Tag Info

Use this tag for questions about phenols, their synthesis, analysis, properties and more. Do not use the 'alcohols' tag for phenols due to the differences between the two classes.

Phenols are defined by IUPAC as compounds that include a hydroxy group bound to a benzene or other aromatic arene ring. These compounds typically differ strongly from in both their properties and reactivity which warrants distinction even though both contain hydroxy groups.

The most notable difference between alcohols and phenols is the latter’s resistance towards typical oxidation reactions such as the Jones oxidation. However, given favourable surroundings, the oxidation of phenols to form quinones is often observable.

The simplest phenol, the name-giving hydroxybenzene, was first isolated from coal tar in 1834. It was initially termed Karbolsäure due to its acidic nature by its discoverer Friedlieb Ferdinand Runge.

Its acidic nature ($\mathrm{p}K_\mathrm{a} = 9.95$) is due to the high electronegativity of an $\mathrm{sp^2}$ hybridised carbon atom which stabilises the oxyanion and, to a lesser degree, due to the possibility of the negative charge being delocalised across the phenyl ring — see the mesomeric structures depicted in figure 1.

Figure 1: Mesomeric structures of the phenolate ion. The image was originally taken from Wikipedia, where a full list of authors is available.

This delocalisation of oxygen’s lone pair into the phenyl ring – a $+M$ effect — is also the reason why phenols react much more readily in electrophilic aromatic substitutions than unsubstituted benzene. The hydroxy group is ortho- and para-directing so that e.g. mononitration will lead to a mixture of 2-nitrophenol and 4-nitrophenol.

Industrially, phenol is produced via the cumene-phenol process or Hock process, wherein cumene (isopropylbenzene) is oxidised in a radical reaction with atmospheric oxygen to give cumene hydroperoxide which rearranges to give phenol and acetone. In 2003, 7 million tonnes of phenol were produced by this process annually.

Phenols also form a wide class of natural products. They are usually classified as polyketides (with a typical 1,3-hydroxy functionalisation) or as derived from the shikimic acid pathway as tyrosine is. Listing the different types of phenols observed in nature would exceed the margin of this tag wiki.

Similarly, a large number of pharmaceutically active substances are phenols, one of the simplest being paracetamol.