Vinyl alcohol


Vinyl alcohol, also called ethenol, is the simplest enol. With the formula 2CHH, it is a labile compound that converts to acetaldehyde. It is not a precursor to polyvinyl alcohol.

Synthesis

Vinyl alcohol can be formed by the pyrolytic elimination of water from ethylene glycol at a temperature of 900 °C and low pressure.

Tautomerization of vinyl alcohol to acetaldehyde

Under normal conditions, vinyl alcohol converts to acetaldehyde:
At room temperature, acetaldehyde is more stable than vinyl alcohol by 42.7 kJ/mol:
proceeds via the intermediacy of a vinyl alcohol complex.
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 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.
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.

Relationship to poly(vinyl alcohol)

Because of the instability of vinyl alcohol, the thermoplastic polyvinyl alcohol is made indirectly by polymerization of vinyl acetate followed by hydrolysis of the ester bonds :

As a ligand

Several metal complexes are known that contain vinyl alcohol as a ligand. One example is PtCl.

Occurrence in interstellar medium

Vinyl alcohol was detected in the molecular cloud Sagittarius B. Its stability in the interstellar medium shows that its tautomerization does not happen unimolecularly.