Alcohols, Phenols and Ethers

1. Classification, Naming & Structure

Alcohol R–OH (OH on sp³ carbon), phenol Ar–OH (OH on an aromatic ring carbon), ether R–O–R′ (O between two carbon groups). Alcohols and ethers with the same formula are functional isomers (C₂H₅OH and CH₃OCH₃).

Also count the OH groups: mono-, di- (glycol), tri- (glycerol) hydric. Allylic (OH on sp³ C next to C=C) and benzylic (next to a ring) alcohols are especially reactive; vinylic alcohol (OH on C=C) is unstable.

Naming. Alcohol: replace –e of the alkane by –ol, number the chain from the end nearest OH (propan-2-ol). Phenol: parent name is phenol; substituents get numbers (2-methylphenol = o-cresol). Ether: alkoxyalkane, the smaller group becomes alkoxy, the longer chain is the parent (CH₃OC₂H₅ = methoxyethane; C₆H₅OCH₃ = methoxybenzene, anisole).

Structure. The C–O–H angle in methanol is 108.9° and the C–O–C angle in methoxymethane is 111.7° (bigger, because the two alkyl groups repel). The C–O bond in phenol (136 pm) is shorter than in methanol (142 pm) because the oxygen lone pair is in resonance with the ring, which gives the C–O bond partial double bond character.

2. Preparation of Alcohols and Phenols

Alcohols

1. Hydration of alkenes. (a) H₂O/H⁺ follows Markovnikov's rule: propene gives propan-2-ol. (b) Hydroboration–oxidation (B₂H₆, then H₂O₂/OH⁻) is anti-Markovnikov: propene gives propan-1-ol.
2. Reduction of carbonyls. Aldehyde gives 1° alcohol; ketone gives 2° alcohol (H₂/Ni, NaBH₄ or LiAlH₄). Carboxylic acids and esters need the stronger LiAlH₄ to give 1° alcohols; NaBH₄ cannot reduce them.
3. Grignard reagent + carbonyl, then H₃O⁺: HCHO gives a 1° alcohol; any other aldehyde gives a 2° alcohol; a ketone gives a 3° alcohol.

Phenols

1. Haloarene: chlorobenzene + NaOH at 623 K and 300 atm, then H⁺.
2. Benzenesulphonic acid: fused with NaOH, then H⁺.
3. Diazonium salt: warm with water (or dilute acid) to give phenol.
4. Cumene process (industrial): cumene + O₂ gives cumene hydroperoxide; dilute acid then gives phenol + acetone (two useful products).

3. Physical Properties

• Boiling point: alcohols and phenols form intermolecular hydrogen bonds, so their b.p. is far above ethers, haloalkanes and alkanes of similar mass. Ethers cannot H-bond with each other. Among isomeric alcohols, b.p. falls with branching (1° > 2° > 3°) because the molecule becomes more spherical and the surface area, hence the van der Waals force, decreases.
• Solubility in water: lower alcohols, phenol (slightly) and ethers (a little) dissolve because they H-bond with water. Solubility decreases as the alkyl part gets larger.

4. Acidity of Alcohols and Phenols

Both react with active metals (2ROH + 2Na → 2RONa + H₂). Only phenol also reacts with NaOH; alcohols do not (they are weaker acids than water).

Why phenol is more acidic than alcohol: the phenoxide ion is stabilised by resonance, the negative charge being delocalised over the ring. In an alkoxide the charge stays on oxygen and the +I effect of the alkyl group pushes electron density onto it, which destabilises it.

Alcohol order: 1° > 2° > 3° (more alkyl groups, more +I, less stable alkoxide). The board answer is water > ROH.

Substituents on phenol: electron-withdrawing groups (–NO₂) at o/p stabilise the phenoxide and raise acidity (2,4,6-trinitrophenol, picric acid, is as strong as a mineral acid); electron-donating groups (–CH₃, –OCH₃) destabilise it and lower acidity. Order: p-nitrophenol > phenol > p-cresol.

