Alcohols, Phenols and Ethers

The One Thing

The bonding environment of oxygen determines whether a compound is an alcohol, phenol or ether, and therefore controls its nomenclature, physical properties, acidity and characteristic reactions. These behaviours are explained principally by hydrogen bonding, resonance, steric effects, substituent effects and the structure of the carbon groups attached to oxygen.

Reactions, Processes and Experiments

Key Terms

• Alcohol: An organic compound containing a hydroxyl group, −OH, bonded to a saturated carbon atom; its general representation is R−OH.
• Phenol: A compound in which the hydroxyl group is directly attached to an aromatic ring, commonly represented as C6H5OH.
• Ether: An organic compound in which an oxygen atom is bonded to two alkyl or aryl groups, represented as R−O−R′.
• Primary, secondary and tertiary alcohols: Alcohols are classified according to whether the carbon bearing −OH is attached to one, two or three other carbon atoms, respectively.
• Monohydric, dihydric and trihydric alcohols: Alcohols containing one, two or three hydroxyl groups, respectively.
• Alkoxy group: The group R−O− formed by removing the hydrogen atom from an alcohol; examples include methoxy, −OCH3, and ethoxy, −OC2H5.
• IUPAC nomenclature of alcohols: Select the longest chain containing the carbon bonded to −OH, number it to give −OH the lowest locant, and use the suffix -ol. Examples include propan-1-ol and propan-2-ol. In cyclic alcohols, the carbon bearing −OH is assigned position 1, as in cyclohexanol.
• IUPAC nomenclature of phenols: The parent compound is phenol; substituents on the ring are numbered to give the hydroxyl group position 1 and the substituents the lowest possible numbers.
• IUPAC nomenclature of ethers: Ethers are named as alkoxyalkanes, with the larger carbon chain as the parent and the smaller group with oxygen as the alkoxy substituent.
• Hydrogen bonding: Attraction involving hydrogen attached to an electronegative atom and a lone pair on another electronegative atom; it raises the boiling points of alcohols and phenols.
• Acidity of alcohols and phenols: Alcohols and phenols can lose the hydrogen of the hydroxyl group to form alkoxide or phenoxide ions; phenols are generally more acidic because phenoxide ion is resonance-stabilised.
• Resonance in phenoxide ion: The negative charge formed after phenol loses H+ is delocalised over the oxygen and certain carbon atoms of the aromatic ring, increasing stability.
• Dehydration of alcohols: Removal of water from an alcohol, usually using concentrated acid and heat, commonly produces an alkene.
• Esterification: Reaction of an alcohol with a carboxylic acid in the presence of concentrated sulfuric acid to form an ester and water.
• Williamson ether synthesis: Preparation of ethers by reacting a sodium alkoxide with a primary alkyl halide through an SN2 substitution reaction.
• Kolbe reaction: Sodium phenoxide reacts with carbon dioxide under pressure followed by acidification to form mainly salicylic acid.
• Reimer-Tiemann reaction: Phenol reacts with chloroform and aqueous sodium hydroxide to produce mainly o-hydroxybenzaldehyde, also called salicylaldehyde.
• Cleavage of ethers: Ethers react with concentrated hydrogen halides, especially HI or HBr, to form an alcohol and an alkyl halide or, under stronger conditions, two alkyl halides.
• General formula of saturated monohydric alcohols: CnH2n+1OH or CnH2n+2O.
• General structural formula of an alcohol: R−OH.
• General structural formula of a phenol: Ar−OH, where Ar represents an aromatic group.
• General structural formula of an ether: R−O−R′.
• Cumene process: Industrial preparation of phenol in which cumene is oxidised and then decomposed to phenol and acetone.
• Anisole: An aryl alkyl ether represented as C6H5OCH3; its alkoxy group directs electrophilic substitution mainly to the ortho and para positions.

Easily Confused

• Alcohol and phenol: In an alcohol, −OH is bonded to a saturated carbon atom; in a phenol, −OH is directly attached to an aromatic ring.
• Phenol and ethanol acidity: Phenol reacts with aqueous sodium hydroxide because phenoxide ion is resonance-stabilised; ethanol does not react appreciably with aqueous sodium hydroxide.
• Alcohol and ether: Alcohols contain an O−H bond and can form intermolecular hydrogen bonds; ethers contain R−O−R′ and cannot form strong self-associated hydrogen bonds.
• Primary, secondary and tertiary alcohols: The classification depends on the number of carbon atoms attached to the carbon bearing −OH: one, two or three, respectively.
• Oxidation of primary and secondary alcohols: Primary alcohols give aldehydes and then carboxylic acids, whereas secondary alcohols give ketones.
• Oxidation of tertiary alcohols and other alcohols: Tertiary alcohols resist oxidation under ordinary conditions because the carbon bearing −OH has no hydrogen atom.
• Williamson synthesis and ether cleavage: Williamson synthesis forms ethers from sodium alkoxides and primary alkyl halides by SN2 substitution; ether cleavage breaks ethers using concentrated HI or HBr.
• Dehydration temperature and ether formation: Heating alcohols with concentrated sulfuric acid at about 413 K favours symmetrical ether formation, whereas higher temperatures favour alkene formation.
• Ortho/para direction in phenol and anisole: Both −OH in phenol and −OCH3 in anisole activate the aromatic ring and direct electrophilic substitution mainly to the ortho and para positions, but they belong to different compound classes.
• Phenol nitration conditions: Dilute nitric acid gives o-nitrophenol and p-nitrophenol, whereas concentrated nitric acid can give 2,4,6-trinitrophenol.
• Alkyl and aryl ether cleavage: In an unsymmetrical ether with one primary side, SN2 attack occurs at the less hindered alkyl group; the aromatic carbon of an aryl–O bond is not normally attacked by SN2.

What Gets Asked

• Classification and nomenclature: Identify whether a structure is an alcohol, phenol or ether; classify an alcohol as primary, secondary or tertiary; and apply the relevant IUPAC rules. Marks are lost by numbering an alcohol chain incorrectly, failing to give −OH the lowest locant, or naming an ether as an ordinary alcohol.
• Physical-property explanations: Explain boiling points and water solubility using hydrogen bonding, molecular mass, surface area and branching. Marks are lost by claiming that ethers form strong self-associated hydrogen bonds despite lacking an O−H bond.
• Acidity comparisons: Rank water, phenol and alcohols using the order water greater than phenol greater than alcohols, and explain phenol’s acidity through resonance stabilisation of phenoxide ion. Marks are lost by ignoring the effects of electron-withdrawing −NO2 groups and electron-releasing alkyl groups.
• Alcohol reactions: Predict products of oxidation, dehydration, reactions with sodium, hydrogen halides and thionyl chloride. Marks are lost by oxidising primary, secondary and tertiary alcohols identically or by overlooking Saytzeff's rule in dehydration.
• Phenol reactions and tests: Identify products of bromine water, nitration, zinc dust, sodium hydroxide, the Kolbe reaction and the Reimer-Tiemann reaction, including the violet neutral ferric chloride test. Marks are lost by omitting the white precipitate of 2,4,6-tribromophenol or confusing salicylic acid with salicylaldehyde.
• Ether preparation and cleavage: Apply Williamson ether synthesis, the 413 K preparation of symmetrical ethers and cleavage with HI or HBr. Marks are lost by using secondary or tertiary alkyl halides in Williamson synthesis without considering elimination, or by ignoring steric effects and the distinction between alkyl and aryl–O bonds.