Alcohols, Phenols and Ethers

Alcohols, Phenols and Ethers

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alcohols are R–OH compounds with a hydroxyl on an sp³ (saturated) carbon. Phenols carry the –OH directly on an aromatic sp² carbon (Ar–OH). Ethers have a C–O–C linkage (R–O–R or R–O–Ar). Boiling points fall in the order alcohols/phenols ≫ ethers ≫ alkanes of similar mass because of intermolecular hydrogen bonding in –OH compounds. Acidic strength: carboxylic acid > phenol > water > primary alcohol (phenoxide is resonance-stabilised over the ring). Alcohols + Na release H₂; phenols + NaOH form soluble sodium phenoxide, distinguishing them from alcohols. ECAT tests mostly MCQs on Lucas test, Williamson synthesis, dehydration, and oxidation patterns.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Classification and Nomenclature

Monohydric alcohols are 1°, 2° or 3° depending on whether the C–OH carbon is bonded to 1, 2 or 3 other carbons. IUPAC names drop the -e of the parent alkane and add -ol (methanol, ethanol, propan-2-ol). For ethers the smaller alkyl becomes the alkoxy substituent of the larger chain (methoxyethane, not ethyl methyl ether in IUPAC). Phenols keep the parent name with -ol or the older “phenol” stem (o-cresol, catechol, resorcinol, hydroquinone, phloroglucinol).

Physical Properties and H-Bonding

O–H…O hydrogen bonding requires a hydrogen attached to the electronegative O. Ethers lack O–H, so even though their oxygen has lone pairs they cannot donate H-bonds — hence diethyl ether (M = 74) boils at 35 °C while butan-1-ol (M = 74) boils at 118 °C. Phenols are denser than alcohols because the aromatic ring packs efficiently.

Acidity Order

Phenol’s pKₐ ≈ 10, water ≈ 15.7, ethanol ≈ 16. Phenoxide is stabilised by delocalisation of the negative charge into the benzene ring (five resonance structures). Alcohols form alkoxides only with active metals (Na, K), not with NaOH; phenols dissolve in aqueous NaOH because they are acidic enough. Carboxylic acids (pKₐ ≈ 4–5) are still stronger than phenols — a common confusion point.

Reactions of Alcohols

• With Na: 2 R–OH + 2 Na → 2 R–ONa + H₂↑ (tests for active hydrogen).
• With PCl₅: R–OH + PCl₅ → R–Cl + POCl₃ + HCl (white fumes).
• Lucas test (anh. ZnCl₂ + conc. HCl): 3° alcohol gives turbidity immediately, 2° in 5–10 min, 1° shows no reaction at room temperature (SN1 vs SN2 governed by carbocation stability: 3° > 2° > 1°).
• Oxidation: 1° → aldehyde → carboxylic acid (PCC stops at aldehyde; KMnO₄ goes to acid); 2° → ketone; 3° resists oxidation.
• Dehydration: at 413 K acid-catalysed gives ether (2 R–OH → R–O–R); at 443 K intramolecular gives alkene.

Reactions of Phenols

• Soluble in NaOH (not NaHCO₃ — distinguishes from carboxylic acids).
• Neutral FeCl₃ → violet/purple/blue colour (characteristic test).
• Activated ring → electrophilic aromatic substitution (bromination with Br₂/H₂O gives 2,4,6-tribromophenol instantly, no catalyst needed).

Williamson Synthesis

R–O⁻ Na⁺ + R′–X → R–O–R′ + NaX (SN2). The alkoxide should ideally be 1° to avoid E2 elimination; 3° alkyl halides give alkenes, not ethers — a classic ECAT trap.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Acid-Catalysed Dehydration Mechanism

Protonation of –OH makes it a good leaving group (H₂O); a 3° carbocation forms, then a β-hydrogen is lost to give the alkene. At lower temperature bimolecular attack of a second alcohol molecule on the carbocation gives the ether. This is why temperature control decides product: 413 K → ether, 443 K → alkene.

Ether Cleavage

Ethers are inert to most reagents but are cleaved by excess conc. HI or HBr at high temperature. With a simple ether and HI, the smaller alkyl group leaves as RI (SN2 attack on the less hindered carbon) and the larger fragment becomes the alcohol: R–O–R′ + HI → R–I + R′–OH. With HI in excess, both fragments become alkyl iodides.

Phenol vs Alcohol: Why Phenols Are More Acidic

Alkoxide’s negative charge has no resonance stabilisation, so it stays localised on oxygen. Phenoxide delocalises charge into ortho and para positions of the ring — verified experimentally by equal C–O bond lengths in solid phenoxide salts (≈ 1.27 Å, between single and double bond). This makes phenol ~10⁶× more acidic than ethanol.

ECAT-Specific Strategy

ECAT Chemistry allocates ~3 % marks to this chapter. Question patterns focus on: (i) reagent identification in conversion series (alcohol → aldehyde → acid, alcohol → alkene, phenol → 2,4,6-tribromophenol), (ii) reagent matching for Williamson synthesis and Lucas test, (iii) ranking boiling points and acidities of given structures. One single correct MCQ typically appears; time budget ~1.5 min.

Common Mistakes

• Writing dehydration as producing ethers unconditionally — always specify temperature.
• Calling ethers “reactive like alcohols” — ethers resist bases, oxidisers and reducing agents.
• Assuming tertiary alcohols are easily oxidised — they break down via C–C cleavage only under harsh conditions.
• Forgetting that phenols do NOT give the Lucas test (no sp³ C–OH).

Practice Prompts

1. Arrange ethanol, phenol, acetic acid and water in increasing order of acidity and justify with one line of reasoning each.
2. Propose a Williamson route to tert-butyl ethyl ether; identify why this route fails and give the actual major product.

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