Vinyl alcohol

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. 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. Vinyl alcohol can be formed by the pyrolytic elimination of water from ethylene glycol at a temperature of 900 °C and low pressure. 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: 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 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. 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): Several metal complexes are known that contain vinyl alcohol as a ligand. One example is Pt(acac)(η2-C2H3OH)Cl. Vinyl alcohol was detected in the molecular cloud Sagittarius B. Its stability in the (dilute) interstellar medium shows that its tautomerization does not happen unimolecularly.