Quick revision notes for Class 12 Chemistry Chapter 7 – Alcohols, Phenols and Ethers, covering classification, nomenclature, preparation, acidity trends, key name reactions, and ether chemistry. Ideal for last-minute board exam revision.
Classification and Nomenclature
Alcohols (R–OH) are classified as primary, secondary or tertiary based on the number of carbon groups attached to the carbinol carbon; phenols have –OH directly on an aromatic ring; ethers (R–O–R’) have oxygen bonded to two carbon groups, symmetrical (same groups) or unsymmetrical (different groups).
Preparation and Physical Properties
Alcohols are prepared from alkenes (acid-catalysed hydration – Markovnikov; oxymercuration–demercuration – Markovnikov, no rearrangement; hydroboration–oxidation – anti-Markovnikov) or from alkyl halides/Grignard reagents. Phenol is manufactured industrially from cumene (cumene process, via cumene hydroperoxide+dilute acid, co-producing acetone) or from chlorobenzene (Dow’s process). Alcohols and phenols show strong intermolecular hydrogen bonding, giving them much higher boiling points and water solubility than comparable hydrocarbons or ethers of similar molecular mass (e.g. ethanol, bp 351K, vs. isomeric methoxymethane, bp≈250K).
Acidity of Phenols
Phenol (pKa≈10) is far more acidic than an alcohol because the phenoxide ion is resonance-stabilised by delocalisation into the ring, while an alkoxide has no such stabilisation. Electron-withdrawing ring substituents (e.g. –NO2) further stabilise the phenoxide and increase acidity (picric acid, with three –NO2 groups, is a very strong acid); electron-donating groups (e.g. –OCH3, alkyl) decrease acidity relative to phenol.
Key Name Reactions
Kolbe’s reaction: sodium phenoxide+CO2, then acidify → salicylic acid. Reimer–Tiemann reaction: phenol+CHCl3/NaOH (via dichlorocarbene) → salicylaldehyde. Williamson ether synthesis: sodium alkoxide+a primary alkyl halide (SN2) → an ether; using a secondary/tertiary halide instead causes E2 elimination to an alkene rather than substitution. Lucas test: conc. HCl/anhyd. ZnCl2 distinguishes 1°/2°/3° alcohols by turbidity onset time (3° immediate, 2° ~5 min, 1° no reaction in the cold).
Oxidation of Alcohols
Primary alcohols oxidise to aldehydes with mild agents (PCC) or all the way to carboxylic acids with strong agents (acidified KMnO4/K2Cr2O7, excess). Secondary alcohols oxidise only to ketones. Tertiary alcohols, having no hydrogen on the carbinol carbon, resist oxidation under these conditions.
Ethers
Ethers are generally unreactive except toward strong acids like HI/HBr, which cleave the C–O bond: with a dialkyl ether, the smaller/less hindered alkyl group is converted to the alkyl iodide; with an aryl alkyl ether like anisole, only the alkyl–oxygen bond breaks (never the aryl–oxygen bond), giving phenol+an alkyl iodide. The –OR group in aromatic ethers (e.g. anisole) is strongly activating and ortho/para-directing in electrophilic aromatic substitution, due to resonance donation of an oxygen lone pair into the ring.
Continue Revising — NCERT Solutions for Class 12 Chemistry:
Chapter 1: Solutions | Chapter 2: Electrochemistry | Chapter 3: Chemical Kinetics | Chapter 4: The d- and f-Block Elements | Chapter 5: Coordination Compounds | Chapter 6: Haloalkanes and Haloarenes | Chapter 7: Alcohols, Phenols and Ethers
