Qualitative Tests for Phenols

Phenols have an −OH group bonded directly to a benzene ring. Two properties are used to identify them.1. Phenol is a weak acid. It is more acidic than an alcohol but less acidic than a carboxylic acid.• Phenol reacts with a strong base, sodium hydroxide, to form a salt and water:C₆H₅OH + NaOH → C₆H₅ONa + H₂O• Phenol is not acidic enough to react with sodium carbonate, so there is no effervescence. Carboxylic acids do react with carbonates, so this distinguishes a phenol from a carboxylic acid.2. Phenol reacts with bromine water at room temperature, without a halogen carrier. The orange bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms:C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBrThis is electrophilic substitution. It happens easily because a lone pair on the oxygen is partially delocalised into the ring, increasing its electron density.Worked example: three colourless liquids are ethanol, phenol (dissolved in water) and ethanoic acid. Add sodium carbonate: only ethanoic acid fizzes. Add bromine water to the other two: phenol gives a white precipitate; ethanol gives no change.The white precipitate distinguishes phenol from an alkene, which also decolourises bromine water but gives no precipitate.