Alcohols and Ethers

🟢 Lite — Quick Review (1h–1d)

Rapid summary of alcohol and ether nomenclature, properties, and reactions for quick revision.

Alcohols — Classification and Nomenclature:

Alcohols contain the –OH functional group attached to a saturated carbon. They are classified as:

• Primary (1°): –OH attached to carbon bonded to only ONE other carbon (e.g., ethanol, CH₃CH₂OH)
• Secondary (2°): –OH attached to carbon bonded to TWO other carbons (e.g., propan-2-ol, (CH₃)₂CHOH)
• Tertiary (3°): –OH attached to carbon bonded to THREE other carbons (e.g., 2-methylpropan-2-ol)

IUPAC naming: replace the –e of the parent alkane with –ol. The position of –OH is indicated by the lowest possible number. Examples: methanol (CH₃OH), ethanol (C₂H₅OH), propan-1-ol (CH₃CH₂CH₂OH), propan-2-ol (CH₃CH(OH)CH₃).

Physical Properties of Alcohols:

Boiling points increase with molecular weight due to hydrogen bonding. Among isomers, branching lowers boiling point (2-methylpropan-2-ol has lower bp than butan-1-ol due to reduced surface area). Alcohols with ≤3 carbons are miscible with water in all proportions — the –OH group forms strong H-bonds with water. As chain length increases, hydrophobic alkyl effect dominates and miscibility decreases.

Ethers — Structure and Nomenclature:

Ethers have the general formula R–O–R′, where two alkyl/aryl groups are attached to oxygen. Named as “alkoxy alkane”: the smaller alkyl group is named as an alkoxy substituent. Examples: methoxymethane (CH₃OCH₃, also called dimethyl ether), ethoxyethane (C₂H₅OC₂H₅, also called diethyl ether), methoxyethane (CH₃OCH₂CH₃).

Symmetrical ethers have two identical R groups; unsymmetrical ethers have different R groups.

⚡ Exam tip: In UTBK questions, distinguish between alcohols and ethers by their molecular formulas — alcohols have formula CₙH₂ₙ₊₂O while ethers have CₙH₂ₙ₊₂O (same general formula but different connectivity).

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🟡 Standard — Regular Study (2d–2mo)

Deeper coverage of reactions, mechanisms, and problem-solving strategies.

Chemical Reactions of Alcohols:

1. Combustion: Complete combustion produces CO₂ and H₂O. $C_2H_5OH + 3O_2 → 2CO_2 + 3H_2O$ (exothermic, ΔH = –1367 kJ/mol)

2. Oxidation:

   • Primary alcohol → aldehyde (with PCC) → carboxylic acid (with K₂Cr₂O₇/H₂SO₄)
   • Secondary alcohol → ketone (with K₂Cr₂O₇/H₂SO₄)
   • Tertiary alcohol: resistant to oxidation (no α-hydrogen) Test: Acidified K₂Cr₂O₇ (orange) turns green in presence of primary/secondary alcohols.

3. Dehydration: With concentrated H₂SO₄ at 140°C, alcohols lose water to form alkenes (E1 mechanism). More substituted alkene is the major product (Zaitsev’s rule). At 180°C with lower H₂SO₄ concentration, ethers form instead.

4. Esterification: Alcohol + carboxylic acid ⇌ ester + H₂O (acid-catalyzed, reversible) $RCOOH + R’OH ⇌ RCOOR’ + H_2O$

5. Reaction with Na metal: 2ROH + 2Na → 2RONa + H₂↑ (test for –OH group)

Reactions of Ethers:

Ethers are chemically inert (no H-bonding, no –OH group). They undergo:

1. Cleavage by HBr/HI: Unsymmetrical ether: R–O–R′ + HI → RI + R′OH (the smaller alkyl becomes alkyl halide) Symmetrical ether: R–O–R + 2HI → 2RI + H₂O

2. Peroxide formation: Ethers form peroxides on exposure to air/light (explosive upon heating) — old samples must be tested before distillation.

⚡ Exam tip: In UTBK organic questions, alcohol dehydration to alkene uses concentrated H₂SO₄ at 140°C — at lower temperature (180°C), the product is ether instead. Watch for this temperature-dependent selectivity.

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🔴 Extended — Deep Study (3mo+)

Complete mechanistic understanding and advanced problem types.

Mechanism of Alcohol Dehydration (E1):

Step 1: Protonation — The –OH group is protonated by strong acid, making it an excellent leaving group (H₂O). Step 2: Carbocation formation — The C–OH bond breaks, generating a carbocation intermediate. Step 3: Elimination — A base (HSO₄⁻ or H₂O) removes a β-hydrogen, forming the C=C double bond.

Tertiary carbocations are most stable (tertienary > secondary > primary), so tertiary alcohols dehydrate most readily. Primary alcohols require harsh conditions and may undergo rearrangement.

Distinguishing Tests for Alcohols:

Test	Primary	Secondary	Tertiary
Lucas test (ZnCl₂/HCl)	No reaction (slow)	Turbidity in 5-10 min	Turbidity immediately
Oxidation (K₂Cr₂O₇)	Green (carboxylic acid)	Green (ketone)	No change
With Na metal	Vigorous H₂ evolution	Moderate H₂	Slow H₂

Glycol and Polyhydroxy Alcohols:

Ethane-1,2-diol (ethylene glycol) has two –OH groups. It is miscible with water and has high boiling point (197°C). Used as antifreeze in car radiators. Toxic if ingested — causes kidney failure.

Propane-1,2,3-triol (glycerol/glycerin) is a trihydroxy alcohol with viscosity and hygroscopic properties. Used in pharmaceuticals, food, and explosives (nitroglycerin).

Phenols vs Alcohols:

Phenols (Ar–OH) differ from aliphatic alcohols because the lone pair on oxygen is delocalized into the aromatic ring, making the O–H bond more acidic. Phenol turns litmus blue and reacts with NaOH to form phenoxide ions. Alcohols do not react with NaOH. Phenol has a pKa ≈ 10 while ethanol has pKa ≈ 16 — phenol is ~10⁶ times more acidic.

⚡ Exam tip: UTBK often tests the distinction between phenol and alcohol using reactions with NaOH (phenol reacts, alcohol doesn’t) or FeCl₃ solution (phenol gives violet color, alcohol gives no color).

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