
| ChEMBL = 76101
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C2H4O/c1-2-3/h2-3H,1H2
}}|Section2={{Chembox Properties
| Formula = C<sub>2</sub>H<sub>4</sub>O
| MolarMass = 44.053 g/mol
| Density =
| MeltingPt =
| BoilingPt =
}}
| Section3 = {{Chembox Related
| OtherCompounds = [[Allyl alcohol]]
}}
}}

'''Vinyl alcohol''', also called '''ethenol''' (IUPAC name; not ethanol) or '''ethylenol''', is the simplest [[enol]]. With the [[chemical formula|formula]] {{chem2|auto=1|CH2CHOH}}, it is a labile compound that converts to [[acetaldehyde]] immediately upon isolation near room temperature.<ref name=March>{{March6th|page=101}}</ref> It is not a practical precursor to any compound.

==Synthesis==
Vinyl alcohol can be formed by the pyrolytic [[elimination reaction|elimination]] of water from [[ethylene glycol]] at a temperature of 900&nbsp;°C and low pressure. Such processes are of no practical importance.<ref>{{cite book|last1 = Clayden|first1 = Jonathan|authorlink1 = Jonathan Clayden|last2 = Greeves|first2 = Nick|last3 = Warren|first3 = Stuart|title = Organic Chemistry|edition = 2nd|pages = 456–57|publisher = [[Oxford University Press]]|year = 2012|isbn = 978-0-19-927029-3}}</ref>

==Tautomerization of vinyl alcohol to acetaldehyde==
Under normal conditions, vinyl alcohol converts ([[tautomer]]izes) to [[acetaldehyde]]:
:[[File:Ethenol–ethanal tautomérisation.svg|250px]]{{Clear left}}

At room temperature, acetaldehyde ({{chem2|H3CC(O)H}}) is more stable than vinyl alcohol ({{chem2|1=H2C=CHOH}}) by 42.7 kJ/mol.<ref>{{cite web|author=R.D. Johnson III|title=CCCBDB NIST Standard Reference Database.|url=https://cccbdb.nist.gov/ |access-date=2014-08-30}}</ref> Vinyl alcohol gas isomerizes to the aldehyde with a half-life of 30 min at room temperature.<ref name=March/>

:{{chem2|1=H2C=CHOH -> H3CC(O)H}} 

[[File:WackerCycleKeith&HenryImp.png|thumb|232px|The industrial synthesis of [[acetaldehyde]] ([[Wacker process]]) proceeds via the intermediacy of a vinyl alcohol complex.<ref name=K&H>{{cite journal|first1 = J. A.|last1 = Keith|first2 = P. M.|last2 = Henry|journal = [[Angew. Chem. Int. Ed.]]|title = The Mechanism of the Wacker Reaction: A Tale of Two Hydroxypalladations|year = 2009|volume = 48|issue = 48|pages = 9038–9049|doi = 10.1002/anie.200902194|pmid = 19834921}}</ref>]]

The uncatalyzed [[keto–enol tautomerism]] by a 1,3-hydrogen migration is forbidden by the [[Woodward–Hoffmann rules]] and therefore has a high activation barrier and is not a significant pathway at or near room temperature. However, even trace amounts of acids or bases (including water) can catalyze the reaction. Even with rigorous precautions to minimize adventitious moisture or proton sources, vinyl alcohol can only be stored for minutes to hours before it isomerizes to acetaldehyde. ([[Carbonic acid]] is another example of a substance that is stable when rigorously pure, but decomposes rapidly due to catalysis by trace moisture.)

The tautomerization can also be catalyzed via photochemical process. These findings suggest that the keto–enol tautomerization is a viable route under atmospheric and stratospheric conditions, relevant to a role for vinyl alcohol in the production of organic acids in the atmosphere.<ref>{{cite journal|last1 = Heazlewood|first1 = B. R.|last2 = Maccarone|first2 = A. T.|last3 = Andrews|first3 = D. U.|last4 = Osborn|first4 = D. L.|last5 = Harding|first5 = L. B.|last6 = Klippenstein|first6 = S. J.|last7 = Jordan|first7 = M. J. T.|last8 = Kable|first8 = S. H.|title = Near-threshold H/D exchange in CD<sub>3</sub>CHO photodissociation|journal = [[Nature Chemistry]]|year = 2011|volume = 3| issue=6 |pages = 443−448|doi =10.1038/nchem.1052| pmid=21602858 | bibcode=2011NatCh...3..443H }}</ref><ref>{{cite journal | last1 = Andrews | first1 = D. U. | last2 = Heazlewood | first2 = B. R. | last3 = Maccarone | first3 = A. T. | last4 = Conroy | first4 = T. | last5 = Payne | first5 = R. J. | last6 = Jordan | first6 = M. J. T. | last7 = Kable | first7 = S. H. | year = 2012 | title = Photo-tautomerization of acetaldehyde to vinyl alcohol: A potential route to tropospheric acids | journal = [[Science (journal)|Science]] | volume = 337 | issue = 6099 | pages = 1203–1206 | doi = 10.1126/science.1220712 | pmid = 22903524 | bibcode = 2012Sci...337.1203A | s2cid = 42079807 }}</ref>

