Alcohols, Phenols and Ethers
Alcohols, Phenols and Ethers
The exam tests preparation, acidity, characteristic reactions and tests that distinguish the three classes.
Core rules
- Alcohol acidity in solution: 1° > 2° > 3°, because alkyl groups destabilise the alkoxide.
- Phenol is more acidic than alcohols (resonance-stabilised phenoxide) but less acidic than carboxylic acids.
- Lucas test: 3° alcohol turbid immediately, 2° in about 5 min, 1° only on heating.
- Victor Meyer test distinguishes 1°, 2° and 3° alcohols by colour.
- Phenol reactions: Reimer–Tiemann (CHCl₃/NaOH), Kolbe (CO₂/NaOH), bromination and coupling with diazonium salts.
- Williamson ether synthesis: best with 1° halides; 3° halides give elimination.
Worked example Why is phenol more acidic than ethanol? Phenol loses H⁺ to give phenoxide, in which the negative charge is delocalised onto the ring through resonance. Ethoxide has no such delocalisation, so phenoxide is more stable and phenol has a larger Kₐ (pKₐ about 10 versus about 16).
Traps
- Saying phenol gives a positive Lucas test; it does not behave like an alcohol.
- Forgetting that HI cleavage of an ether gives the iodide from the smaller alkyl group.
- Assuming all alcohols dehydrate at the same rate; the order is 3° > 2° > 1°.