Alcohol, Ether and Phenol Notes for Class 12, IIT JEE & NEET

Phenol (pKa ≈ 10) is more acidic than water (pKa ≈ 15.7), which is more acidic than alcohols (pKa ≈ 16–18). Phenol's higher acidity arises from resonance stabilisation of the phenoxide ion. This order — phenol > water > alcohol — is tested almost every year in both JEE Main and NEET.

The Lucas test uses ZnCl₂ and conc. HCl to convert an alcohol to an alkyl chloride. Tertiary alcohols form stable tertiary carbocations (SN1 mechanism) instantly at room temperature, turning the solution turbid immediately. Secondary alcohols react slowly, and primary alcohols require heating to react at all.

Williamson ether synthesis is an SN2 reaction between sodium alkoxide and an alkyl halide to form an ether. A primary alkyl halide must be used because secondary and tertiary halides undergo elimination (E2) instead of substitution when attacked by the strongly basic alkoxide. The product would be an alkene, not an ether.

Both reactions use the same alcohol and conc. H₂SO₄. At 413 K (lower temperature), intermolecular dehydration of two alcohol molecules gives an ether. At 443 K (higher temperature), intramolecular dehydration gives an alkene. Temperature is the deciding variable — this is a standard JEE Main question format.

The –OH group donates electron density into the benzene ring through resonance, activating the ring towards electrophilic attack. It directs incoming electrophiles to the ortho and para positions. This is why bromination of phenol with Br₂(aq) gives 2,4,6-tribromophenol directly without a Lewis acid catalyst, unlike benzene bromination.