Definition
Phenols are organic compounds consisting of aromatics bearing one or more hydroxyl groups (Ar-OH). The simplest representative is phenol: this in contrast to alcohols, which are bonded to an aliphatic radical. For example, benzyl alcohol is an alcohol and not a phenol.
Nomenclature
The names of phenols are formed with the suffix –phenol, e.g., 2-methylphenol. Monomethylated phenol is called cresol (cresol). A distinction is made between o-, m- and p-cresol. The refers to position 2, 3 and 4 (see below).
Representative
Some known phenols are listed below:
- Aminophenol
- Bisphenol A
- Catechol
- Capsaicin
- Cresol
- Dopamine
- Estradiol
- Flavonoids
- Hydroquinone
- Levodopa
- Naphthol
- Phenol
- Phenolphthalein
- Picric acid
- Polyphenols
- Propofol
- Pyrogallol
- Resorcinol
- Salicylic acid
- Thymol
- Tyrosine
- Vanillin
- Vitamin E
Properties
Phenols are stronger acids (proton donors) than alcohols. Phenol has a pKa of 10. In comparison, the pKa of ethanol is much higher at about 16. The reason is the delocalization of the negative charge by the aromatics. The salts of phenols are called phenolates. Nitrated phenols have an even lower pKa. The pKa of picric acid is about 0.4! Phenols are hydrogen bond donors and acceptors. Therefore, they have comparatively high boiling points and melting points and exist as liquids or solids.
Reactions
Acid-base reactions:
- Phenol + NaOH (sodium hydroxide) sodium phenolate + H2O (water)
Electrophilic substitution:
- For example, with phenol and dilute nitric acid (nitration). This produces nitrophenol.
- Bromination of phenol with bromine (Br2).
Like alcohols, phenols can form esters with carboxylic acids. The best known example is acetylsalicylic acid, an acetylated salicylic acid. Shown here is the reaction with acetic anhydride: Ether:
- Arylethers can be formed with phenols.
In pharmacy
Many active pharmaceutical ingredients are phenols and phenols also play an important role as excipients, for example, as antioxidants (eg, butylated hydroxyanisole, butylated hydroxytoluene) and preservatives. Phenols are also used as disinfectants (e.g., 2-phenylphenol).
