Alcohols Phenol Ether

Alcohols Phenol Ether

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alcohols (R–OH) carry a hydroxyl group on an sp3 carbon and are classified as 1°/2°/3° by the number of carbons attached to the C–OH carbon. Phenols (Ar–OH) have –OH directly bonded to an aromatic sp2 carbon, while ethers (R–O–R′) have oxygen bridging two alkyl/aryl groups. Acidity follows the order H2O < ROH < phenol < o-/p-nitrophenol, driven by resonance stabilization of the phenoxide ion (pKa ≈ 10 vs ≈ 16 for ethanol). Phenol gives a violet colour with neutral FeCl3 — a textbook identification test. Williamson synthesis (R–ONa + R′–X → R–O–R′) is the standard route to ethers. For CUET, memorise acidic strength order, the Lucas test for 1°/2°/3° alcohols, and the reaction of phenol with bromine water (2,4,6-tribromophenol).

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature and Classification

Alcohols use the suffix –ol (e.g. propan-1-ol), phenols use –phenol (e.g. 2-nitrophenol), and ethers are named as alkoxyalkanes (e.g. methoxyethane). Monohydric alcohols carry one –OH; dihydric (glycols) and trihydric (glycerol) carry more. On the C–OH carbon, alcohols are primary (1°), secondary (2°) or tertiary (3°).

Physical Properties

Boiling points rise with molecular mass and H-bonding capacity. Among isomers, phenol’s b.p. is higher than aliphatic alcohols of similar mass because of stronger intermolecular H-bonding. o-Nitrophenol shows intramolecular H-bonding, lowering its b.p. and making it steam-volatile — a classic CUET fact.

Acidic Strength

Phenol (pKa ≈ 10) is far more acidic than water (15.7) or ethanol (16) because the phenoxide ion is resonance-stabilized over the ring. Electron-withdrawing groups like –NO2 at ortho/para positions further stabilize phenoxide, sharpening acidity. However, phenol is weaker than carbonic acid and does not liberate CO2 from NaHCO3 — yet it dissolves in aqueous NaOH.

Alcohol Reactions

• With Na: 2R–OH + 2Na → 2R–ONa + H2.
• With PCl5 / SOCl2: conversion to alkyl halides.
• Lucas test (HCl/ZnCl2): 3° alcohols react immediately, 2° within minutes, 1° shows no reaction at room temperature.
• Esterification: R–COOH + R′–OH ⇌ R–COOR′ + H2O (conc. H2SO4 catalyst).

Phenol Reactions

• NaOH: gives sodium phenoxide (acidic enough).
• FeCl3 (neutral): violet/blue colour — diagnostic test.
• Bromine water: 2,4,6-tribromophenol (white precipitate).
• Kolbe–Schmitt (CO2, NaOH, pressure): salicylic acid.
• Reimer–Tiemann (CHCl3, NaOH, then H⁺): salicylaldehyde.

Ether Preparation and Cleavage

Williamson synthesis (R–ONa + R′–X → R–O–R′) works best with primary alkyl halides; tertiary halides give elimination (E2). Cleavage with excess HI: alkyl aryl ethers break the alkyl–O bond, giving phenol + alkyl iodide because the aryl–O bond is reinforced by resonance and does not break.

Typical CUET Question Patterns

Direct MCQs on acidity comparison, the FeCl3 test, Lucas reactivity order, and Williamson synthesis preference. Assertion-reason questions on intramolecular H-bonding in o-nitrophenol appear frequently.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism: Phenoxide Resonance

When phenol loses H⁺, the resulting phenoxide carries negative charge delocalized over the ortho and para carbons of the ring (five resonance structures). This delocalization lowers the energy of the conjugate base, shifting the equilibrium toward dissociation. By contrast, alkoxide has no such resonance, so ethanol is a much weaker acid. Substituents modulate this: –NO2 at ortho/para withdraws charge through –M and –I effects, further stabilizing phenoxide; –CH3 or –OCH3 (electron-donating) destabilizes it, weakening acidity.

Edge Cases in Williamson Synthesis

The reaction proceeds via SN2, so the alkyl halide must be methyl or primary. A common trap is writing the alkoxide of a tertiary alcohol with a primary halide — this fails because tertiary alkoxides are strong bases that force E2 elimination on the primary halide. Conversely, aryl halides cannot be used because the C(sp2)–X bond resists SN2; unsymmetrical ethers with aryl groups must therefore be made from phenoxide + primary alkyl halide.

Ether Cleavage Nuances

With excess HI, the mechanism depends on the alkyl group: a primary alkyl ether follows SN2, giving phenol + R–I; a tertiary alkyl ether follows SN1 because the 3° carbocation is stable. Mixed alkyl aryl ethers (e.g. anisole, C6H5–O–CH3) yield phenol + CH3I, never iodobenzene, because the aryl–O bond has partial double-bond character from lone-pair donation.

Reactions Linking the Three Families

Phenols can be converted to ethers via Williamson, and alcohols to ethers via acid-catalysed dehydration (140 °C gives symmetrical ether; 170 °C gives alkene — another CUET favourite). Distillation of phenol with Zn dust gives benzene — proof of the aromatic ring.

Common Mistakes

• Assuming phenol reacts with NaHCO3 — it does not; it is weaker than carbonic acid.
• Forgetting that phenol turns blue litmus red while alcohols do not.
• Predicting Williamson with tertiary halides — elimination dominates.

Practice Prompts

1. Arrange phenol, ethanol, water and o-nitrophenol in increasing pKa, and justify using resonance and inductive effects.
2. Anisole (C6H5OCH3) is treated with excess HI. Identify the products and state whether the mechanism at the methyl carbon is SN1 or SN2.

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.