Phenols

Phenol (C₆H₅OH) is a weak acid. It reacts with reactive metals and with alkali: $2\text{C}_6\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_6\text{H}_5\text{ONa} + \text{H}_2$ $\text{C}_6\text{H}_5\text{OH} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{ONa} + \text{H}_2\text{O}$

But phenol is too weak to react with carbonates (it cannot displace CO₂) — this distinguishes it from a carboxylic acid.

Why is phenol acidic? When phenol loses H⁺, the negative charge on the phenoxide ion is delocalised into the benzene ring, stabilising the ion. A more stable anion means phenol dissociates more readily than an alcohol (whose alkoxide is not stabilised). However, in a carboxylate ion the charge is shared over two electronegative oxygens — even more effective — so a carboxylic acid is stronger than phenol. The order is: $\text{carboxylic acid} > \text{phenol} > \text{water} > \text{alcohol}$

Unlike benzene, phenol reacts with bromine water at room temperature without a catalyst, decolourising the orange bromine and forming a white precipitate: $\text{C}_6\text{H}_5\text{OH} + 3\text{Br}_2 \rightarrow \text{C}_6\text{H}_2\text{Br}_3\text{OH} + 3\text{HBr}$ (2,4,6-tribromophenol).

Why so reactive? The lone pairs on the oxygen of –OH overlap with the ring's delocalised π system, donating electron density into the ring. This makes the ring more electron-rich than benzene, so it attracts electrophiles more strongly (and stabilises the intermediate better). The –OH is therefore an activating, 2,4,6-directing group.

A simple confirmatory test for a phenol is to add neutral iron(III) chloride (FeCl₃) solution: a phenol gives a violet (purple) colouration. Most alcohols give no such colour.

Together with its reactions (acidic toward NaOH but not carbonates; decolourising bromine water without a catalyst), the FeCl₃ test lets you distinguish a phenol from both an alcohol and a carboxylic acid.