Vinyl alcohol

Vinyl alcohol
Names
Preferred IUPAC name
Ethenol
Other names
Hydroxyethene
Hydroxyethylene
Identifiers
3D model (JSmol)
ChEMBL
ChemSpider
ECHA InfoCard 100.008.350
Properties
C2H4O
Molar mass 44.053 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Vinyl alcohol, also called ethenol (IUPAC name), is the simplest enol. With the formula CH2CHOH, it is a labile compound that converts to acetaldehyde. It is not a precursor to polyvinyl alcohol.

Tautomerization of acetaldehyde to vinyl alcohol

Under normal conditions, vinyl alcohol converts (tautomerizes) to acetaldehyde:

At room temperature, acetaldehyde (H3CC(O)H) is more stable than vinyl alcohol (H2C=CHOH) by 42.7 kJ/mol:[1]

H2C=CHOH → H3CC(O)H                 ΔH298,g = −42.7 kJ/mol

The uncatalyzed keto-enol tautomerization 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 isomerizing to acetaldehyde. (Carbonic acid is another example of a substance that is kinetically 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.[2][3]

It can be formed by the pyrolytic elimination of water from ethylene glycol at a temperature of 900 °C and low pressure.[4]

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):

n CH2=CHOAc → (CH2−CHOAc)n
(CH2−CHOAc)n + n H2O → (CH2−CHOH)n + n HOAc

As a ligand

Several metal complexes are known that contain vinyl alcohol as a ligand. One example is Pt(acac)(η2-C2H3OH)Cl.[5]

The industrial synthesis of acetaldehyde ("Wacker process") proceeds via the intermediacy of a vinyl alcohol complex.[6]

Occurrence in interstellar medium

Vinyl alcohol was detected in the molecular cloud Sagittarius B.[7] Its stability in the (dilute) interstellar medium shows that its tautomerization is not unimolecular.

See also

References

  1. ↑ R.D. Johnson III. "CCCBDB NIST Standard Reference Database". Retrieved 2014-08-30.
  2. ↑ Heazlewood, B. R.; Maccarone, A. T.; Andrews, D. U.; Osborn, D. L.; Harding, L. B.; Klippenstein, S. J.; Jordan, M. J. T.; Kable, S. H. "Near-threshold H/D exchange in CD3CHO photodissociation." Nat. Chem. 2011, 3, 443−448. doi:10.1038/nchem.1052
  3. ↑ Andrews, D. U.; Heazlewood, B. R.; Maccarone, A. T.; Conroy, T.; Payne, R. J.; Jordan, M. J. T.; Kable, S. H. "Photo-tautomerization of acetaldehyde to vinyl alcohol: A potential route to tropospheric acids." Science 2012, 337, 1203−1206. doi:10.1126/science.1220712
  4. ↑ Clayden, Jonathan; Greeves, Nick; Warren, Stuart. Organic Chemistry, 2nd edition, pp. 456-57. Oxford University Press, 2012. ISBN 978-0-19-927029-3.
  5. ↑ F. A. Cotton, J. N. Francis, B. A. Frenz, M. Tsutsui "Structure of a dihapto(vinyl alcohol) complex of platinum(II)" Journal of the American Chemical Society, 1973, volume 95, p. 2483-6. doi:10.1021/ja00789a011
  6. ↑ J. A. Keith, P. M. Henry (2009). "The Mechanism of the Wacker Reaction: A Tale of Two Hydroxypalladations". Angew. Chem. Int. Ed. 48: 9038–9049. doi:10.1002/anie.200902194.
  7. ↑ "Scientists Toast the Discovery of Vinyl Alcohol in Interstellar Space". National Radio Astronomy Observatory. 2001-10-01. Retrieved 2006-12-20.
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