Alcohols, Phenols & Ethers

Concept overview

Alcohols (R–OH), phenols (Ar–OH) and ethers (R–O–R') share a single oxygen, but their chemistry diverges sharply. Alcohols react as weak nucleophiles at the O, weak acids at the OH proton, and can be dehydrated to alkenes or oxidised to carbonyls. Phenols are far more acidic than alcohols (pK_a ~10 vs ~16) because the phenoxide anion is resonance-stabilised across the aromatic ring. Ethers are relatively unreactive but undergo cleavage by HI or HBr and can react as Lewis bases in ether-metal complexes.

The Williamson synthesis is the workhorse preparation of ethers: alkoxide + primary alkyl halide → ether (an SN2 reaction). Choose the primary alkyl halide as the electrophile; using the wrong pairing (bulky alkoxide + secondary halide) gives elimination as the main product.

Phenols have their own reaction set. The Reimer-Tiemann reaction converts phenol to salicylaldehyde using CHCl₃ + NaOH. The Kolbe reaction gives salicylic acid from phenol + CO₂ under high pressure. Phenol undergoes rapid electrophilic aromatic substitution at ortho and para positions — an –OH is a strong activator that turns benzene into a super-reactive substrate for halogenation and nitration.

Alcohols undergo three canonical reaction families. Dehydration to alkenes under H₂SO₄ or Al₂O₃ (goes via an E1 mechanism for 2° and 3° alcohols). Oxidation — primary alcohols → aldehydes → carboxylic acids (KMnO₄, K₂Cr₂O₇, or PCC for stopping at aldehyde). Secondary alcohols → ketones. Tertiary alcohols resist oxidation because the α-carbon has no H to remove.

Key reactions & principles

• Acidity order: carboxylic > phenol > water > alcohol
• Phenol pK_a: ~10 (much more acidic than alcohols due to resonance)
• Williamson ether synthesis: R–O⁻ + R'–X → R–O–R' + X⁻ (SN2)
• Lucas test: 3° alcohol reacts fastest with HCl/ZnCl₂; 1° slowest
• PCC oxidation: 1° alcohol → aldehyde (stops there); 2° alcohol → ketone
• Reimer-Tiemann: Phenol + CHCl₃/NaOH → salicylaldehyde
• Kolbe reaction: Phenoxide + CO₂ / high P → salicylic acid

Common mistakes to avoid

• Attempting Williamson synthesis with a tertiary alkyl halide — elimination dominates. Always use the primary halide as the electrophile.
• Confusing oxidation stopping points — KMnO₄ takes a primary alcohol all the way to a carboxylic acid; PCC stops at aldehyde.
• Dehydrating a primary alcohol at low temperature — you need higher T (170°C+) for primary alcohols; secondary and tertiary dehydrate at lower T.
• Assigning phenol as a weak acid — phenol is much stronger than water (pK_a 10 vs 15.7), which is why it dissolves in NaOH but not NaHCO₃.

JEE Main relevance

JEE Main asks 1–2 questions from this chapter every paper — one on preparation (Williamson synthesis, Reimer-Tiemann, Kolbe) and one on acidity ordering or oxidation-product prediction. All archetypes are clean pattern-matching.

JEE Advanced relevance

Advanced problems combine alcohol/phenol chemistry with mechanism — for instance, retrosynthesis of a complex ether via a Williamson step, or predicting the major EAS product on a substituted phenol given competing directive effects. The pinacol-pinacolone rearrangement is another Advanced favourite.

Previous-year questions (PYQ)

This chapter's PYQs favour named reactions (Williamson, Reimer-Tiemann, Kolbe) and acidity orderings. Practise both in the JEETutorAI PYQ bank filtered chapter-wise.