Board exam focus — Alcohols, Phenols and Ethers (CBSE & HBSE)

Preparation and reactions of alcohols and phenols, acidity comparison, reactions of phenol (electrophilic substitution), Williamson synthesis of ethers, industrial importance.

Alcohols - Preparation and Properties

Alcohols

Classification

• Primary (1°): R-CH2-OH (e.g., CH3CH2OH)
• Secondary (2°): R-CHOH-R (e.g., (CH3)2CHOH)
• Tertiary (3°): R3C-OH (e.g., (CH3)3COH)

Monohydric (1 OH), dihydric (2 OH = glycols), trihydric (3 OH = glycerol/triols)

Preparation

1. From Alkenes: Acid-catalyzed hydration: RCH=CH2 + H2O (H+) → RCH(OH)CH3 (Markovnikov) Hydroboration-oxidation: RCH=CH2 + BH3 → RCH2CH2OH (anti-Markovnikov)

2. From Carbonyl Compounds: Reduction: RCHO + [H] → RCH2OH (primary) RCOR + [H] → RCHOHR (secondary) Reducing agent: LiAlH4 (strongest), NaBH4 (mild, preferred for ketones), H2/catalyst

Grignard reaction: RMgX + HCHO → RCH2OH (primary) RMgX + RCHO → R-CHOH-R (secondary) RMgX + RCOR → R3COH (tertiary)

3. From Haloalkanes: RX + KOH (aq.) → ROH + KX (SN2)

4. Fermentation (Industrial): (C6H12O6) → 2C2H5OH + 2CO2 (yeast, invertase, zymase) Sucrose → glucose + fructose → ethanol

Physical Properties

H-bonding: OH group forms strong H-bonds. Boiling points: Much higher than corresponding hydrocarbons. BP: 1° > 2° > 3° for same molecular weight (branching decreases H-bonding area) Solubility: Lower alcohols (C1-C4) miscible with water; higher alcohols insoluble.

Chemical Properties

Reaction with Na: 2ROH + 2Na → 2RONa + H2 (Na faster with primary) Acidity: H2O > 1° > 2° > 3° alcohol (alkyl groups donate electrons, destabilize alkoxide)

Esterification: ROH + RCOOH ⇌ RCOOR + H2O (H+ catalyst)

Dehydration to alkene: ROH (conc. H2SO4, 170 C) → alkene + H2O Dehydration to ether: 2ROH (conc. H2SO4, 140 C) → ROR + H2O

Oxidation: 1° alcohol → aldehyde (with PCC or K2Cr2O7 acidic) → carboxylic acid 2° alcohol → ketone (with K2Cr2O7) 3° alcohol → no oxidation (no H on C-OH carbon)

Lucas Test (for identification): Lucas reagent = anhydrous ZnCl2 + conc. HCl 3° alcohol: turbidity immediate (SN1, stable carbocation) 2° alcohol: turbidity in 5 min 1° alcohol: no turbidity (SN2, very slow or requires heat)

Phenols - Preparation and Reactions

Phenols

Preparation of Phenol

1. From benzene sulfonic acid: C6H5SO3H + NaOH (fusion, 300 C) → C6H5ONa + Na2SO3 C6H5ONa + HCl → C6H5OH + NaCl

2. From chlorobenzene (Dow process): C6H5Cl + NaOH (300 C, 200 atm) → C6H5ONa → C6H5OH

3. Cumene process (industrial, most important): Step 1: Benzene + propylene (H+) → cumene (isopropylbenzene) Step 2: Cumene + O2 → cumene hydroperoxide (ROOH) Step 3: Cumene hydroperoxide + H2SO4 (dilute) → phenol + acetone

4. From diazonium salts: Ar-N2+Cl- + H2O (boil) → ArOH + N2 + HCl

Physical Properties

Higher mp than alcohols (due to intermolecular H-bonds in crystal). Phenol slightly soluble in water; forms H-bonds with water. Weakly acidic (pKa = 10)

