Phenols

Phenols are molecules that have a hydroxyl group attached to the carbon atom of an aromatic ring.

Nomenclature

By definition, phenol is hydroxybenzene. Phenol is a common name for the compound. Its IUPAC name would be benzenol, derived in the same manner as the IUPAC names for aliphatic alcohols.

When a phenol molecule is substituted with additional groups, either the ortho, meta, para system or the numbering system can be employed. In either case, if the parent molecule is referred to as a phenol, the nomenclature being used is the common system.

In IUPAC nomenclature, the parent molecule is called benzenol, and substituents are always numbered with the OH group being given the understood first position. For the compounds below, the first name listed is the common name and the second is the IUPAC name.





Certain phenols are referred to by common names. For example, methyl phenols are called cresols. In the illustrations below, the first name under each compound is its common name, and the bottom name is its IUPAC name.





Similarly, hydroxyphenols have common names, which are listed first under each of the following illustrations, while the IUPAC names are listed last.




Physical properties

Low molecular weight phenols are normally liquids or low melting solids. Due to hydrogen bonding, most low molecular weight phenols are water‐soluble. Phenols tend to have higher boiling points than alcohols of similar molecular weight because they have stronger intermolecular hydrogen bonding.

Acidity

Phenols show appreciable acidity (p K a = 8 −10). For example, phenol reacts with aqueous NaOH as follows.





This is a typical neutralization reaction.

Because of their high acidity, phenols are often called carbolic acids. The phenol molecule is highly acidic because it has a partial positive charge on the oxygen atom due to resonance, and the anion that is formed by loss of a hydrogen ion is also resonance stabilized.




Resonance structures of phenol

Notice that three of the four contributing structures possess a positive charge on the oxygen atom of the molecule. Thus, the true hybrid structure must possess a partial positive charge. Because oxygen is an electronegative element, the electrons in the oxygen‐hydrogen bond orbital are attracted to the oxygen atom, resulting in a partially positive hydrogen.

Loss of a hydrogen ion to a base creates a phenoxide ion that is resonance stabilized.





Notice that upon removal of the hydroxy hydrogen by a base, the phenoxide anion results. This anion is resonance stabilized by delocalization of an electron pair throughout the molecule, as shown by the contributing structures.