Phenols and their Reactions - SS2 Chemistry Lesson Note
Phenols are a class of organic compounds characterised by the presence of a hydroxyl (-OH) group attached to an aromatic ring. They exhibit distinct properties and reactivity compared to alcohols due to the aromatic nature of the ring.
Nomenclature of Phenols:
The systematic nomenclature of phenols follows the rules established by the International Union of Pure and Applied Chemistry (IUPAC). The parent compound is named as a derivative of benzene, with the "-ol" suffix denoting the hydroxyl group. Substituents are identified by their position on the benzene ring and named using prefixes such as "ortho-," "meta-," or "para-."
Examples of IUPAC Names:
● Phenol: The simplest and most common phenol, also known as hydroxybenzene.
● 2-Chlorophenol: A phenol with a chlorine substituent at the ortho position.
Properties of Phenols:
Phenols exhibit unique physical and chemical properties due to the presence of the hydroxyl group on an aromatic ring.
● Hydrogen Bonding: Phenols can form intermolecular hydrogen bonds due to the presence of the hydroxyl group, resulting in higher boiling points compared to hydrocarbons of similar molecular weight.
● Acidic Nature: Phenols are weak acids due to the ability of the hydroxyl group to donate a proton (H+). The presence of the aromatic ring stabilises the phenoxide ion formed after deprotonation.
● Water Solubility: Phenols are soluble in water to some extent due to hydrogen bonding but are less soluble than alcohols.
Reactions of Phenols:
Phenols can undergo a variety of reactions, including substitution, oxidation, and condensation reactions. These reactions are influenced by the aromaticity and acidity of the phenol.
Electrophilic Substitution Reactions:
Phenols can undergo electrophilic substitution reactions due to the presence of an electron-rich aromatic ring. Common reactions include:
● Halogenation: Phenols react with halogens in the presence of a Lewis acid catalyst (such as FeCl3) to form halogenated phenols.
● Nitration: Phenols react with nitric acid in the presence of a sulfuric acid catalyst to form nitrophenols.
● Sulfonation: Phenols react with sulfuric acid to form phenol sulfonic acids.
Oxidation Reactions:
Phenols can be oxidised to form quinones or aromatic compounds with additional oxygen atoms. Oxidising agents such as potassium permanganate (KMnO4) or chromic acid (H2CrO4) are commonly used. The oxidation of phenols can lead to the formation of highly coloured compounds, such as benzoquinones.
Condensation Reactions:
Phenols can undergo condensation reactions, particularly with aldehydes and ketones, to form compounds known as phenolic ethers or acetals. These reactions are catalysed by acids and result in the formation of ethers or cyclic acetals.
Reactions with Bases:
Phenols can react with strong bases, such as sodium hydroxide (NaOH), to form water-soluble sodium phenoxides. This reaction is important in the synthesis of various compounds and pharmaceuticals.
Applications of Phenols:
Phenols have diverse applications in different industries and fields:
● Antiseptics and Disinfectants: Phenols, such as carbolic acid (also known as phenol), are used as antiseptics and disinfectants due to their antimicrobial properties.
● Pharmaceuticals: Phenols serve as building blocks in the synthesis of pharmaceutical compounds and drugs.
● Polymers: Phenolic resins, derived from phenols, are used in the production of plastics, adhesives, and insulation materials.
Phenols are aromatic compounds containing a hydroxyl group attached to a benzene ring. They possess unique properties, including acidity and reactivity, due to the aromatic nature of the ring and the presence of the hydroxyl group. Phenols undergo various reactions, such as electrophilic substitution, oxidation, and condensation reactions, which make them versatile precursors in organic synthesis. Understanding the properties and reactions of phenols enables their effective utilisation in industries, such as pharmaceuticals, polymers, and disinfection products.