OC Ch 04 — Day-Before Review

4.1 & 4.2 — Structure, Classification & IUPAC

Slide p.1–11

Alcohol: compound with –OH bonded to an sp3 carbon. Oxygen is also sp3 (two lone pairs), bond angle 108.5°.

1° (primary): C–OH bonded to 1 carbon. Examples: methanol (CH₃OH), ethanol, 1-propanol.
2° (secondary): C–OH bonded to 2 carbons. Example: 2-propanol.
3° (tertiary): C–OH bonded to 3 carbons. Example: tert-butanol.

KEY EXAM FACT: There is NO quaternary alcohol. A quaternary carbon has 4 C–C bonds — it cannot bond to –OH. The statement "there is no 4° alcohol" is TRUE. (2019 Past Paper III.7)

IUPAC naming: (1) Find longest chain with C–OH. (2) Change -ane → -ol. (3) Number from end giving –OH the lowest locant. (4) Multiple –OH: -diol, -triol (keep -e: ethanediol).

Physical properties: –OH forms intermolecular H-bonds (O–H···O), so alcohols have much higher bp than alkanes of similar MW. C₁–C₄ miscible with water; solubility decreases as chain length increases (hydrophobic tail dominates).

4.4A — Reaction with Na Metal

Slide p.16

ROH + Na → RO⁻Na⁺ + ½ H₂↑

The –OH proton is acidic enough for Na to abstract it (pKₐ ROH ~16–18).
Less vigorous than Na + water (water more acidic than alcohol).
Product sodium alkoxide (RO⁻Na⁺) is a strong base — used in Williamson synthesis.
Phenol reacts faster than alcohols (pKₐ ~10, stronger acid).

4.4B — HX Reactions & Lucas Test

Slide p.20–23

ROH + HX → R–X + H₂O. Reactivity of HX: HI > HBr > HCl.

Lucas test: ZnCl₂ in conc. HCl. Converts –OH to –Cl. Alkyl chloride is insoluble → turbidity.
3°: immediate turbidity (stable 3° carbocation, fast S⁼1)
2°: turbid in 5–10 min (slower S⁼1)
1°: no turbidity at RT; requires heat (no stable carbocation, must use S⁼2)

Mechanism (3°): ZnCl₂ (Lewis acid) coordinates to –OH oxygen → water leaves → 3° carbocation → Cl⁻ attacks → R–Cl (turbid).

4.4C — Dehydration (Ether & Alkene)

Slide p.24–27

140°C — intermolecular dehydration → ether:

2 ROH + H₂SO₄ (140°C) → R–O–R + H₂O

Ethanol + ethanol → diethyl ether. Industrial production of ethers.

170°C — intramolecular dehydration → alkene:

ROH + H₂SO₄ (170°C) → alkene + H₂O

E2-like mechanism via protonation of –OH → good leaving group (H₂O); base removes β-H.

Zaitsev's rule: The major product of dehydration (and other eliminations) is the more-substituted alkene (more stable due to hyperconjugation).
Example: 2-butanol → H₂SO₄/170°C → 2-butene (83%) + 1-butene (17%). The more substituted product (2-butene) is favoured.

4.4D — Oxidation Ladder

Slide p.29–31

Starting material	Reagent	Product	Further?
1° alcohol	PCC / CH₂Cl₂	Aldehyde (RCHO)	No (PCC cannot go further)
1° alcohol	KMnO₄/H⁺ or K₂Cr₂O₄/H₂SO₄	Carboxylic acid (RCOOH)	Acid is end-point
2° alcohol	Any Cr/Mn oxidant	Ketone (RCOR')	No (no further oxidation)
3° alcohol	Any oxidant	No reaction	— No α-H

Why 3° = no reaction? Oxidation of alcohol requires removal of an H from the α-carbon (the C bearing –OH). A tertiary carbon bearing –OH has no H on that carbon, so oxidation cannot proceed.

4.5 — Phenols

Slide p.33–42

Structure: –OH directly on benzene ring. Phenol (C₆H₅OH) is the simplest phenol.

