ChemistryModerate Weightage ★★★Class 12
Alcohols, Phenols & Ethers
Classification, Lucas test, acidity comparison, oxidation, Williamson ether synthesis, and Reimer-Tiemann — expect 2–3 EAPCET questions every year.
2–3Questions in EAPCET
~3%Paper Weightage
8Key Reactions
3Mistake Traps
Concept Core
Alcohols, phenols, and ethers — preparation, reactions, and acidity comparisons.
Acidity Comparison — The Most-Tested Concept
Stability of conjugate base (anion) determines acid strength. More stable the anion → stronger the acid.
Phenol (pKa ≈ 10) > Water (pKa ≈ 15.7) > Alcohols (pKa ≈ 16–18)
Why phenol is most acidic: Phenoxide ion (C₆H₅O⁻) is stabilised by resonance delocalisation of negative charge into the aromatic ring → 5 resonance structures. Alkoxide (RO⁻) has no such stabilisation.
Effect of substituents: EWG (−NO₂, −Cl) at o/p on phenol ring → increase acidity. EDG (−CH₃, −OCH₃) → decrease acidity.
Lucas Test (1°, 2°, 3° Alcohol Distinction)
Lucas reagent: Anhydrous ZnCl₂ + conc. HCl
Reaction: R-OH + HCl → R-Cl (turbidity due to insoluble alkyl chloride)
| Alcohol | Observation | Reason |
|---|
| 3° (tertiary) | Immediate turbidity | SN1 via stable 3° carbocation |
| 2° (secondary) | Turbidity in ~5 min | Slower SN1 |
| 1° (primary) | No turbidity at RT | No stable carbocation; needs heat |
Reactions of Alcohols
With Na: 2R-OH + 2Na → 2R-ONa + H₂↑ (H₂ gas evolved)
Oxidation: 1° → RCHO (aldehyde, PCC) → RCOOH (KMnO₄)
2° → R-CO-R' (ketone)
3° → no oxidation (no H on C-OH)
Dehydration: R-OH → alkene (conc. H₂SO₄, 170°C) [Zaitsev applies]
Etherification: 2R-OH → R-O-R + H₂O (conc. H₂SO₄, 140°C)
Esterification: R-OH + R'COOH ⇌ R'COOR + H₂O (H₂SO₄ catalyst)
Phenol — Distinctive Reactions
FeCl₃ test: PhOH + FeCl₃ → violet/purple complex (diagnostic test)
Kolbe's reaction: PhONa + CO₂ (pressure) → o-HOC₆H₄COONa (salicylate → aspirin)
Reimer-Tiemann: PhOH + CHCl₃/NaOH (aq.) → o-HO-C₆H₄-CHO (salicylaldehyde)
Bromination: PhOH + Br₂(aq.) → 2,4,6-tribromophenol (white ppt) — no Lewis acid needed
Phenol is much more reactive toward EAS than benzene (−OH is powerful o/p activator via +M effect).
Williamson Ether Synthesis
R-O⁻Na⁺ + R'-X → R-O-R' + NaX
Critical rule: R'-X must be a primary alkyl halide (or methyl). Using 2° or 3° R'-X → E2 elimination (not substitution), because alkoxide is a strong base.
For unsymmetrical ethers: the larger/more branched group should come from the alcohol side; the smaller/primary group from the alkyl halide side.
Ether Properties and Cleavage
Ethers are relatively inert. Strong acid (HI) cleaves them:
R-O-R' + HI → R-I + R'-OH (SN2 if both primary)
Mixed ether with 3° R: R-I + R'-OH (SN1 at 3° carbon)
HI is most reactive (I⁻ is best nucleophile + strongest acid of HX series). HBr next; HCl least effective.
Ethers are good solvents (Grignard reagent preparation requires dry ether).
Formula Vault
Alcohol, phenol, and ether reactions for EAPCET.
