Phenols are a class of organic compounds consisting of a hydroxyl group (-OH) attached directly to an aromatic hydrocarbon ring, usually a benzene ring.
They are derived from and named after phenol, the simplest member of the class.
Phenols are known for their antioxidant, antiseptic, and preservative properties and are widely used in various industries, such as pharmaceuticals, plastics, resins, and cosmetics.
Types and Classification of Phenols
Phenols can be classified into different categories based on the number of hydroxyl groups, the type of aromatic ring, and the presence of other substituents.
The common types and classifications include:
Monophenols
These phenols have one hydroxyl group attached to the aromatic ring.
Examples include phenol itself and cresols.
Polyphenols
These phenols have two or more hydroxyl groups attached to the aromatic ring.
They can be further divided into different categories:
Diphenols: They have two hydroxyl groups. Examples include catechol, resorcinol, and hydroquinone.
Triphenols: They have three hydroxyl groups. An example is phloroglucinol.
Tetraphenols: They have four hydroxyl groups. An example is pyrogallol.
Pentaphenols: They have five hydroxyl groups. An example is gallic acid.
Substituted Phenols
Phenols can also be classified based on the presence of other substituents on the aromatic ring.
Examples include:
Alkylphenols: Phenols with alkyl groups.
Halophenols: Phenols with halogen atoms.
Nitrophenols: Phenols with nitro groups.
Aminophenols: Phenols with amino groups.
Applications of Phenols:
Phenols are versatile compounds with wide-ranging applications across various industries:
Pharmaceuticals: Phenols serve as precursors in drug synthesis, including analgesics like aspirin, antipyretics, and antiseptics such as hexachlorophene.
Plastics and Resins: Phenol-formaldehyde resins are key components in adhesives and plastic production, notably in Bakelite.
Cosmetics: Phenolic compounds are used as preservatives and antioxidants in cosmetics and personal care products.
Food Industry: Phenolic antioxidants like butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are used in food packaging and processing to prevent spoilage and extend shelf life.
Chemical Industry: Phenols are intermediates in the synthesis of various chemicals, including dyes, detergents, and herbicides.
Examples of Phenols:
Phenol: The simplest phenol, used as a disinfectant and in the production of phenolic resins.
Cresols: Monophenols with a methyl group, used in disinfectants and solvents.
Catechol: A diphenol used as a precursor in pharmaceuticals and dyes.
Resorcinol: A diphenol used in resins, adhesives, and pharmaceuticals.
Hydroquinone: A diphenol used as a reducing agent, photographic developer, and chemical precursor.
Gallic Acid: A pentaphenol with antioxidant properties.
General Methods of Preparation of Phenol
From Cumene (Isopropylbenzene) - Cumene Hydroperoxide Process
Cumene is oxidized to cumene hydroperoxide, which is cleaved using an acid catalyst (e.g., sulfuric acid) to yield phenol and acetone.
Reaction:
C6H5CH(CH3)2 + O2 → C6H5CH(OOH)CH3 → C6H5OH + (CH3)2CO
From Chlorobenzene - Dow Process
Chlorobenzene reacts with aqueous sodium hydroxide at high temperature and pressure to form sodium phenoxide, which is acidified to yield phenol.
Reaction:
C6H5Cl + NaOH → C6H5ONa + NaCl → C6H5OH + H2O + NaCl
From Aniline - Raschig Process
Aniline is diazotized with nitrous acid to form diazonium salt, which is hydrolyzed to produce phenol and nitrogen gas.
Reaction:
C6H5NH2 + HNO2 → C6H5N2+Cl- → C6H5OH + N2
From Benzene Sulfonic Acid
Benzene is sulfonated using concentrated sulfuric acid to form benzene sulfonic acid, which is treated with hot aqueous sodium hydroxide to form sodium phenoxide, followed by acidification to yield phenol.
Reaction:
C6H6 + H2SO4 → C6H5SO3H → C6H5ONa + H2O + Na2SO4 → C6H5OH + H2O + Na2SO4
Important Reactions of Phenol
Electrophilic Aromatic Substitution
Phenol undergoes electrophilic aromatic substitution reactions more readily than benzene due to the electron-donating nature of the hydroxyl group, which activates the aromatic ring toward electrophiles.
A) Nitration:
Phenol reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid to form ortho- and para-nitrophenols.
Reaction:
C6H5OH + HNO3 → C6H4(OH)(NO2) + H2O
B) Halogenation:
Phenol reacts with halogens in the presence of a suitable catalyst to form ortho- and para-halophenols.
Reaction:
C6H5OH + X2 → C6H4(OH)(X) + HX (X=Cl, Br, or I)
C) Friedel-Crafts Alkylation and Acylation:
Phenol undergoes Friedel-Crafts reactions with alkyl halides and acyl halides in the presence of a Lewis acid catalyst (e.g., AlCl3).
Reaction:
C6H5OH + RCl → C6H4(OH)(R) + HCl(Alkylation)
Reactions of the Hydroxyl Group
Phenol's hydroxyl group is reactive and can undergo several important chemical transformations.
A) Esterification:
Phenol reacts with carboxylic acids or their derivatives (e.g., acid chlorides) to form esters, usually in the presence of a catalyst such as concentrated sulfuric acid or pyridine.
Reaction:
C6H5OH + RCOOH → C6H5OCOR + H2O
B) Ether Formation:
Phenol reacts with alkyl halides in the presence of a base, such as potassium hydroxide, to form phenyl ethers.
Reaction:
C6H5OH+R-X→C6H5OR+HX
Oxidation Reactions
Phenol can be oxidized to various products depending on the conditions and oxidizing agents used.
A) Oxidation to Quinones:
Phenol is oxidized to form quinones, such as benzoquinone, in the presence of oxidizing agents like chromic acid or potassium dichromate.
Reaction:
C6H5OH+[O]→C6H4O2
B) Oxidative Cleavage to Dicarboxylic Acids:
Phenol undergoes oxidative cleavage to form dicarboxylic acids, such as muconic acid, in the presence of strong oxidizing agents like potassium permanganate.
Reaction:
C6H5OH+2[O]→HOOC-C6H2−COOH
Reduction Reactions
Phenol can be reduced to simpler aromatic compounds.
A) Reduction to Benzene:
Phenol is reduced to benzene by treating it with zinc dust at high temperatures.
Reaction:
C6H5OH+Zn→C6H6+ZnO