5. Reactions of Alcohols

A. O–H bond breaks (alcohol acts as an acid or nucleophile)

• With Na/K gives alkoxide + H₂.
• Esterification: ROH + R′COOH ⇌ R′COOR + H₂O (conc. H₂SO₄, reversible). With acid chlorides or anhydrides the reaction is faster and goes to completion; acetylation of salicylic acid gives aspirin (2-acetoxybenzoic acid).

B. C–O bond breaks

(i) With HX (Lucas test). Conc. HCl + anhydrous ZnCl₂ turns alcohols into alkyl chlorides through an SN1-type carbocation, so 3° > 2° > 1°. Tertiary alcohol: turbid at once. Secondary: turbid in about 5 minutes. Primary: no turbidity at room temperature (only on heating). Other reagents: PCl₅, PCl₃, and SOCl₂ (best, the by-products SO₂ and HCl are gases).

(ii) Dehydration (elimination).

Write the mechanism in three steps: (1) protonation of OH, (2) slow loss of water to give a carbocation, (3) loss of H⁺ from the adjacent carbon to give the alkene. Ease of dehydration: 3° > 2° > 1°. At 413 K with excess ethanol, ethoxyethane forms instead (the protonated alcohol is attacked by a second alcohol molecule in an SN2 step; this works only for 1° alcohols).

(iii) Oxidation.

6. Reactions of Phenols

The –OH group donates its lone pair into the ring (+R), so phenol is strongly activated and the attack goes to the ortho and para positions. Assertion "–OH is meta-directing" is always false.

• Nitration: dilute HNO₃ at low temperature gives o- + p-nitrophenol (separated by steam distillation, since the ortho isomer is volatile). Conc. HNO₃ gives 2,4,6-trinitrophenol (picric acid).
• Halogenation: Br₂ in CS₂ or CHCl₃ at low temperature gives mainly o/p-bromophenol (mono). Bromine water gives a white ppt of 2,4,6-tribromophenol (no catalyst is needed).
• Kolbe reaction: sodium phenoxide + CO₂ (400 K, 4–7 atm), then H⁺ gives salicylic acid.
• Reimer–Tiemann reaction: phenol + CHCl₃ + aq NaOH (340 K), then H⁺ gives salicylaldehyde. The electrophile is dichlorocarbene, :CCl₂.
• Zn dust distillation removes OH and gives benzene. Oxidation with Na₂Cr₂O₇/H₂SO₄ gives benzoquinone.
• Phenol gives a violet colour with neutral FeCl₃ (identification test).

7. Ethers

Preparation

1. Dehydration of 1° alcohol (excess alcohol, conc. H₂SO₄, 413 K).
2. Williamson synthesis: R–ONa + R′–X → R–O–R′ + NaX (SN2). Use a primary alkyl halide. Tertiary halides eliminate to give alkenes.

Reactions

1. Cleavage by HI (HI > HBr > HCl): R–O–R′ + HI → R–I + R′–OH; excess HI converts the alcohol to a second RI.
   • Both groups primary: the iodide forms on the smaller group (SN2).
   • One group tertiary: the iodide forms on the tertiary carbon (SN1).
   • Aryl alkyl ether (anisole): the alkyl–O bond breaks and phenol + alkyl iodide form, because the aryl–O bond has double bond character.
2. Electrophilic substitution of anisole: –OR is o/p-directing and activating. Halogenation (Br₂ in ethanoic acid, no catalyst), nitration (conc. HNO₃/H₂SO₄), Friedel–Crafts alkylation and acylation (anhydrous AlCl₃) all give o- and p-products, with p- as the major product.

8. Quick Sheet & Last-Minute Checklist

Before the exam, check you can:

• Write the acidity order (phenol > water > ROH) with the resonance reason.
• Give b.p. reasons: H-bonding, branching, o- vs p-nitrophenol.
• Write the three-step dehydration mechanism, and say which step is slow.
• Predict products of 1°, 2°, 3° alcohols with PCC, KMnO₄ and Cu at 573 K.
• Write Kolbe and Reimer–Tiemann with reagents and the products.
• Pick the correct halide for a Williamson synthesis and predict the HI cleavage product.