OC Ch 04 · Alcohols · Panic Mode

5 Minute Cram

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Walking into the hall. Lucas test order. Oxidation ladder. Acidity order. Zaitsev. Phenol tests. Don’t overthink.

1. Lucas test — the order

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Alcohol classResult with ZnCl₂/HCl
3° (tertiary)Turbid immediately
2° (secondary)Turbid in 5–10 min
1° (primary)No reaction at RT (requires heat)
Why 3° fastestS⁼1 — stable tertiary carbocation forms instantly.

2. Oxidation ladder

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1° alcohol+ PCC → aldehyde  |  + KMnO₄ → carboxylic acid
2° alcohol+ any oxidant → ketone
3° alcoholNo reaction (no α-H)
PCC trickPCC stops at aldehyde. KMnO₄ goes all the way to acid.

3. Dehydration temperature rule

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140°CIntermolecular → ether (R–O–R)
170°CIntramolecular → alkene
ZaitsevMore substituted alkene = major product. 2-butanol → 2-butene (83%) + 1-butene (17%).

4. Acidity order

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Strongest → weakestRCOOH > H₂CO₃ > Phenol > H₂O > ROH
Key implicationPhenol dissolves in NaOH but CO₂ reprotoates it (precipitate). Phenol does NOT react with Na₂CO₃.

5. Phenol identification tests

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FeCl₃→ Violet colour (phenol only)
Br₂ water→ White ppt of 2,4,6-tribromophenol
Na metal→ H₂↑ (both phenols and alcohols react)

6. Williamson ether synthesis & thiol oxidation

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WilliamsonNaOR + 1° R'X → R–O–R' + NaX  (S₂; must use 1° halide)
Ether peroxideTest: KI/starch paper turns blue. Danger: explosive.
Thiol oxidation2 RSH + I₂ → RSSR + 2 HI  (cysteine → cystine)
Disulfide reductionRSSR + Zn/H⁺ → 2 RSH

7. T/F traps

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TRUE"No quaternary alcohol" — a quaternary C has 4 C-bonds, no room for –OH. (2019 III.7)
FALSE"3° alcohols oxidise to ketones." → 3° = NO REACTION.
FALSE"PCC oxidises primary alcohol to carboxylic acid." → PCC stops at aldehyde.
FALSE"Phenol reacts with Na₂CO₃." → Phenol is weaker acid than H₂CO₃; no CO₂ gas.
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McMurry & Ballantine 8e Ch 17 · TMU Slide pp.1–63 · Past Paper 2019 III.7