Acidity Order
PhOH > H₂O > ROH
pKa: phenol~10; water~15.7; alcohol~16–18
Lucas Test Order
3° (instant) > 2° (~5 min) > 1° (none)
ZnCl₂ + conc. HCl reagent
1° Alcohol Oxidation
PCC → RCHO; KMnO₄ → RCOOH
PCC = pyridinium chlorochromate (stops at aldehyde)
2° Alcohol Oxidation
KMnO₄ / K₂Cr₂O₇ → ketone
Ketones resist further oxidation
3° Alcohol
Resistant to oxidation
No H on the C bearing OH
Williamson Synthesis
RONa + R'X(primary) → ROR' + NaX
R'X must be primary to avoid E2
Reimer-Tiemann
PhOH + CHCl₃/NaOH → o-OH-C₆H₄-CHO
Electrophile: :CCl₂ (dichlorocarbene)
FeCl₃ Test
Phenol + FeCl₃ → violet colour
Diagnostic; alcohols give no colour
Worked Examples
5 problems — acidity, Lucas test, oxidation, Williamson, and substituent effect on phenol acidity.
EasyArrange in decreasing acidity: phenol, ethanol, water▾
Arrange C₆H₅OH, C₂H₅OH, and H₂O in decreasing order of acidity.
1
Acidity = stability of conjugate base (anion after losing H⁺).
2
Phenoxide C₆H₅O⁻: resonance-stabilised over 5 structures → most stable → most acidic.
3
Hydroxide OH⁻: moderately stable.
4
Ethoxide C₂H₅O⁻: alkyl +I destabilises → least stable → least acidic.
5
Order: C₆H₅OH > H₂O > C₂H₅OH
✓ C₆H₅OH > H₂O > C₂H₅OH
EasyIdentify: which alcohol gives immediate turbidity with Lucas reagent?▾
Three alcohols: n-propanol (1°), isopropanol (2°), tert-butanol (3°). Which reacts immediately with Lucas reagent?
1
Lucas reagent (ZnCl₂ + conc. HCl) tests via SN1 mechanism.
2
3° alcohols form stable tertiary carbocations → fastest SN1 → immediate turbidity.
3
tert-Butanol is the 3° alcohol.
✓ tert-Butanol (3°) — immediate turbidity via stable 3° carbocation
MediumPropose Williamson synthesis for methyl tert-butyl ether (MTBE)▾
Suggest a Williamson synthesis for CH₃-O-C(CH₃)₃ (methyl tert-butyl ether).
1
Option 1: (CH₃)₃CO⁻Na⁺ + CH₃Br → MTBE + NaBr ✓ [CH₃Br is primary — SN2 works]
2
Option 2: CH₃O⁻Na⁺ + (CH₃)₃CBr → MTBE? ✗ [(CH₃)₃CBr is tertiary → E2 gives isobutene!]
3
Only Option 1 works. The primary alkyl halide must provide the R' group.
✓ Use (CH₃)₃CONa + CH₃Br; tertiary bromide + methoxide gives elimination not ether
EAPCET LevelCompare acidity: phenol vs p-nitrophenol vs p-methylphenol▾
Arrange phenol, p-nitrophenol, and p-methylphenol in increasing order of acidity.
1
p-Nitrophenol: −NO₂ is strongly electron-withdrawing (−M, −I) at para position → extra stabilises phenoxide anion → most acidic.
2
Phenol: base case. Phenoxide stabilised by ring resonance alone.
3
p-Methylphenol (p-cresol): −CH₃ is electron-donating (+I) → destabilises phenoxide → least acidic.
4
Increasing order: p-CH₃-C₆H₄-OH < C₆H₅OH < O₂N-C₆H₄-OH
✓ Increasing acidity: p-cresol < phenol < p-nitrophenol
Trap Question3° alcohols cannot be oxidised — does this mean they cannot react with K₂Cr₂O₇?▾
Will tert-butanol react with acidified K₂Cr₂O₇? A student says no — is this correct?