Acidity & acid-base:

Acidity order: RCOOH > H₂CO₃ > phenol > H₂O > ROH
Phenoxide (C₆H₅O⁻) resonance-stabilised → stronger acid than ROH.
Reacts with NaOH → sodium phenoxide (soluble).
CO₂ reprotoates phenoxide → phenol precipitates (turbid).
Does NOT react with Na₂CO₃ (phenol weaker acid than H₂CO₃).

Identification tests:

FeCl₃ solution → violet colour (diagnostic for phenol)
Br₂ water (no catalyst) → white ppt of 2,4,6-tribromophenol
(instantaneous; –OH is a powerful o/p director)
Esterification with acyl chloride (RCOCl) → ester + HCl

4.6 — Ethers

Slide p.47–59

Structure: R–O–R'. Relatively unreactive (no acidic H, not easily oxidised). Used as solvents (diethyl ether, THF).

Williamson ether synthesis:
NaOR + primary R'X → ROR' + NaX
Mechanism: S₂ (backside attack on primary C).
Why primary R'X? Secondary/tertiary R'X → elimination (E2) instead of substitution because RO⁻ is a strong base.
Example: NaOEt + CH₃Br → EtOCH₃ + NaBr

Ether peroxide hazard:
Diethyl ether + O₂ (air, light) → autoxidation → explosive peroxides.
Test: KI/starch paper turns blue (peroxide oxidises I⁻ → I₂).
Cleavage with HI (excess):
R–O–R + 2 HI → 2 R–I + H₂O
(Both C–O bonds cleave under excess HI)

4.7 — Thiols & Disulfides

Slide p.60–63

Thiol (R–SH): sulfur analogue of alcohol. More acidic than alcohol (S–H bond weaker). Lower bp than comparable alcohol (S–H···S-H weaker H-bonds). Pungent odour.

Oxidation to disulfide:
2 RSH + I₂ → R–S–S–R + 2 HI
(mild oxidation; iodine sufficient)
Reduction of disulfide:
RSSR + Zn/H⁺ → 2 RSH
(or β-mercaptoethanol in biochemistry)

Biological significance:
Amino acid cysteine (side chain –CH₂SH) oxidises to cystine (disulfide bridge).
Disulfide bonds = covalent cross-links in protein tertiary/quaternary structure.
Insulin: 3 disulfide bonds.
Hair waving: reduce SS bonds, reshape, re-oxidise.

★ Walk-In Algorithm

For exam hall

Given an alcohol:
1. Identify degree (count C-bonds on C–OH): 1° / 2° / 3°
2. Lucas test → 3°=instant, 2°=5-10min, 1°=heat only
3. Dehydration temp? → 140°C=ether, 170°C=alkene (Zaitsev = more substituted)
4. Oxidation? → 1° + PCC=aldehyde, 1° + KMnO₄=acid, 2°=ketone, 3°=NONE
5. Is it a phenol? → FeCl₃ test (violet), Br₂ water (white ppt), reacts NaOH not Na₂CO₃
6. Thiol reactions? → I₂ oxidises RSH→RSSR; Zn/H⁺ reduces back

★ T/F Final Drill

Self-test

Statement	Answer	Reason
There is no quaternary alcohol (2019 III.7)	TRUE	4° C has 4 C-bonds, no H, cannot carry –OH
A tertiary alcohol is oxidised to a ketone	FALSE	3° = no reaction (no α-H)
PCC oxidises a primary alcohol to a carboxylic acid	FALSE	PCC stops at aldehyde stage
Phenol reacts with Na₂CO₃ to give CO₂	FALSE	Phenol weaker acid than H₂CO₃; no gas
FeCl₃ gives violet colour with phenol	TRUE	Diagnostic test for phenol
Williamson synthesis uses NaOR + tertiary R'X	FALSE	Must use primary R'X; tertiary gives elimination
Diethyl ether in air forms explosive peroxides	TRUE	Autoxidation; KI/starch test for detection
Cystine is formed by reduction of two cysteine residues	FALSE	Cystine = oxidation product (disulfide bond)