OC Ch 04 · Alcohols · Targeted Review

15 Minute Review

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Day before or morning-of. Hit each card, confirm you can explain why, not just the answer.

1. Alcohol structure & classification

~2 min

Alcohol = R–OH (sp3 O, angle 108.5°). Phenol = Ar–OH. Enol = C=C–OH (unstable).

Degree by substitution at C–OH:

  • 1° — C–OH bonded to 1 carbon (e.g. ethanol)
  • 2° — C–OH bonded to 2 carbons (e.g. 2-propanol)
  • 3° — C–OH bonded to 3 carbons (e.g. tert-butanol)
  • No 4° alcohol — a quaternary C has no H and no room for –OH

IUPAC rules:

  • Longest chain containing C–OH → change -e to -ol
  • Number from the end giving –OH the lowest locant
  • Multiple –OH: -diol, -triol
H-bonding: Alcohols have higher bp than analogous hydrocarbons (O–H···O). C₁–C₄ miscible with water; solubility falls with chain length.

2. Alcohol reactions — master table

~4 min
ReactionReagent / ConditionsProductNotes
Na metalNa(s), RTAlkoxide RO⁻Na⁺ + H₂↑Less vigorous than Na + H₂O
HX (Lucas)ZnCl₂/HClAlkyl chloride R–Cl3°=instant; 2°=5–10min; 1°=heat
Dehydration (ether)H₂SO₄, 140°CR–O–RIntermolecular
Dehydration (alkene)H₂SO₄, 170°CAlkene (Zaitsev)More substituted alkene major
Oxidation 1°PCC / CH₂Cl₂Aldehyde (stops here)PCC = controlled
Oxidation 1°KMnO₄/H⁺ or K₂Cr₂O₄Carboxylic acidOver-oxidation
Oxidation 2°Any Cr/Mn reagentKetoneCannot go further
Oxidation 3°—No reactionNo α-H

3. Phenols

~3 min

Acidity: RCOOH > H₂CO₃ > phenol > H₂O > ROH. Phenoxide anion is resonance-stabilised (charge delocalised into ring).

Identification tests:

  • FeCl₃ solution → violet
  • Br₂ water → 2,4,6-tribromophenol (white ppt, instant, no catalyst needed)

Acid-base:

  • + NaOH → sodium phenoxide (soluble)
  • + CO₂/H₂O → phenol reprotonated (turbid)
  • Does NOT react with Na₂CO₃ (phenol weaker acid than H₂CO₃)
Exam trap: Phenol + Na₂CO₃ → no reaction (no CO₂ gas). Only carboxylic acids liberate CO₂ from Na₂CO₃.

4. Ethers

~2 min

Structure: R–O–R'. Relatively unreactive. IUPAC: alkoxy-alkane (methoxymethane = dimethyl ether).

Williamson synthesis:

  • NaOR + 1° R'X → ROR' + NaX
  • S₂ mechanism — must use primary halide
  • 2° or 3° R'X → elimination instead

Hazard & cleavage:

  • Autoxidation in air → explosive peroxides
  • Test: KI/starch paper (turns blue)
  • HI (excess) → 2 R–I + H₂O (both C–O bonds cleave)
  • HBr also cleaves ethers

5. Thiols & disulfides

~2 min

Thiol (R–SH):

  • More acidic than alcohols (S–H weaker than O–H)
  • Weaker H-bonds → lower bp than comparable alcohols
  • Strong odour (skunk, garlic)

Oxidation/reduction:

  • 2 RSH + I₂ → RSSR + 2 HI (disulfide)
  • Reduction: RSSR + Zn/H⁺ → 2 RSH
  • Biology: cysteine (SH) → cystine (SS); stabilises protein 3°/4° structure
Clinical link: Insulin = 3 disulfide bonds. Hair perming = reduce disulfides (break), reshape, re-oxidise (reform).

6. Past-paper drill (2019)

~2 min
StatementT/FReason
There is no quaternary alcohol (III.7)TRUEQuaternary C has 4 C-bonds; no room for –OH
3° alcohol oxidised to ketone by KMnO₄FALSE3° = no reaction (no α-H)
PCC converts 1° alcohol to carboxylic acidFALSEPCC stops at aldehyde
Phenol + Na₂CO₃ → CO₂ gasFALSEPhenol weaker acid than H₂CO₃
2,4,6-tribromophenol is a white precipitateTRUEPhenol + Br₂/H₂O → white ppt
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McMurry & Ballantine 8e Ch 17 · TMU Slide pp.1–63 · Past Paper 2019