Alcohols, Phenols and Ethers
This domain tests acidity differences, distinguishing tests, dehydration and oxidation sequences, and named phenol reactions.
Alcohols
- Grignard addition: HCHO gives a primary alcohol, other aldehydes secondary, ketones tertiary
- dehydration order 3° > 2° > 1°, via carbocation stability
- oxidation: 1° alcohol → aldehyde → carboxylic acid; 2° → ketone; 3° resists oxidation
- Lucas test: 3° alcohol gives immediate turbidity, 2° after about five minutes, 1° no reaction at room temperature
- Victor Meyer test distinguishes 1°, 2° and 3° alcohols by colour
Phenols
- acidic enough to react with NaOH but not with Na2CO3; Ka around 10⁻¹⁰ versus 10⁻¹⁶ for alcohols
- the phenoxide ion is resonance-stabilised
- electron-withdrawing groups raise acidity: p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol (ortho is weakened by intramolecular hydrogen bonding)
- Reimer–Tiemann gives salicylaldehyde; Kolbe gives salicylic acid; coupling with diazonium salts gives azo dyes
Ethers
- Williamson synthesis works best with a primary halide; tertiary halides give elimination instead
- cleavage with HI: primary and secondary ethers react by SN2 giving iodomethane and an alcohol; aryl ethers give phenol and an alkyl iodide
- Zeisel method estimates methoxy groups
Worked example
To distinguish propan-1-ol, propan-2-ol and 2-methylpropan-2-ol, add Lucas reagent. 2-Methylpropan-2-ol turns cloudy at once (tertiary), propan-2-ol turns cloudy only after warming (secondary), and propan-1-ol gives no cloudiness (primary).
Traps that cost marks
- Claiming phenol reacts with NaHCO3 to release CO2; that is carboxylic acid behaviour.
- Reversing the o- and p-nitrophenol acidity comparison.
- Using Williamson coupling on a tertiary halide.