Phenols

Phenols are a class of organic compounds consisting of one or more hydroxyl groups (-OH) bonded directly to an aromatic ring. The simplest member is phenol (also called carbolic acid), with the formula C6H5OH. The attachment of the hydroxyl group to the aromatic system gives phenols distinctive acidity, reactivity, and physical properties compared with ordinary alcohols.

Physically, phenols are capable of hydrogen bonding and are generally more reactive than alkanols. They are weaker acids than carboxylic acids but stronger than most alcohols, with typical pKa values around 9–10 for the parent phenol, enabling deprotonation to form phenoxide anions in basic solution. Water solubility is moderate and decreases with increased hydrophobic substituents. The hydroxyl group is activating toward electrophilic aromatic substitution and directs new substituents to the ortho and para positions relative to itself.

Chemically, phenols undergo a range of reactions. The hydroxyl group makes the aromatic ring more reactive toward electrophilic substitution than benzene, allowing nitration, halogenation, sulfonation, and other activations under milder conditions. Phenols readily form esters with acyl chlorides and react with bases to give phenoxide salts. Oxidation can yield quinones, as in the conversion of hydroquinone to benzoquinone. The acidity and reactivity of phenols can be modulated by ring substituents, enabling wide utility in synthesis and materials science.

Occurrence and uses: Phenols occur in plants as simple phenols and as polyphenols, contributing to flavors, colors, and antioxidant properties. Industrially, phenol is a major chemical feedstock, produced widely via the cumene process, and is a precursor to phenol-formaldehyde resins, bisphenol A, polycarbonates, and other polymers. Phenolic compounds also serve as disinfectants, antiseptics, and antioxidants in various applications.

Classification and examples: Monohydroxybenzene is phenol itself. Dihydroxybenzenes include catechol (1,2-), resorcinol (1,3-), and hydroquinone (1,4-). Trihydroxybenzenes include pyrogallol (1,2,3-) and phloroglucinol (1,3,5-). In natural systems, polyphenols such as tannins and flavonoids are important due to their biological activities and structural diversity.