The –OH and –O– families
Alcohols have an –OH group attached to an sp³ carbon: ethanol, CH₃CH₂OH.
Phenols have an –OH group attached directly to a benzene ring: phenol, C₆H₅OH.
Ethers have an oxygen bonded to two carbon groups, R–O–R′: methoxymethane, CH₃OCH₃.
Alcohols are monohydric, dihydric (ethane-1,2-diol, glycol) or trihydric (propane-1,2,3-triol, glycerol) depending on the number of –OH groups. Monohydric alcohols are primary, secondary or tertiary depending on the carbon bearing the –OH.
Preparation of alcohols
Acid-catalysed hydration of alkenes (follows Markovnikov's rule): CH₃CH=CH₂ + H₂O → CH₃CH(OH)CH₃.
Hydroboration-oxidation: B₂H₆ followed by H₂O₂/OH⁻ adds water anti-Markovnikov, giving primary alcohols from terminal alkenes.
Reduction of carbonyl compounds with H₂/Pd, NaBH₄ or LiAlH₄: aldehydes give primary alcohols, ketones give secondary alcohols. Carboxylic acids are reduced to primary alcohols by LiAlH₄.
Grignard reagents with carbonyl compounds: methanal gives a primary alcohol, other aldehydes give secondary, and ketones give tertiary alcohols.
Preparation of phenol
From haloarenes: chlorobenzene with NaOH at 623 K and 300 atm, then acid.
From benzenesulphonic acid: fused with NaOH, then acidified.
From diazonium salts: warming with water.
From cumene (isopropylbenzene): oxidation to cumene hydroperoxide, then acid treatment gives phenol and acetone. This is the main industrial method.
Physical properties
Alcohols and phenols have much higher boiling points than hydrocarbons or ethers of similar mass, because of intermolecular hydrogen bonding. Lower alcohols are soluble in water, as they form hydrogen bonds with water; solubility falls as the hydrocarbon part gets larger.
Ethers can't hydrogen bond with each other, so their boiling points are close to those of alkanes, but they are somewhat soluble in water because they can accept hydrogen bonds.
Acidity
Alcohols are very weak acids. They react with active metals: 2ROH + 2Na → 2RONa + H₂. Acidity order: primary > secondary > tertiary, because alkyl groups push electrons towards oxygen.
Phenols are more acidic than alcohols (and than water). The phenoxide ion is stabilised by resonance, spreading the negative charge over the ring. Phenol reacts with NaOH; ethanol does not.
Electron-withdrawing groups (–NO₂) at ortho and para positions increase acidity: 2,4,6-trinitrophenol (picric acid) is a strong acid.
Electron-donating groups (–CH₃) decrease acidity.
Phenol is still weaker than carboxylic acids: it doesn't react with NaHCO₃.
Reactions of alcohols
Breaking the O–H bond: esterification with carboxylic acids (acid catalyst) or acid chlorides gives esters.
Breaking the C–O bond:
With HX: R–OH + HX → R–X. Lucas test: with Lucas reagent (conc. HCl + anhydrous ZnCl₂), tertiary alcohols give turbidity immediately, secondary ones in about 5 minutes, and primary ones not at room temperature.
Dehydration with conc. H₂SO₄: ethanol at 443 K gives ethene. The ease of dehydration is 3° > 2° > 1°.
Oxidation:
Primary alcohols → aldehydes (with mild oxidant PCC) → carboxylic acids (with KMnO₄ or acidified K₂Cr₂O₇).
Secondary alcohols → ketones.
Tertiary alcohols resist oxidation.
Heating alcohol vapour over copper at 573 K gives the same results by dehydrogenation; tertiary alcohols are dehydrated to alkenes instead.
Reactions of phenols
The –OH group is strongly activating and ortho-para directing for electrophilic substitution.
Nitration: dilute HNO₃ at low temperature gives o- and p-nitrophenol (separated by steam distillation; the ortho isomer is steam-volatile due to intramolecular hydrogen bonding). Concentrated HNO₃ gives picric acid.
Bromination: with bromine water, phenol immediately gives a white precipitate of 2,4,6-tribromophenol. In CS₂ at low temperature, monobromophenols form.
Kolbe's reaction: sodium phenoxide + CO₂ under pressure, then acid, gives salicylic acid (2-hydroxybenzoic acid), used to make aspirin.
Reimer-Tiemann reaction: phenol + CHCl₃ + NaOH, then acid, gives salicylaldehyde (2-hydroxybenzaldehyde).
Reduction: heating with zinc dust gives benzene.
Oxidation with chromic acid gives benzoquinone.
Test: phenol gives a violet colour with neutral FeCl₃ solution.
Ethers
Preparation:
Dehydration of alcohols: ethanol with conc. H₂SO₄ at 413 K gives ethoxyethane (at 443 K, ethene forms instead). This works only for primary alcohols making symmetrical ethers.
Williamson synthesis: an alkyl halide reacts with a sodium alkoxide: R–X + R′–ONa → R–O–R′ + NaX. It works for both symmetrical and unsymmetrical ethers. Use a primary alkyl halide; tertiary halides give alkenes by elimination instead.
Reactions:
Ethers are fairly unreactive. They are cleaved by HI: R–O–R′ + HI → R–I + R′–OH. With a methyl alkyl ether, the iodide goes to the methyl group by SN2 (for primary/secondary groups). For an aryl alkyl ether like anisole, the products are phenol and an alkyl iodide, because the aryl C–O bond has partial double-bond character.
The –OR group of anisole is ortho-para directing; anisole undergoes halogenation, nitration and Friedel-Crafts reactions.
Important compounds
Methanol ("wood spirit") is very poisonous; small amounts cause blindness, larger amounts death.
Ethanol is made by fermentation of sugars with yeast. Industrial ethanol is made unfit to drink by adding copper sulphate and pyridine (denatured alcohol).
Summary
Phenol is more acidic than alcohols due to resonance in phenoxide; –NO₂ groups increase acidity.
Lucas test: 3° immediate, 2° in about 5 min, 1° no reaction at room temperature.
Primary alcohols oxidise to aldehydes then acids; secondary to ketones; tertiary resist.
Phenol gives a white precipitate with bromine water and violet colour with FeCl₃; Kolbe gives salicylic acid; Reimer-Tiemann gives salicylaldehyde.
Williamson synthesis: alkoxide + primary alkyl halide → ether.