4-Vinylphenol is an organic compound with the formula C<sub>2</sub>H<sub>3</sub>C<sub>6</sub>H<sub>4</sub>OH. It is the most studied of the three isomeric vinylphenols. It is a white volatile solid.
Upon contact with iron oxide at 500 ðC, 4-ethylphenol undergoes dehydrogenation to give 4-vinyllphenol:
It is found in wine and beer. It is produced by the spoilage yeast Brettanomyces. When it reaches concentrations greater than the sensory threshold, it can give the wine aromas described as barnyard, medicinal, band-aids, and mousy. In wine, 4-vinylphenol can react with other molecules, such as anthocyanidins, to produce new chemical compounds. In white wines vinylphenols are dominant (4-vinylphenol 70-1 150 üg/L, 4-vinylguaiacol 10-490 üg/L) whereas, in red wines, it is the corresponding ethyl phenols.
The enzyme cinnamate decarboxylase converts p-coumaric acid to 4-vinylphenol. 4-Vinylphenol is further reduced to 4-ethylphenol by the enzyme vinyl phenol reductase. Coumaric acid is sometimes added to microbiological media, enabling the positive identification of Brettanomyces by smell.