Vinyl alcohol can be stabilized by controlling the water concentration in the system and utilizing the kinetic favorability of the [[deuterium]]-produced [[kinetic isotope effect]] (''k''<sub>H<sup>+</sup></sub>/''k''<sub>D<sup>+</sup></sub> = 4.75, ''k''<sub>H<sub>2</sub>O</sub>/''k''<sub>D<sub>2</sub>O</sub> = 12). Deuterium stabilization can be accomplished through hydrolysis of a [[ketene]] precursor in the presence of a slight stoichiometric excess of [[heavy water]] (D<sub>2</sub>O). Studies show that the tautomerization process is significantly inhibited at ambient temperatures ( ''k''<sub>t</sub> ≈ 10<sup>−6</sup> M/s), and the [[half-life]] of the enol form can easily be increased to ''t''<sub>1/2</sub> = 42 minutes for first-order hydrolysis kinetics.<ref>{{cite journal |title= Investigations into the Chemistry of Thermodynamically Unstable Species. The Direct Polymerization of Vinyl Alcohol, the Enolic Tautomer of Acetaldehyde |first1= Anna K. |last1= Cederstav |first2= Bruce M. |last2= Novak |journal= [[Journal of the American Chemical Society]] |year= 1994 |volume= 100 |issue= 9 |pages= 4073–4074 |doi= 10.1021/ja00088a051}}</ref>

==Relationship to poly(vinyl alcohol)==
Because of the instability of vinyl alcohol, the [[thermoplastic]] [[polyvinyl alcohol]] (PVA or PVOH) is made indirectly by polymerization of [[vinyl acetate]] followed by [[hydrolysis]] of the [[ester]] bonds (Ac = acetyl; HOAc = acetic acid):<ref name=Ullmann>{{Ullmann |first1=Manfred L. |last1=Hallensleben| name-list-style = vanc |title=Polyvinyl Compounds, Others |year=2000 |doi=10.1002/14356007.a21_743}}</ref>
:{{chem2|1=''n'' CH2=CHOAc -> (CH2\sCHOAc)_{''n''} }}
:{{chem2|(CH2\sCHOAc)_{''n''} + ''n'' H2O -> (CH2\sCHOH)_{''n''} + ''n'' HOAc}}

==As a ligand==
Several [[coordination complex|metal complexes]] are known that contain vinyl alcohol as a [[ligand]]. One example is Pt(acac)(η<sup>2</sup>-C<sub>2</sub>H<sub>3</sub>OH)Cl.<ref>{{cite journal | last1 = Cotton | first1 = F. A. | authorlink1 = F. Albert Cotton| last2 = Francis | first2 = J. N. | last3 = Frenz | first3 = B. A. | last4 = Tsutsui | first4 = M. | year = 1973 | title = Structure of a dihapto(vinyl alcohol) complex of platinum(II) | journal = [[Journal of the American Chemical Society]] | volume = 95 | issue = 8 | pages = 2483–6 | doi = 10.1021/ja00789a011 }}</ref>

==Occurrence in interstellar medium==
Vinyl alcohol was detected in the [[molecular cloud]] [[Sagittarius B2|Sagittarius B]] in 2001, the last of the three stable [[isomer]]s of {{chem|C|2|H|4|O}} (after acetaldehyde and [[ethylene oxide]]) to be detected in space.<ref>{{cite press release|title = Scientists Toast the Discovery of Vinyl Alcohol in Interstellar Space|publisher = [[National Radio Astronomy Observatory]]|date = October 1, 2001|url = https://www.nrao.edu/pr/2001/vinylalco/|access-date = December 20, 2006|first = Charles|last = Blue}}</ref><ref name = SpaceVA /> Its stability in the (dilute) [[interstellar medium]] shows that its tautomerization does not happen [[molecularity|unimolecularly]],<ref name = SpaceVA>{{cite journal|journal = [[The Astrophysical Journal]]|volume = 561|issue = 2|pages = L207–L210|year = 2001|title = Microwave Detection of Interstellar Vinyl Alcohol, {{chem|CH|2|{{=}}CHOH}}|first1 = B. E.|last1 = Turner|first2 = A. J.|last2 = Apponi|doi = 10.1086/324762|doi-access = free}}</ref> a fact attributed to the size of the [[activation energy]] barrier to the rearrangement being insurmountable at temperatures present in interstellar space.<ref name = ChemPhysChem2021 />  The vinyl alcohol to acetaldehyde rearrangement is the only keto-enol tautomerisation to have been detected in deep space, induced by the provision of secondary electrons from [[galactic cosmic rays]].<ref name = ChemPhysChem2021>{{cite journal|journal = [[ChemPhysChem]]|title = Interstellar Enolization-Acetaldehyde ({{chem|CH|3|CHO}}) and Vinyl Alcohol ({{chem|H|2|CCH(OH)}}) as a Case Study|first1 = N. Fabian|last1 = Kleimeier|first2 = Ralf I.|last2 = Kaiser|year = 2021| volume=22 | issue=12 | pages=1229–1236 |doi = 10.1002/cphc.202100111| pmid=33913232 | s2cid=233447839 }}</ref>

==References==
{{Reflist}}

{{Molecules detected in outer space}}
{{Alcohols}}
[[Category:Enols]]
[[Category:Vinyl compounds]]
[[Category:Organic compounds with 2 carbon atoms]]