Unit 7: Alcohols, Phenols and Ethers

Oxygen-containing organic compounds — essential in industry, medicine, and daily life

7.1 Classification

Classification of Alcohols, Phenols and Ethers

Alcohols (R-OH)

-OH group on sp³ carbon. Classified as primary (1°), secondary (2°), or tertiary (3°). Also allylic, benzylic, and vinylic.

Phenols (Ar-OH)

-OH group directly on aromatic ring. More acidic than alcohols due to resonance stabilisation of phenoxide ion.

Ethers (R-O-R')

Oxygen between two carbon atoms. Symmetrical (R=R') or unsymmetrical (R≠R').

7.2 Nomenclature

Alcohols

IUPAC: replace 'e' of alkane with 'ol' — methanol, ethanol, propan-2-ol. Polyhydric: ethane-1,2-diol, propane-1,2,3-triol.

Phenols

Phenol is accepted IUPAC name. Substituted: o-cresol (2-methylphenol), p-cresol (4-methylphenol). Dihydroxy: catechol (1,2), resorcinol (1,3), hydroquinone (1,4).

Ethers

Common: alkyl groups + ether (ethylmethyl ether). IUPAC: alkoxyalkane (methoxyethane, ethoxybenzene).

7.3 Preparation

Preparation of Alcohols, Phenols and Ethers

        Grignard route to alcohols: Formaldehyde → 1° alcohol | Other aldehydes → 2° alcohol | Ketones → 3° alcohol

        Williamson synthesis: Best method for mixed ethers — R-X (1°) + NaOR' → R-O-R'

7.4 Reactions of Alcohols

C-O Bond Cleavage

With HX: R-OH + HX → R-X + H₂O (Lucas test: ZnCl₂/HCl distinguishes 1°, 2°, 3°)
With SOCl₂: R-OH + SOCl₂ → R-Cl + SO₂ + HCl (best method — gaseous byproducts)
Dehydration: R-CH₂CH₂-OH → R-CH=CH₂ + H₂O (H₂SO₄, 443K — Zaitsev's rule)

O-H Bond Cleavage

Active metals: 2R-OH + 2Na → 2RONa + H₂ (reactivity: 1° > 2° > 3°)
Esterification: R-OH + R'-COOH ⇌ R'-COOR + H₂O (conc. H₂SO₄)

Oxidation

Primary: R-CH₂OH → R-CHO → R-COOH
Secondary: R₂CHOH → R₂C=O (ketone)
Tertiary: No oxidation (no α-hydrogen)

7.5 Reactions of Phenols

        -OH group is strongly activating and ortho/para directing in electrophilic aromatic substitution.

        Bromine water gives white precipitate of 2,4,6-tribromophenol (test for phenols).

        FeCl₃ gives violet colour (test for phenols).

Important Named Reactions

Kolbe's reaction: Phenol + CO₂/NaOH → salicylic acid (o-hydroxybenzoic acid)
Reimer-Tiemann reaction: Phenol + CHCl₃/NaOH → salicylaldehyde (o-hydroxybenzaldehyde)
Coupling reaction: Phenol + ArN₂⁺ → azo dye (orange/red)

7.6 Ethers

        Key points about ethers:
        Ethers are relatively unreactive — no reaction with Na, NaOH, or NaHCO₃
C-O bond cleavage occurs with strong acids (HI, HBr)
Ethers form explosive peroxides on prolonged air exposure — test with KI/starch paper before distillation
Diethyl ether was historically used as general anaesthetic

    

Summary

Alcohols, phenols, and ethers are classified by the number of -OH groups and structure. Alcohols undergo C-O and O-H bond cleavage reactions, oxidation to carbonyl compounds, and dehydration. Phenols are more acidic than alcohols due to resonance stabilisation of phenoxide, undergo electrophilic substitution at ortho/para positions, and give characteristic colour tests. Ethers are prepared by dehydration of alcohols or Williamson synthesis, and undergo C-O cleavage with HI/HBr. Named reactions include Kolbe's, Reimer-Tiemann, Williamson synthesis, and Lucas test.