Acidic Character (Comparison)

Acidity: C6H5OH (pKa 10) >> ROH (pKa 16-18) Phenol is stronger acid than alcohols because:

1. Phenoxide ion (C6H5O-) is stabilized by resonance (lone pair delocalized into ring)
2. Alcoholate ion (RO-) has no resonance stabilization

Effect of substituents on phenol acidity:

• Electron-withdrawing groups (NO2, CN) at ortho/para: increase acidity (stabilize C6H5O- by delocalizing negative charge further)
• Electron-donating groups (CH3, OH) at ortho/para: decrease acidity
• p-Nitrophenol (pKa 7.1) > phenol (pKa 10) > p-methylphenol (pKa 10.2)

Chemical Reactions

1. Electrophilic Aromatic Substitution (EAS): OH group is strongly ortho/para directing (activating). Bromination: C6H5OH + Br2(aq.) → 2,4,6-tribromophenol (white ppt, no catalyst needed!) (Direct bromination of benzene needs FeBr3 catalyst; phenol doesn't)

2. Nitration: C6H5OH + dil. HNO3 → ortho-nitrophenol + para-nitrophenol (mixture)

3. Kolbe's Reaction (with CO2): C6H5ONa + CO2 (125 C, 5 atm) → sodium salicylate Sodium salicylate + HCl → salicylic acid → aspirin (acetylation)

4. Reimer-Tiemann Reaction (with CHCl3): C6H5OH + CHCl3 + KOH → 2-hydroxy-benzaldehyde (salicylaldehyde)

5. Esterification: C6H5OH + CH3COCl → C6H5OCOCH3 + HCl (phenyl acetate)

6. Azo Coupling: C6H5OH + C6H5N2+Cl- → C6H5-N=N-C6H4-OH (azo dye, orange-red) Occurs at ortho or para to OH group.

Ethers - Preparation and Reactions

Ethers

Introduction

Ethers: R-O-R' (symmetrical: both R same; unsymmetrical: R ≠ R') Common: diethyl ether (CH3CH2-O-CH2CH3), THF, dioxane.

Preparation

1. Dehydration of Alcohols (Williamson's Method - Industrial): 2ROH (conc. H2SO4, 140 C) → ROR + H2O Limited to symmetrical ethers.

2. Williamson's Synthesis (Lab, for unsymmetrical ethers): RONa + R'X → ROR' + NaX (SN2) Sodium alkoxide + alkyl halide → ether Best with primary R'X (SN2 favored; 3° would give elimination)

Example: C2H5ONa + CH3Br → C2H5-O-CH3 (methyl ethyl ether) + NaBr

Physical Properties

• Lower boiling points than comparable alcohols (no H-bonding in ethers)
• Slightly polar but cannot H-bond with each other
• Diethyl ether bp 34.6 C (very volatile, fire hazard!)
• Slightly soluble in water (lone pairs on O can H-bond with water)

Chemical Properties

1. Acid Cleavage with HI or HBr: R-O-R' + HI → ROH + R'I (one molecule) R-O-R' + excess HI → RI + R'I + H2O Mechanism: I- attacks more reactive carbon (SN2 on 1°; SN1 on 3°)

2. Cleavage with HI: Diethyl ether + 2HI → 2CH3CH2I + H2O

3. Peroxide formation: Ethers + O2 (air) → ether peroxides (ROOR') Dangerous! Must be checked for peroxides before distilling.

4. Electrophilic aromatic substitution in aryl ethers: Anisole (C6H5OCH3) + Br2 (FeBr3) → ortho and para bromoanisolePara product predominant.

Epoxides (Oxiranes)

Three-membered cyclic ethers. Preparation: RCH=CH2 + mCPBA → epoxide High ring strain → very reactive. Opened by nucleophiles under acid or base conditions.

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