Vinyl alcohol

Tautomerization of vinyl alcohol to acetaldehyde

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. Vinyl alcohol gas isomerizes to the aldehyde with a half-life of 30 min at room temperature.

H₂C=CHOH → H₃CC(O)H

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.

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_H⁺/k_D⁺ = 4.75, k_H2O/k_D2O = 12). Deuterium stabilization can be accomplished through hydrolysis of a ketene precursor in the presence of a slight stoichiometric excess of heavy water (D₂O). Studies show that the tautomerization process is significantly inhibited at ambient temperatures (k_t ≈ 10⁻⁶ M/s), and the half-life of the enol form can easily be increased to t_1/2 = 42 minutes for first-order hydrolysis kinetics.

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.

Occurrence in interstellar medium

Vinyl alcohol was detected in the molecular cloud Sagittarius B in 2001, the last of the three stable isomers of C
₂H
₄O (after acetaldehyde and ethylene oxide) to be detected in space. Its stability in the (dilute) interstellar medium shows that its tautomerization does not happen unimolecularly, a fact attributed to the size of the activation energy barrier to the rearrangement being insurmountable at temperatures present in interstellar space. 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.