1
The trap: 3° alcohols resist OXIDATION under mild conditions because there is no H on the carbon bearing −OH.
2
K₂Cr₂O₇ (acidified) is a strong oxidising agent. Under very forcing conditions (concentrated acid, high temperature), C–C bonds can break (combustion-like reactions).
3
However, for the purpose of EAPCET and standard organic chemistry: 3° alcohols do NOT give colour change with K₂Cr₂O₇ under mild conditions.
4
1° → orange to green (RCHO or RCOOH formed). 2° → orange to green (ketone). 3° → no colour change (no oxidation).
✓ Correct in standard context — 3° alcohols don't oxidise with K₂Cr₂O₇ under lab conditions (no H on C-OH)
Mistake DNA
3 common errors from EAPCET distractor analysis.
📊
Alcohols Are More Acidic Than Water
Alcohols (pKa 16–18) are LESS acidic than water (pKa 15.7). Alkyl groups (+I) destabilise the alkoxide anion.
❌ Wrong
Acidity: ROH > H₂O ✗
(alkyl groups push e⁻
onto O⁻ — destabilise!)
✓ Correct
H₂O > ROH ✓
Alkyl groups: +I effect ✓
→ destabilise RO⁻ anion ✓
Makes alcohol weaker acid
The +I effect of alkyl groups increases electron density on O⁻, making it less stable (more basic). Less stable anion = weaker conjugate acid = alcohol is weaker acid than water.
🔬
Lucas: 1° Alcohols React Quickly
1° alcohols show NO turbidity at room temperature. They can only react (via SN2) on heating.
❌ Wrong
1° alcohol with Lucas:
immediate turbidity ✓ ✗
(only 3° gives immediate!)
✓ Correct
1° at RT: no turbidity ✓
2° at RT: turbidity ~5 min ✓
3° at RT: immediate ✓
The reaction proceeds through carbocation (SN1). Primary carbocations are extremely unstable — so 1° alcohols essentially don't react at room temperature. Heating forces SN2 eventually.
⚗️
Williamson with Secondary/Tertiary Alkyl Halide Works
With 2° or 3° R'X, alkoxide (strong base) causes E2 elimination instead of substitution. Only primary R'X gives ether.
❌ Wrong
(CH₃)₃CBr + NaOCH₃:
Ether formed ✗
(E2 gives isobutene!)
✓ Correct
Primary R'X only ✓
(CH₃)₃CO⁻ + CH₃Br ✓
→ MTBE via SN2 ✓
Alkoxide is a strong base (pKa of alcohol ~16). With hindered (2° or 3°) substrates, E2 elimination dominates over SN2. Use primary alkyl halide for clean ether formation.
Chapter Intelligence
Alcohols/phenols/ethers connect to GOC (acidity theory), carbonyls (oxidation products), and haloalkanes (Williamson).
EAPCET Weightage (2019–2024)
Lucas test interpretation~7 Williamson ether synthesis~6 Phenol reactions (FeCl₃, Kolbe, R-T)~5 Oxidation of 1°/2°/3° alcohols~4 High-Yield PYQ Patterns
PhOH > H₂O > ROH acidityLucas test: 3° → immediateWilliamson: primary R'X essentialEWG increases phenol acidityFeCl₃ → violet with phenolIodoform test for CH₃CH(OH)RPCC stops at aldehyde, KMnO₄ goes to acid
Exam Strategy
- Acidity order: phenol > water > alcohols. For substituted phenols: EWG at o/p increases acidity; EDG decreases acidity. This is a very frequent direct question.
- Lucas test: memorise 3° = immediate, 2° = 5 minutes, 1° = no reaction at RT. The question gives you the observation and asks you to identify the alcohol type.
- Williamson synthesis: ALWAYS use primary alkyl halide. The alkoxide provides the -O- part; the primary R'X provides the other R' by SN2.
- FeCl₃ test is diagnostic for phenols (violet/purple). Alcohols give no colour. This appears in identification problem sets regularly.