Alcohols contain –OH on an sp3 carbon; phenols have –OH directly on a benzene ring; ethers have an oxygen bridging two carbon groups (R–O–R). All are important in industry and synthesis.
Alcohols
Classification: Primary (–OH on C bonded to 1C), secondary (2C), tertiary (3C).
Physical properties: High boiling points due to intermolecular O–H...O hydrogen bonding. Lower alcohols (up to C3) are fully miscible with water.
- 1.Preparation:
- 2.Hydration of alkenes (H3PO4 catalyst): CH2=CH2 + H2O → C2H5OH (Markovnikov)
- 3.Reduction of carbonyl compounds: RCHO or RCOR' + NaBH4 or LiAlH4 → alcohol
- 4.Grignard + carbonyl: RMgX + HCHO → primary alcohol (after H3O^+ workup)
- Key reactions:
- Dehydration (conc. H2SO4): at 170°C → alkene; at 140°C → ether.
- Oxidation: 1° → aldehyde → acid (K2Cr2O7); 2° → ketone; 3° resistant.
- With Na: 2ROH + 2Na → 2RONa + H2 (acidity: methanol > 1° > 2° > 3°).
- Esterification: ROH + R'COOH -(H^+)→ ester + H2O (reversible).
- Lucas test (ZnCl2/conc. HCl): 3° → immediate turbidity; 2° → 5 min; 1° → no turbidity at RT.
Phenols
The –OH group on the benzene ring donates lone pairs by +M effect: (a) weakens O–H bond → phenols are more acidic than alcohols (pKa ~10 vs ~16); (b) activates ring toward EAS at ortho/para.
Preparation: Cumene process (industrial): cumene oxidised to hydroperoxide → acid cleavage → phenol + acetone.
- Key reactions:
- With NaOH → sodium phenoxide (C6H5ONa); does NOT react with Na2CO3.
- Bromination with Br2(aq): instant 2,4,6-tribromoaniline (no catalyst needed).
- Kolbe-Schmitt: C6H5ONa + CO2 (400K, 4-7 atm) → sodium salicylate.
- Reimer-Tiemann: Phenol + CHCl3 + NaOH → salicylaldehyde (via :CCl2 intermediate).
Ethers
Formula: R–O–R'. IUPAC: alkoxyalkane. Boiling points much lower than alcohols (no O–H H-bonding).
Williamson synthesis: R'O-Na^+ + RX (primary) → R'–O–R + NaX (SN2; best for unsymmetrical ethers).
Cleavage with HI (excess): R–O–R' → 2RI + H2O.
Worked Examples
Lucas test for tert-butanol?
tert-butanol is 3°. Forms stable 3° carbocation instantly with ZnCl2/HCl.
Answer: Immediate turbidity (insoluble tert-butyl chloride forms).
Why is phenol more acidic than ethanol?
Phenoxide ion is resonance-stabilised by the benzene ring; ethoxide is not.
Answer: pKa of phenol (~10) << ethanol (~16), so phenol is a stronger acid.
Dehydration of ethanol at 170°C vs 140°C.
At 170°C: intramolecular dehydration → CH2=CH2 (elimination).
At 140°C: intermolecular dehydration → C2H5OC2H5 (ether formation).
Answer: 170°C gives ethene; 140°C gives diethyl ether.
Williamson synthesis of anisole (methoxybenzene).
C6H5ONa + CH3I → C6H5OCH3 + NaI.
Answer: Anisole (methoxybenzene).
Product when propan-2-ol is oxidised with acidified K2Cr2O7.
Propan-2-ol is 2°; secondary alcohols oxidise to ketones.
Answer: Propanone (acetone).
Why do ethers have lower boiling points than alcohols of similar mass?
Ethers lack O–H; only weak dipole-dipole forces exist. No intermolecular H-bonding.
Answer: Ethers cannot form intermolecular H-bonds, so they have much lower boiling points.
Product of excess HI with diethyl ether.
First HI: C2H5OC2H5 → C2H5I + C2H5OH. Second HI: C2H5OH → C2H5I.
Answer: 2 mol C2H5I + H2O.
Common mistakes
> Watch out: Phenol does NOT react with Na2CO3 (phenol is weaker acid than carbonic acid). Dehydration at 140°C gives ether; at 170°C gives alkene. Williamson synthesis uses a primary alkyl halide to avoid E2 side reactions.
Summary
Alcohols: high BP due to H-bonding; oxidise (1° → acid, 2° → ketone, 3° resistant); dehydrate to alkene or ether. Phenols: more acidic than alcohols; activates ring toward EAS; key reactions include Kolbe-Schmitt and Reimer-Tiemann. Ethers prepared by Williamson synthesis; cleaved by HI.