Phenol

Phenol, also known as carbolic acid, phenolic acid, or benzenol, is an aromatic organic compound composed of a phenyl ring bonded to a hydroxyl group. With the molecular formula C₆H₅OH, it appears as a white crystalline solid that is moderately volatile and notably corrosive. Once obtained primarily from coal tar, phenol is today manufactured in large quantities from petroleum-based feedstocks, with global production exceeding several million tonnes per year. Its industrial significance lies in its role as a precursor to a wide range of synthetic materials, resins, polymers, pharmaceuticals, and agrochemicals.

Physical and Chemical Properties

Phenol is appreciably soluble in water, with around 84 g dissolving in 1 litre to form a homogeneous mixture at appropriate mass ratios. Its sodium salt, sodium phenoxide, exhibits far greater water solubility. Phenol is combustible; when heated it produces flammable vapours that form explosive mixtures with air. Its flammability and ability to cause severe chemical burns necessitate careful handling.
Chemically, phenol is a weak acid, typically displaying a pH near 5–6 in aqueous solution. In alkaline conditions, particularly at pH values above 8, phenol partially ionises to form the phenolate (phenoxide) anion. Phenol is more acidic than aliphatic alcohols, a property attributed either to resonance stabilisation of the phenoxide ion or to inductive effects associated with sp² hybridisation of the aromatic ring. Comparative studies show that resonance alone does not fully account for this difference, especially when solvation effects are considered.
Hydrogen-bonding interactions are prominent in non-aqueous solvents such as carbon tetrachloride, where phenol readily forms adducts with various Lewis acids or bases. It is classified as a hard acid under HSAB theory. Phenol also exhibits keto–enol tautomerism, but the equilibrium overwhelmingly favours the aromatic enol form because tautomerisation would disrupt aromaticity. Only substituted systems, such as annulated rings or polyhydroxy phenols, display meaningful tautomeric effects under special conditions.

Reactivity and Important Reactions

Phenol is highly reactive toward electrophilic aromatic substitution. The hydroxyl group strongly activates the benzene ring by donating electron density, favouring substitution at the ortho and para positions. Halogenation therefore occurs readily, often producing polysubstituted derivatives unless reaction conditions are tightly controlled.

• Nitration with dilute nitric acid yields mixtures of 2-nitrophenol and 4-nitrophenol, while concentrated nitric acid can produce picric acid.
• Friedel–Crafts alkylation may proceed without added catalyst due to the activating effect of the hydroxyl group, enabling phenol to react with alkyl halides, alkenes, ketones, and related reagents.
• Esterification, such as the Schotten–Baumann reaction with benzoyl chloride, forms aromatic esters under alkaline conditions.
• Reduction of phenol vapour over zinc produces benzene, and methylation with diazomethane forms anisole.
• Solutions of phenol react with iron(III) chloride to produce strongly coloured complexes, a classical qualitative test for phenolic groups.

Phenoxides behave as resonance-stabilised enolates, exhibiting hard nucleophilic character at oxygen and softer nucleophilicity at the ortho and para carbon positions.

Industrial Production

Given phenol’s commercial value, numerous synthetic routes have been explored; however, the cumene (Hock) process dominates modern manufacture, accounting for the vast majority of global output. It entails the oxidation of cumene to cumene hydroperoxide, followed by acid-catalysed cleavage to yield phenol and acetone. The economic viability of this process relies on steady demand for both products.
A closely related route uses cyclohexylbenzene, generating phenol and cyclohexanone via hydroperoxide formation and rearrangement. Cyclohexanone is an essential precursor for nylon production.
Other oxidative pathways include:

• Direct oxidation of benzene, which remains impractical on a commercial scale.
• Nitrous oxide oxidation, a potentially greener method but hampered by costly oxidant generation.
• Electrosynthesis, in which alternating current promotes benzene oxidation to phenol.
• Toluene oxidation, developed by Dow, which proceeds through a benzoate intermediate.
• Cyclohexylbenzene autoxidation, also yielding cyclohexanone and phenol.

Historical Methods

Earlier industrial processes relied on coal tar distillation or the hydrolysis of substituted benzene derivatives. One influential route, developed in the early twentieth century, involved the alkaline fusion of benzenesulfonic acid, a method based on the work of Charles-Adolphe Wurtz and August Kekulé. Although these routes have largely been superseded by more economical technologies, they played a foundational role in the emergence of phenol chemistry.