Alcohols, Phenols & Ethers

Alcohols, phenols, and ethers are oxygen-containing organic compounds with diverse chemistry. Alcohols (R-OH) and phenols (Ar-OH) contain the hydroxyl group; ethers (R-O-R') have an oxygen bridge between two carbon groups.

Classification of Alcohols

Alcohols are classified based on the carbon bearing -OH: Primary (1°) — OH on a carbon attached to one other carbon (CH₃CH₂OH). Secondary (2°) — OH on a carbon attached to two other carbons ((CH₃)₂CHOH). Tertiary (3°) — OH on a carbon attached to three other carbons ((CH₃)₃COH). This classification determines reactivity patterns for oxidation, dehydration, and substitution reactions.

Key alcohol structures:

Ethanol (1° alcohol):

Isopropanol (2° alcohol):

tert-Butanol (3° alcohol):

Preparation of Alcohols

1. Hydration of alkenes: (a) Acid-catalysed (Markovnikov): CH₂=CH₂ + H₂O (H₂SO₄) → CH₃CH₂OH. For unsymmetrical alkenes, Markovnikov addition gives 2° or 3° alcohol. (b) Hydroboration-oxidation (anti-Markovnikov): RCH=CH₂ + BH₃ → (RCH₂CH₂)₃B + H₂O₂/NaOH → RCH₂CH₂OH (primary alcohol from terminal alkene — syn addition).

2. Grignard reagent reactions: RMgX + HCHO → RCH₂OH (1° alcohol). RMgX + R'CHO → RR'CHOH (2° alcohol). RMgX + R'COR" → RR'R"COH (3° alcohol). RMgX + ethylene oxide → RCH₂CH₂OH (extends chain by 2C).

3. Reduction: Aldehydes → 1° alcohols (NaBH₄ or LiAlH₄). Ketones → 2° alcohols. Carboxylic acids/esters → 1° alcohols (LiAlH₄ only — NaBH₄ cannot reduce acids/esters).

4. From carbonyl compounds: Cannizzaro reaction (HCHO + conc. NaOH → CH₃OH + HCOONa). Tischenko reaction (2RCHO + Al(OEt)₃ → RCOOCH₂R, an ester).

Physical Properties of Alcohols

Boiling points: Alcohols have higher b.p. than corresponding alkanes, ethers, and halides due to intermolecular hydrogen bonding. Order: 1° > 2° > 3° (for same molecular weight — better H-bonding in 1°). Lower alcohols (methanol, ethanol) are miscible with water; solubility decreases as hydrocarbon chain lengthens.

Reactions of Alcohols

1. Acidity: Alcohols are weakly acidic. Acidity order: H₂O > 1° > 2° > 3° (in gas phase, reversed due to inductive effect). In solution, steric effects and solvation dominate. With Na: 2ROH + 2Na → 2RONa + H₂.

2. Esterification: ROH + R'COOH (H+, heat) → R'COOR + H₂O (Fischer esterification, reversible). Mechanism: nucleophilic acyl substitution.

3. Oxidation: 1° ROH → RCHO (PCC, mild) → RCOOH (KMnO₄, K₂Cr₂O₇). 2° ROH → R₂CO (ketone). 3° ROH → resistant (strong oxidation → C-C cleavage). Reagents: PCC (stops at aldehyde), Jones reagent (CrO₃/H₂SO₄), KMnO₄.

4. Dehydration: ROH (conc. H₂SO₄, heat) → alkene + H₂O. Ease of dehydration: 3° > 2° > 1° (stability of carbocation intermediate). Zaitsev's rule: more substituted alkene is the major product. Temperature matters: ethanol at 443 K → ethene; ethanol at 413 K → diethyl ether.

5. Substitution with HX: ROH + HX → RX + H₂O. Reactivity of HX: HI > HBr > HCl. Reactivity of alcohol: 3° > 2° > 1° (SN₁ for 3°, SN₂ for 1°). Lucas test: ZnCl₂/conc. HCl — 3° gives immediate turbidity, 2° in 5-10 min, 1° requires heating.

