Alcohols, Phenols and Ethers

The One Thing

The properties and reactions of alcohols, phenols, and ethers are determined by the oxygen-containing functional group and its molecular environment. Hydrogen bonding, polarity, resonance stabilisation, carbon structure, and aromatic substitution therefore control their physical properties, acidity, preparation, and reactivity.

Reactions, Processes and Experiments

Key Terms

• Alcohol: An organic compound containing a hydroxyl group (-OH) attached to a saturated carbon atom; its general representation is R-OH.
• Phenol: An aromatic compound in which the hydroxyl group is directly attached to a benzene ring; its general representation is Ar-OH.
• Ether: An organic compound containing an oxygen atom bonded to two alkyl or aryl groups; its general representation is R-O-R′.
• Primary alcohol: An alcohol in which the carbon bearing the -OH group is attached to one other carbon atom, represented as R-CH2OH.
• Secondary alcohol: An alcohol in which the carbon bearing the -OH group is attached to two other carbon atoms, represented as R2CHOH.
• Tertiary alcohol: An alcohol in which the carbon bearing the -OH group is attached to three other carbon atoms, represented as R3COH.
• Hydrogen bonding: Attraction between hydrogen bonded to an electronegative atom and a lone pair on another electronegative atom; it raises the boiling points of alcohols and phenols.
• Acidity of phenol: Phenol is weakly acidic because loss of H+ produces a resonance-stabilised phenoxide ion.
• Resonance: Delocalisation of electrons among several atoms; in the phenoxide ion, the negative charge is spread over oxygen and the ortho and para positions of the ring.
• Lucas test: A test using concentrated hydrochloric acid and anhydrous zinc chloride to distinguish alcohols by the rate of turbidity formation: tertiary alcohols react fastest, followed by secondary and primary alcohols.
• Esterification: Reaction of an alcohol with a carboxylic acid in the presence of concentrated sulfuric acid to form an ester and water.
• Dehydration: Removal of water from an alcohol, usually using concentrated sulfuric acid and heat, to form an alkene.
• Williamson ether synthesis: Preparation of ethers by reacting a sodium alkoxide with an alkyl halide through an SN2 reaction: RONa + R′X → ROR′ + NaX.
• Kolbe-Schmitt reaction: Reaction of sodium phenoxide with carbon dioxide under pressure followed by acidification to produce mainly salicylic acid.
• Reimer-Tiemann reaction: Reaction of phenol with chloroform and aqueous sodium hydroxide to introduce a formyl group mainly at the ortho position, producing salicylaldehyde.
• Azo coupling: Electrophilic coupling of phenol with a diazonium salt in a mildly alkaline medium to form brightly coloured azo compounds.
• Ether cleavage: Breaking of the C–O bond in ethers by concentrated hydrogen halides, especially HI or HBr, to form an alcohol and an alkyl halide or two alkyl halides.

Easily Confused

• Alcohols and phenols: In alcohols, -OH is attached to a saturated carbon atom; in phenols, -OH is directly attached to an aromatic ring.
• Phenol and most alcohols with aqueous sodium hydroxide: Phenol reacts with aqueous NaOH to form sodium phenoxide, whereas most alcohols do not react appreciably.
• Phenol and alcohol acidity: Phenol is more acidic than alcohols because the phenoxide ion is resonance-stabilised; alkoxide ions lack comparable resonance stabilisation.
• Primary, secondary, and tertiary alcohols: The classification depends on whether the carbon bearing -OH is attached to one, two, or three other carbon atoms.
• Oxidation of primary and secondary alcohols: Primary alcohols give aldehydes and then carboxylic acids on further oxidation; secondary alcohols give ketones.
• Oxidation and dehydration of tertiary alcohols: Tertiary alcohols resist oxidation under mild conditions but dehydrate more readily than secondary and primary alcohols.
• Williamson synthesis and tertiary halides: Williamson ether synthesis works best with primary alkyl halides; tertiary halides commonly undergo elimination rather than SN2 substitution.
• Esterification and dehydration: Esterification forms an ester from an alcohol and a carboxylic acid, whereas dehydration removes water from an alcohol to form an alkene.
• Industrial dehydration of ethanol at 413 K and 443 K: At about 413 K, ethanol forms diethyl ether; at about 443 K, it forms ethene.
• Ether cleavage at alkyl and aryl groups: In unsymmetrical ethers, SN2 cleavage generally occurs at a methyl or primary alkyl group; aryl–oxygen bonds are not usually cleaved by SN2 attack.
• Alcohols and ethers in boiling-point comparisons: Alcohols form intermolecular hydrogen bonds and generally have higher boiling points, whereas ethers cannot hydrogen-bond with one another as effectively.
• Azo coupling and Reimer-Tiemann reaction: Azo coupling forms brightly coloured azo compounds with a diazonium salt, whereas the Reimer-Tiemann reaction introduces a formyl group to produce salicylaldehyde.

What Gets Asked

• Definition and structural questions: Identify alcohols, phenols, and ethers from R-OH, Ar-OH, and R-O-R′; the common error is failing to distinguish an aromatic Ar-OH group from an alcohol.
• Classification and naming questions: Classify an alcohol as primary, secondary, or tertiary and apply IUPAC naming by selecting the longest chain containing -OH, numbering it to give -OH the lowest locant, and using the suffix -ol; ethers are commonly named as alkoxyalkanes.
• Physical-property explanations: Explain why alcohols and phenols generally have higher boiling points than comparable hydrocarbons and ethers, and why lower alcohols are miscible with water; the mark-losing error is overlooking hydrogen bonding and the effect of increasing hydrocarbon-chain length.
• Acidity comparisons: Compare water, alcohols, and phenol, and explain the effects of -NO2 and -CH3 substituents; the central distinction is resonance stabilisation of phenoxide, with electron-withdrawing -NO2 increasing acidity and electron-donating -CH3 decreasing it.
• Reaction prediction and observation questions: Use the violet or purple ferric chloride test, the white precipitate with bromine water, and the Lucas-test turbidity order; the common error is assigning an observation from one test to a different experiment.
• Mechanism and product questions: Predict products from oxidation, dehydration, esterification, Williamson ether synthesis, Kolbe-Schmitt reaction, Reimer-Tiemann reaction, azo coupling, and ether cleavage; the main slips are reversing the alcohol oxidation products, ignoring Saytzeff's rule, or choosing a tertiary halide for Williamson synthesis.