6. With PCl₅, PCl₃, SOCl₂: ROH + SOCl₂ → RCl + SO₂ + HCl (Darzen's reaction — cleanest, gaseous by-products).

Phenols

Structure: Hydroxyl group attached directly to benzene ring. The lone pair on oxygen is delocalised into the ring (resonance), making the O-H bond weaker (more acidic than alcohols) and activating the ring for electrophilic substitution.

Phenol:

p-Nitrophenol (increased acidity, pKa 7.15):

Acidity: Phenol (pKa ~10) > H₂O > alcohols. Electron-withdrawing groups (NO₂, CN) increase acidity; electron-donating groups (CH₃, OCH₃) decrease it. p-Nitrophenol (pKa 7.15) is much more acidic than phenol due to resonance stabilisation of the phenoxide by -NO₂.

Preparation: (1) From cumene (industrial): C₆H₅CH(CH₃)₂ + O₂ → C₆H₅OH + (CH₃)₂CO (phenol + acetone). (2) From diazonium salt: ArN₂+ + H₂O (warm) → ArOH + N₂ + H+. (3) Dow's process: C₆H₅Cl + NaOH (623 K, 300 atm) → C₆H₅ONa + HCl.

Reactions of phenols:

1. Kolbe's reaction (Kolbe-Schmitt): C₆H₅ONa + CO₂ (125°C, 4-7 atm) → sodium salicylate → salicylic acid (ortho-hydroxybenzoic acid). At higher temperature (higher T), para-product predominates.

Salicylic acid (ortho-hydroxybenzoic acid):

2. Reimer-Tiemann reaction: C₆H₅OH + CHCl₃ + NaOH → o-hydroxybenzaldehyde (salicylaldehyde). Intermediate: dichlorocarbene (:CCl₂). With CCl₄ instead of CHCl₃: salicylic acid is formed.

3. Fries rearrangement: Phenyl ester + AlCl₃ → o- and p-acylphenol. Low temperature favours para; high temperature favours ortho.

4. Electrophilic substitution: Phenol is strongly activating (ortho/para director). Bromination: C₆H₅OH + Br₂(aq) → 2,4,6-tribromophenol (white ppt) — no catalyst needed. Nitration: dilute HNO₃ gives mix of o- and p-nitrophenol. Friedel-Crafts does NOT work directly on phenol (OH coordinates to Lewis acid catalyst).

5. Liebermann's test: Phenol + NaNO₂ + conc. H₂SO₄ → deep blue/green colour (turns red with NaOH).

Ethers

Structure: R-O-R' (symmetric) or R-O-R' (asymmetric). Named as alkoxyalkanes (IUPAC).

Preparation: Williamson synthesis: RONa + R'X → R-O-R' + NaX. Best with primary R'X (SN₂). If R'X is tertiary, elimination dominates — use the tertiary group as alkoxide instead. This is the most important ether synthesis for JEE.

Dehydration of alcohols: 2ROH (conc. H₂SO₄, 413 K) → R-O-R + H₂O. Only for symmetric ethers from primary alcohols.

Reactions of ethers:

1. Cleavage by HI: R-O-R' + excess HI → RI + R'I + H₂O. With limited HI: larger/more substituted group preferentially leaves as iodide. Mechanism: protonation of O, then SN₂ (1° alkyl) or SN₁ (3° alkyl) attack by I-.

2. Electrophilic substitution on anisole (C₆H₅OCH₃): -OCH₃ is activating (+M effect), ortho/para director. Bromination, nitration, Friedel-Crafts all give predominantly ortho/para products.

Anisole (methyl phenyl ether):

3. Zeisel's method: R-O-R' + HI → RI + R'OH; then RI + AgNO₃ → AgI (yellow ppt). Used to detect and estimate methoxy/ethoxy groups.

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